Crystal structure of CTP-ligated T state aspartate transcarbamoylase at 2.5 A resolution: implications for ATCase mutants and the mechanism of negative cooperativity

Illuminating Protein Allostery by Chemically Accurate Contact Response Analysis (ChACRA)

Decoding allostery at the atomic level is essential for understanding the relationship between a protein's sequence, structure, and dynamics. Recently, we have shown that decomposing temperature responses of inter-residue contacts can reveal allosteric couplings and provide useful insight into the functional dynamics of proteins. The details of this Chemically Accurate Contact Response Analysis (ChACRA) are presented here along with its application to two well-known allosteric proteins. The first protein, IGPS, is a model of ensemble allostery that lacks clear structural differences between the active and inactive states. We show that the application of ChACRA reveals the experimentally identified allosteric coupling between effector and active sites of IGPS. The second protein, ATCase, is a classic example of allostery with distinct active and inactive structural states. Using ChACRA, we directly identify the most significant residue level interactions underlying the enzyme's cooperative behavior. Both test cases demonstrate the utility of ChACRA's unsupervised machine learning approach for dissecting allostery at the residue level.

Biological Assembly Comparison with VAST

The VAST+ algorithm is an efficient, simple, and elegant solution to the problem of comparing the atomic structures of biological assemblies. Given two protein assemblies, it takes as input all the pairwise structural alignments of the component proteins. It then clusters the rotation matrices from the pairwise superpositions, with the clusters corresponding to subsets of the two assemblies that may be aligned and well superposed. It uses the Vector Alignment Search Tool (VAST) protein-protein comparison method for the input structural alignments, but other methods could be used, as well. From a chosen cluster, an "original" alignment for the assembly may be defined by simply combining the relevant input alignments. However, it is often useful to reduce/trim the original alignment, using a Monte Carlo refinement algorithm, which allows biologically relevant conformational differences to be more readily detected and observed. The method is easily extended to include RNA or DNA molecules. VAST+ results may be accessed via the URL https://www.ncbi.nlm.nih.gov/Structure , then entering a PDB accession or terms in the search box, and using the link [VAST+] in the upper right corner of the Structure Summary page.

Methodological improvements for the analysis of domain movements in large biomolecular complexes

Domain movements play a prominent role in the function of many biomolecules such as the ribosome and F₀F₁-ATP synthase. As more structures of large biomolecules in different functional states become available as experimental techniques for structure determination advance, there is a need to develop methods to understand the conformational changes that occur. DynDom and DynDom3D were developed to analyse two structures of a biomolecule for domain movements. They both used an original method for domain recognition based on clustering of "rotation vectors". Here we introduce significant improvements in both the methodology and implementation of a tool for the analysis of domain movements in large multimeric biomolecules. The main improvement is in the recognition of domains by using all six degrees of freedom required to describe the movement of a rigid body. This is achieved by way of Chasles' theorem in which a rigid-body movement can be described as a screw movement about a unique axis. Thus clustering now includes, in addition to rotation vector data, screw-axis location data and axial climb data. This improves both the sensitivity of domain recognition and performance. A further improvement is the recognition and annotation of interdomain bending regions, something not done for multimeric biomolecules in DynDom3D. This is significant as it is these regions that collectively control the domain movement. The new stand-alone, platform-independent implementation, DynDom6D, can analyse biomolecules comprising protein, DNA and RNA, and employs an alignment method to automatically achieve the required equivalence of atoms in the two structures.

From Genome to Structure and Back Again: A Family Portrait of the Transcarbamylases

Enzymes in the transcarbamylase family catalyze the transfer of a carbamyl group from carbamyl phosphate (CP) to an amino group of a second substrate. The two best-characterized members, aspartate transcarbamylase (ATCase) and ornithine transcarbamylase (OTCase), are present in most organisms from bacteria to humans. Recently, structures of four new transcarbamylase members, N-acetyl-l-ornithine transcarbamylase (AOTCase), N-succinyl-l-ornithine transcarbamylase (SOTCase), ygeW encoded transcarbamylase (YTCase) and putrescine transcarbamylase (PTCase) have also been determined. Crystal structures of these enzymes have shown that they have a common overall fold with a trimer as their basic biological unit. The monomer structures share a common CP binding site in their N-terminal domain, but have different second substrate binding sites in their C-terminal domain. The discovery of three new transcarbamylases, l-2,3-diaminopropionate transcarbamylase (DPTCase), l-2,4-diaminobutyrate transcarbamylase (DBTCase) and ureidoglycine transcarbamylase (UGTCase), demonstrates that our knowledge and understanding of the spectrum of the transcarbamylase family is still incomplete. In this review, we summarize studies on the structures and function of transcarbamylases demonstrating how structural information helps to define biological function and how small structural differences govern enzyme specificity. Such information is important for correctly annotating transcarbamylase sequences in the genome databases and for identifying new members of the transcarbamylase family.

An unprecedented porphyrin-pillar[5]arene hybrid ditopic receptor

A porphyrin-pillar[5]arene hybrid host compound with a ditopic receptor nature was synthesized for the first time, which combines a neutral 1,4-bis(imidazol-1-yl)butane guest by means of its two active centers to form a stable supramolecular complex.

From feedback inhibition to allostery: the enduring example of aspartate transcarbamoylase

Aspartate transcarbamoylase (ATCase) of Escherichia coli, the first enzyme of the pyrimidine biosynthetic pathway, is inhibited by CTP and UTP, the nucleotide end-products of the pathway. First discovered by Yates and Pardee in 1956 [Yates R & Pardee AB (1956) J Biol Chem 221, 743-756; Yates RA & Pardee AB (1956) J Biol Chem 221, 757-770], these interactions establish feedback inhibition in vivo, a key means of metabolic regulation by which end-product production by the pathway is adjusted to end-product usage in macromolecule synthesis. Activation of the enzyme by the purine nucleotide ATP may also have regulatory significance. ATCase and threonine deaminase of E. coli were the first enzymes to be characterized with regard to their allosteric properties, namely, sigmoidal saturation with regard to substrates, reflecting cooperative ligand binding at the active site, and inhibition and activation by nucleotides of very different chemical structure from the substrates. In the case of ATCase, the nucleotides bind at regulatory sites located on protein subunits different from those bearing the active sites. The early characterization of ATCase proved useful in the 1965 conceptualization of the allosteric transition by Monod, Wyman, and Changeux [Monod J et al. (1965) J Mol Biol 12, 88-118], and the protein in subsequent years has proved useful in the experimental analysis of the interactions of sites and of conformational changes in allosteric proteins. This is an account of the early years of work on ATCase, up to 1965.

Metal ion involvement in the allosteric mechanism of Escherichia coli aspartate transcarbamoylase

Escherichia coli aspartate transcarbamoylase (ATCase) allosterically regulates pyrimidine nucleotide biosynthesis. The enzyme is inhibited by CTP and can be further inhibited by UTP, although UTP alone has little or no influence on activity; however, the mechanism for the synergistic inhibition is still unknown. To determine how UTP is able to synergistically inhibit ATCase in the presence of CTP, we determined a series of X-ray structures of ATCase·nucleotide complexes. Analysis of the X-ray structures revealed that (1) CTP and dCTP bind in a very similar fashion, (2) UTP, in the presence of dCTP or CTP, binds at a site that does not overlap the CTP/dCTP site, and (3) the triphosphates of the two nucleotides are parallel to each other with a metal ion, in this case Mg(2+), coordinated between the β- and γ-phosphates of the two nucleotides. Kinetic experiments showed that the presence of a metal ion such as Mg(2+) is required for synergistic inhibition. Together, these results explain how the binding of UTP can enhance the binding of CTP and why UTP binds more tightly in the presence of CTP. A mechanism for the synergistic inhibition of ATCase is proposed in which the presence of UTP stabilizes the T state even more than CTP alone. These results also call into question many of the past kinetic and binding experiments with ATCase with nucleotides as the presence of metal contamination was not considered important.

Structure and mechanisms of Escherichia coli aspartate transcarbamoylase

Enzymes catalyze a particular reaction in cells, but only a few control the rate of this reaction and the metabolic pathway that follows. One specific mechanism for such enzymatic control of a metabolic pathway involves molecular feedback, whereby a metabolite further down the pathway acts at a unique site on the control enzyme to alter its activity allosterically. This regulation may be positive or negative (or both), depending upon the particular system. Another method of enzymatic control involves the cooperative binding of the substrate, which allows a large change in enzyme activity to emanate from only a small change in substrate concentration. Allosteric regulation and homotropic cooperativity are often known to involve significant conformational changes in the structure of the protein. Escherichia coli aspartate transcarbamoylase (ATCase) is the textbook example of an enzyme that regulates a metabolic pathway, namely, pyrimidine nucleotide biosynthesis, by feedback control and by the cooperative binding of the substrate, L-aspartate. The catalytic and regulatory mechanisms of this enzyme have been extensively studied. A series of X-ray crystal structures of the enzyme in the presence and absence of substrates, products, and analogues have provided details, at the molecular level, of the conformational changes that the enzyme undergoes as it shifts between its low-activity, low-affinity form (T state) to its high-activity, high-affinity form (R state). These structural data provide insights into not only how this enzyme catalyzes the reaction between l-aspartate and carbamoyl phosphate to form N-carbamoyl-L-aspartate and inorganic phosphate, but also how the allosteric effectors modulate this activity. In this Account, we summarize studies on the structure of the enzyme and describe how these structural data provide insights into the catalytic and regulatory mechanisms of the enzyme. The ATCase-catalyzed reaction is regulated by nucleotide binding some 60 Å from the active site, inducing structural alterations that modulate catalytic activity. The delineation of the structure and function in this particular model system will help in understanding the molecular basis of cooperativity and allosteric regulation in other systems as well.

Allostery and cooperativity in Escherichia coli aspartate transcarbamoylase

The allosteric enzyme aspartate transcarbamoylase (ATCase) from Escherichia coli has been the subject of investigations for approximately 50 years. This enzyme controls the rate of pyrimidine nucleotide biosynthesis by feedback inhibition, and helps to balance the pyrimidine and purine pools by competitive allosteric activation by ATP. The catalytic and regulatory components of the dodecameric enzyme can be separated and studied independently. Many of the properties of the enzyme follow the Monod, Wyman Changeux model of allosteric control thus E. coli ATCase has become the textbook example. This review will highlight kinetic, biophysical, and structural studies which have provided a molecular level understanding of how the allosteric nature of this enzyme regulates pyrimidine nucleotide biosynthesis.

Enzymes, embryos, and ancestors

In the 1950s, cellular regulatory mechanisms were newly recognized; with Arthur Pardee I investigated the initial enzyme of pyrimidine biosynthesis, which he discovered is controlled by feedback inhibition. The protein proved unusual in having separate but interacting sites for substrates and regulators. Howard Schachman and I dissociated the protein into different subunits, one binding regulators and one substrates. The enzyme became an early prime example of allostery. In developmental biology I studied the egg of the frog, Xenopus laevis, characterizing early processes of axis formation. My excellent students and I described cortical rotation, a 30° movement of the egg's cortex over tracks of parallel microtubules anchored to the underlying cytoplasmic core, and we perturbed it to alter Spemann's organizer and effect spectacular phenotypes. The entire sequence of events has been elucidated by others at the molecular level, making Xenopus a prime example of vertebrate axis formation. Marc Kirschner, Christopher Lowe, and I then compared hemichordate (half-chordate) and chordate early development. Despite anatomical-physiological differences, these groups share numerous steps of axis formation, ones that were probably already in use in their pre-Cambrian ancestor. I've thoroughly enjoyed exploring these areas during a 50-year period of great advances in biological sciences by the worldwide research community.

A cooperative Escherichia coli aspartate transcarbamoylase without regulatory subunits

Here we report the isolation, kinetic characterization, and X-ray structure determination of a cooperative Escherichia coli aspartate transcarbamoylase (ATCase) without regulatory subunits. The native ATCase holoenzyme consists of six catalytic chains organized as two trimers bridged noncovalently by six regulatory chains organized as three dimers, c(6)r(6). Dissociation of the native holoenzyme produces catalytically active trimers, c(3), and nucleotide-binding regulatory dimers, r(2). By introducing specific disulfide bonds linking the catalytic chains from the upper trimer site specifically to their corresponding chains in the lower trimer prior to dissociation, a new catalytic unit, c(6), was isolated consisting of two catalytic trimers linked by disulfide bonds. Not only does the c(6) species display enhanced enzymatic activity compared to the wild-type enzyme, but the disulfide bonds also impart homotropic cooperativity, never observed in the wild-type c(3). The c(6) ATCase was crystallized in the presence of phosphate and its X-ray structure determined to 2.10 A resolution. The structure of c(6) ATCase liganded with phosphate exists in a nearly identical conformation as other R-state structures with similar values calculated for the vertical separation and planar angles. The disulfide bonds linking upper and lower catalytic trimers predispose the active site into a more active conformation by locking the 240s loop into the position characteristic of the high-affinity R state. Furthermore, the elimination of the structural constraints imposed by the regulatory subunits within the holoenzyme provides increased flexibility to the c(6) enzyme, enhancing its activity over the wild-type holoenzyme (c(6)r(6)) and c(3). The covalent linkage between upper and lower catalytic trimers restores homotropic cooperativity so that a binding event at one or so active sites stimulates binding at the other sites. Reduction of the disulfide bonds in the c(6) ATCase results in c(3) catalytic subunits that display kinetic parameters similar to those of wild-type c(3). This is the first report of an active c(6) catalytic unit that displays enhanced activity and homotropic cooperativity.

The pathway of product release from the R state of aspartate transcarbamoylase

The pathway of product release from the R state of aspartate transcarbamoylase (ATCase; EC 2.1.3.2, aspartate carbamoyltransferase) has been determined here by solving the crystal structure of Escherichia coli ATCase locked in the R quaternary structure by specific introduction of disulfide bonds. ATCase displays ordered substrate binding and product release, remaining in the R state until substrates are exhausted. The structure reported here represents ATCase in the R state bound to the final product molecule, phosphate. This structure has been difficult to obtain previously because the enzyme relaxes back to the T state after the substrates are exhausted. Hence, cocrystallizing the wild-type enzyme with phosphate results in a T-state structure. In this structure of the enzyme trapped in the R state with specific disulfide bonds, we observe two phosphate molecules per active site. The position of the first phosphate corresponds to the position of the phosphate of carbamoyl phosphate (CP) and the position of the phosphonate of N-phosphonacetyl-l-aspartate. However, the second, more weakly bound phosphate is bound in a positively charged pocket that is more accessible to the surface than the other phosphate. The second phosphate appears to be on the path that phosphate would have to take to exit the active site. Our results suggest that phosphate dissociation and CP binding can occur simultaneously and that the dissociation of phosphate may actually promote the binding of CP for more efficient catalysis.

Synthesis and in vitro evaluation of aspartate transcarbamoylase inhibitors

The design, synthesis, and evaluation of a series of novel inhibitors of aspartate transcarbamoylase (ATCase) are reported. Several submicromolar phosphorus-containing inhibitors are described, but all-carboxylate compounds are inactive. Compounds were synthesized to probe the postulated cyclic transition-state of the enzyme-catalyzed reaction. In addition, the associated role of the protonation state at the phosphorus acid moiety was evaluated using phosphinic and carboxylic acids. Although none of the synthesized inhibitors is more potent than N-phosphonacetyl-l-aspartate (PALA), the compounds provide useful mechanistic information, as well as the basis for the design of future inhibitors and/or prodrugs.

Submicromolar phosphinic inhibitors of Escherichia coli aspartate transcarbamoylase

The design, syntheses, and enzymatic activity of two submicromolar competitive inhibitors of aspartate transcarbamoylase (ATCase) are described. The phosphinate inhibitors are analogs of N-phosphonacetyl-l-aspartate (PALA) but have a reduced charge at the phosphorus moiety. The mechanistic implications are discussed in terms of a possible cyclic transition-state during enzymatic catalysis.

Picosecond dynamics of T and R forms of aspartate transcarbamylase: A neutron scattering study

E. coli aspartate transcarbamylase (ATCase) is a 310 kDa allosteric enzyme which catalyses the first committed step in pyrimidine biosynthesis. The binding of its substrates, carbamylphosphate and aspartate, induces significant conformational changes. This enzyme shows homotropic cooperative interactions between the catalytic sites for the binding of aspartate. This property is explained by a quaternary structure transition from T state (aspartate low affinity) to R state (aspartate high affinity) accompanied by a 5% increase of radius of gyration of ATCase. The same quaternary structure change is observed upon binding of the bisubstrate analogue PALA (N-(phosphonacetyl)-L-aspartate. Owing to the large incoherent neutron scattering cross-section of the hydrogen atom and the abundance of this element in proteins, inelastic neutron scattering gives a global view of protein dynamics as sensed via the individual motions of its hydrogen atoms. We present neutron scattering results of the local dynamics (few angstroms), at short time (few tens of picoseconds), of ATCase in T and R forms. Compared to the T form, we observe an increased mobility of the protein in the R form that we associate to an increase of accessible surface area to the solvent. Beyond this specific result, this highlights the key role of the accessible surface area (ASA) in dynamic contribution to inelastic neutron data in the picosecond time scale. In particular, we want to stress out (i) that a difference at the picosecond time scale does not allow to conclude to a difference in the dynamics at a longer time scale and to address whether the T state is looser than the R state (ii) how challenging is, any comparison in terms of general dynamics (tense or relaxed) between dynamic values deduced from experimental neutron data on proteins with different sequences and therefore ASA. This caveat holds particularly when comparing dynamics of a mesophile with the corresponding extremophile.

Structural basis for ordered substrate binding and cooperativity in aspartate transcarbamoylase

X-ray structures of aspartate transcarbamoylase in the absence and presence of the first substrate carbamoyl phosphate are reported. These two structures in conjunction with in silico docking experiments provide snapshots of critical events in the function of the enzyme. The ordered substrate binding, observed experimentally, can now be structurally explained by a conformational change induced upon the binding of carbamoyl phosphate. This induced fit dramatically alters the electrostatics of the active site, creating a binding pocket for aspartate. Upon aspartate binding, a further change in electrostatics causes a second induced fit, the domain closure. This domain closure acts as a clamp that both facilitates catalysis by approximation and also initiates the global conformational change that manifests homotropic cooperativity.

Monitoring the transition from the T to the R state in E.coli aspartate transcarbamoylase by X-ray crystallography: crystal structures of the E50A mutant enzyme in four distinct allosteric states

A detailed description of the transition that allosteric enzymes undergo constitutes a major challenge in structural biology. We have succeeded in trapping four distinct allosteric states of a mutant enzyme of Escherichia coli aspartate transcarbomylase and determining their structures by X-ray crystallography. The mutant version of aspartate transcarbamoylase in which Glu50 in the catalytic chains was replaced by Ala destabilizes the native R state and shifts the equilibrium towards the T state. This behavior allowed the use of substrate analogs such as phosphonoacetamide and malonate to trap the enzyme in T-like and R-like structures that are distinct from the T-state structure of the wild-type enzyme (as represented by the structure of the enzyme with CTP bound and the R-state structure as represented by the structure with N-(phosphonacetyl)-L-aspartate bound). These structures shed light on the nature and the order of internal structural rearrangements during the transition from the T to the R state. They also suggest an explanation for diminished activity of the E50A enzyme and for the change in reaction mechanism from ordered to random for this mutant enzyme.

Crystal Structure of the Reaction Complex of 3-Deoxy- d - arabino -heptulosonate-7-phosphate Synthase from Thermotoga maritima Refines the Catalytic Mechanism and Indicates a New Mechanism of Allosteric Regulation

3-Deoxy-d-arabino-heptulosonate-7-phosphate synthase (DAHPS) catalyzes the first reaction of the aromatic biosynthetic pathway in bacteria, fungi, and plants, the condensation of phosphoenolpyruvate (PEP) and d-erythrose-4-phosphate (E4P) with the formation of DAHP. Crystals of DAHPS from Thermotoga maritima (DAHPS(Tm)) were grown in the presence of PEP and metal cofactor, Cd(2+), and then soaked with E4P at 4 degrees C where the catalytic activity of the enzyme is negligible. The crystal structure of the "frozen" reaction complex was determined at 2.2A resolution. The subunit of the DAHPS(Tm) homotetramer consists of an N-terminal ferredoxin-like (FL) domain and a (beta/alpha)(8)-barrel domain. The active site located at the C-end of the barrel contains Cd(2+), PEP, and E4P, the latter bound in a non-productive conformation. The productive conformation of E4P is suggested and a catalytic mechanism of DAHPS is proposed. The active site of DAHPS(Tm) is nearly identical to the active sites of the other two known DAHPS structures from Escherichia coli (DAHPS(Ec)) and Saccharomyces cerevisiae (DAHPS(Sc)). However, the secondary, tertiary, and quaternary structures of DAHPS(Tm) are more similar to the functionally related enzyme, 3-deoxy-d-manno-octulosonate-8-phosphate synthase (KDOPS) from E.coli and Aquiflex aeolicus, than to DAHPS(Ec) and DAHPS(Sc). Although DAHPS(Tm) is feedback-regulated by tyrosine and phenylalanine, it lacks the extra barrel segments that are required for feedback inhibition in DAHPS(Ec) and DAHPS(Sc). A sequence similarity search revealed that DAHPSs of phylogenetic family Ibeta possess a FL domain like DAHPS(Tm) while those of family Ialpha have extra barrel segments similar to those of DAHPS(Ec) and DAHPS(Sc). This indicates that the mechanism of feedback regulation in DAHPS(Tm) and other family Ibeta enzymes is different from that of family Ialpha enzymes, most likely being mediated by the FL domain.

240s Loop Interactions Stabilize the T State of Escherichia coli Aspartate Transcarbamoylase

Here the functional and structural importance of interactions involving the 240s loop of the catalytic chain for the stabilization of the T state of aspartate transcarbamoylase were tested by replacement of Lys-244 with Asn and Ala. For the K244A and K244N mutant enzymes, the aspartate concentration required to achieve half-maximal specific activity was reduced to 8.4 and 4.0 mm, respectively, as compared with 12.4 mM for the wild-type enzyme. Both mutant enzymes exhibited dramatic reductions in homotropic cooperativity and the ability of the heterotropic effectors to modulate activity. Small angle x-ray scattering studies showed that the unligated structure of the mutant enzymes, and the structure of the mutant enzymes ligated with N-phosphonacetyl-L-aspartate, were similar to that observed for the unligated and N-phosphonacetyl-L-aspartateligated wild-type enzyme. A saturating concentration of carbamoyl phosphate alone has little influence on the small angle x-ray scattering of the wild-type enzyme. However, carbamoyl phosphate was able to shift the structure of the two mutant enzymes dramatically toward R, establishing that the mutations had destabilized the T state of the enzyme. The x-ray crystal structure of K244N enzyme showed that numerous local T state stabilizing interactions involving 240s loop residues were lost. Furthermore, the structure established that the mutation induced additional alterations at the subunit interfaces, the active site, the relative position of the domains of the catalytic chains, and the allosteric domain of the regulatory chains. Most of these changes reflect motions toward the R state structure. However, the K244N mutation alone only changes local conformations of the enzyme to an R-like structure, without triggering the quaternary structural transition. These results suggest that loss of cooperativity and reduction in heterotropic effects is due to the dramatic destabilization of the T state of the enzyme by this mutation in the 240s loop of the catalytic chain.

Aspartate transcarbamylase from the hyperthermophilic archaeon Pyrococcus abyssi: thermostability and 1.8A resolution crystal structure of the catalytic subunit complexed with the bisubstrate analogue N-phosphonacetyl-L-aspartate

The Pyrococcus abyssi aspartate transcarbamylase (ATCase) shows a high degree of structural conservation with respect to the well-studied mesophilic Escherichia coli ATCase, including the association of catalytic and regulatory subunits. The adaptation of its catalytic function to high temperature was investigated, using enzyme purified from recombinant E.coli cells. At 90 degrees C, the activity of the trimeric catalytic subunit was shown to be intrinsically thermostable. Significant extrinsic stabilization by phosphate, a product of the reaction, was observed when the temperature was raised to 98 degrees C. Comparison with the holoenzyme showed that association with regulatory subunits further increases thermostability. To provide further insight into the mechanisms of its adaptation to high temperature, the crystal structure of the catalytic subunit liganded with the analogue N-phosphonacetyl-L-aspartate (PALA) was solved to 1.8A resolution and compared to that of the PALA-liganded catalytic subunit from E.coli. Interactions with PALA are strictly conserved. This, together with the similar activation energies calculated for the two proteins, suggests that the reaction mechanism of the P.abyssi catalytic subunit is similar to that of the E.coli subunit. Several structural elements potentially contributing to thermostability were identified: (i) a marked decrease in the number of thermolabile residues; (ii) an increased number of charged residues and a concomitant increase of salt links at the interface between the monomers, as well as the formation of an ion-pair network at the protein surface; (iii) the shortening of three loops and the shortening of the N and C termini. Other known thermostabilizing devices such as increased packing density or reduction of cavity volumes do not appear to contribute to the high thermostability of the P.abyssi enzyme.

The allosteric activator Mg-ATP modifies the quaternary structure of the R-state of Escherichia coli aspartate transcarbamylase without altering the T<-->R equilibrium

The allosteric enzyme aspartate transcarbamylase from Escherichia coli (ATCase) displays regulatory properties that involve various conformational changes, including a large quaternary structure rearrangement. This entails a major change in its solution X-ray scattering curve upon binding substrate analogues. We show here that, in the presence of the nucleotide effector ATP, known to stimulate the enzyme activity, the scattering profiles show a marked dependence on the metal bound to ATP. Whereas ATP has no major effect on the scattering pattern of ATCase, a saturating concentration of Mg-ATP notably modifies the scattering profile of the enzyme, either in the absence or in the presence of the bisubstrate analogue N-(phosphonacetyl)-l-aspartate (PALA). The transition with PALA in the presence of this metal-nucleotide complex remains concerted. Furthermore, Mg-ATP, as already observed with ATP, has no detectable direct effect on the T to R transition. The experimental scattering curves in the presence of Mg-ATP were fitted by a modeling approach using rigid body movements of the regulatory subunits and the catalytic trimers in the crystal structures. While the differences observed in the T-state in the presence of Mg-ATP are essentially attributed to the binding per se of the nucleotide, the solution structure of the R-state complexed to Mg-ATP is even more extended along the 3-fold axis than the previously described R solution structure, which is already more stretched out along the same axis than the crystal R structure. Based on the crystal structure of the enzyme in the R-state complexed with free ATP, a proposal is made to account for the effect of magnesium.

The use of nucleotide analogs to evaluate the mechanism of the heterotropic response of Escherichia coli aspartate transcarbamoylase

As an alternative method to study the heterotropic mechanism of Escherichia coli aspartate transcarbamoylase, a series of nucleotide analogs were used. These nucleotide analogs have the advantage over site-specific mutagenesis experiments in that interactions between the backbone of the protein and the nucleotide could be evaluated in terms of their importance for function. The ATP analogs purine 5'-triphosphate (PTP), 6-chloropurine 5'-triphosphate (Cl-PTP), 6-mercaptopurine 5'-triphosphate (SH-PTP), 6-methylpurine 5'-triphosphate (Me-PTP), and 1-methyladenosine 5'-triphosphate (Me-ATP) were partially synthesized from their corresponding nucleosides. Kinetic analysis was performed on the wild-type enzyme in the presence of these ATP analogs along with GTP, ITP, and XTP. PTP, Cl-PTP, and SH-PTP each activate the enzyme at subsaturating concentrations of L-aspartate and saturating concentrations of carbamoyl phosphate, but not to the same extent as does ATP. These experiments suggest that the interaction between N6-amino group of ATP and the backbone of the regulatory chain is important for orienting the nucleotide and inducing the displacements of the regulatory chain backbone necessary for initiation of the regulatory response. Me-PTP and Me-ATP also activate the enzyme, but in a more complex fashion, which suggests differential binding at the two sites within each regulatory dimer. The purine nucleotides GTP, ITP, and XTP each inhibit the enzyme but to a lesser extent than CTP. The influence of deoxy and dideoxynucleotides on the activity of the enzyme was also investigated. These experiments suggest that the 2' and 3' ribose hydroxyl groups are not of significant importance for binding and orientation of the nucleotide in the regulatory binding site. 2'-dCTP inhibits the enzyme to the same extent as CTP, indicating that the interactions of the enzyme to the O2-carbonyl of CTP are critical for CTP binding, inhibition, and the ability of the enzyme to discriminate between ATP and CTP. Examination of the electrostatic surface potential of the nucleotides and the regulatory chain suggest that the complimentary electrostatic interactions between the nucleotides and the regulatory chain are important for binding and orientation of the nucleotide necessary to induce the local conformational changes that propagate the heterotropic effect.

Intramolecular signal transmission in enterobacterial aspartate transcarbamylases II. Engineering co-operativity and allosteric regulation in the aspartate transcarbamylase of Erwinia herbicola

The aspartate transcarbamylase (ATCase) from Erwinia herbicola differs from the other investigated enterobacterial ATCases by its absence of homotropic co-operativity toward the substrate aspartate and its lack of response to ATP which is an allosteric effector (activator) of this family of enzymes. Nevertheless, the E. herbicola ATCase has the same quaternary structure, two trimers of catalytic chains with three dimers of regulatory chains ((c3)2(r2)3), as other enterobacterial ATCases and shows extensive primary structure conservation. In (c3)2(r2)3 ATCases, the association of the catalytic subunits c3 with the regulatory subunits r2 is responsible for the establishment of positive co-operativity between catalytic sites for the binding of aspartate and it dictates the pattern of allosteric response toward nucleotide effectors. Alignment of the primary sequence of the regulatory polypeptides from the E. herbicola and from the paradigmatic Escherichia coli ATCases reveals major blocks of divergence, corresponding to discrete structural elements in the E. coli enzyme. Chimeric ATCases were constructed by exchanging these blocks of divergent sequence between these two ATCases. It was found that the amino acid composition of the outermost beta-strand of a five-stranded beta-sheet in the effector-binding domain of the regulatory polypeptide is responsible for the lack of co-operativity and response to ATP of the E. herbicola ATCase. A novel structural element involved in allosteric signal recognition and transmission in this family of ATCases was thus identified.

The binding of inosine monophosphate to Escherichia coli carbamoyl phosphate synthetase

Carbamoyl phosphate synthetase (CPS) from Escherichia coli catalyzes the formation of carbamoyl phosphate, which is subsequently employed in both the pyrimidine and arginine biosynthetic pathways. The reaction mechanism is known to proceed through at least three highly reactive intermediates: ammonia, carboxyphosphate, and carbamate. In keeping with the fact that the product of CPS is utilized in two competing metabolic pathways, the enzyme is highly regulated by a variety of effector molecules including potassium and ornithine, which function as activators, and UMP, which acts as an inhibitor. IMP is also known to bind to CPS but the actual effect of this ligand on the activity of the enzyme is dependent upon both temperature and assay conditions. Here we describe the three-dimensional architecture of CPS with bound IMP determined and refined to 2.1 A resolution. The nucleotide is situated at the C-terminal portion of a five-stranded parallel beta-sheet in the allosteric domain formed by Ser(937) to Lys(1073). Those amino acid side chains responsible for anchoring the nucleotide to the polypeptide chain include Lys(954), Thr(974), Thr(977), Lys(993), Asn(1015), and Thr(1017). A series of hydrogen bonds connect the IMP-binding pocket to the active site of the large subunit known to function in the phosphorylation of the unstable intermediate, carbamate. This structural analysis reveals, for the first time, the detailed manner in which CPS accommodates nucleotide monophosphate effector molecules within the allosteric domain.

Aspartate carbamoyltransferase from the thermoacidophilic archaeon Sulfolobus acidocaldarius. Cloning, sequence analysis, enzyme purification and characterization

The genes coding for aspartate carbamoyltransferase (ATCase) in the extremely thermophilic archaeon Sulfolobus acidocaldarius have been cloned by complementation of a pyrBI deletion mutant of Escherichia coli. Sequencing revealed the existence of an enterobacterial-like pyrBI operon encoding a catalytic chain of 299 amino acids (34 kDa) and a regulatory chain of 170 amino acids (17.9 kDa). The deduced amino acid sequences of the pyrB and pyrI genes showed 27.6-50% identity with archaeal and enterobacterial ATCases. The recombinant S. acidocaldarius ATCase was purified to homogeneity, allowing the first detailed studies of an ATCase isolated from a thermophilic organism. The recombinant enzyme displayed the same properties as the ATCase synthesized in the native host. It is highly thermostable and exhibits Michaelian saturation kinetics for carbamoylphosphate (CP) and positive homotropic cooperative interactions for the binding of L-aspartate. Moreover, it is activated by nucleoside triphosphates whereas the catalytic subunits alone are inhibited. The holoenzyme purified from recombinant E. coli cells or present in crude extract of the native host have an Mr of 340 000 as estimated by gel filtration, suggesting that it has a quaternary structure similar to that of E. coli ATCase. Only monomers could be found in extracts of recombinant E. coli or Saccharomyces cerevisiae cells expressing the pyrB gene alone. In the presence of CP these monomers assembled into trimers. The stability of S. acidocaldarius ATCase and the allosteric properties of the enzyme are discussed in function of a modeling study.

Simulations of the T <--> R conformational transition in aspartate transcarbamylase

Aspartate transcarbamylase (ATCase) from Escherichia coli is one of the best known allosteric enzymes. In spite of numerous experiments performed by biochemists, no consensus model for the cooperative transition between the tensed (T) and the relaxed (R) forms exists. It is hypothesized, however, that changes in the quaternary structure play a key role in the allosteric properties of oligomeric proteins such as ATCase. Previous normal mode calculations of the two states of ATCase illustrated the type of motions that could be important in initiating the transition. In this work four pathways for the transition were calculated using the targeted molecular dynamics (TMD) method without constraint on the symmetry of the system. The most important quaternary structure changes are the relative rotation and translation of the catalytic trimers and the rotations of the regulatory dimers. The simulations show that these quaternary changes start immediately and finish when about 70% of the transition is completed whereas there are tertiary changes throughout the transition. In agreement with the work of Lipscomb et al., it was found that the relative translation between the catalytic trimers appears to play a central role in allowing the transition to occur. In all the simulations differences are observed in the opening and closing behaviours of the domains in the catalytic and regulatory chains that could provide a structural interpretation for the results of certain site-directed mutagenesis experiments. Overall the motions of the subunits are concerted even though the constraint imposed on the TMD method does not explicitly require that this be so.

Tertiary and quaternary conformational changes in aspartate transcarbamylase: a normal mode study

Aspartate transcarbamylase (ATCase) initiates the pyrimidine biosynthetic pathway in Escherichia coli. Binding of aspartate to this allosteric enzyme induces a cooperative transition between the tensed (T) and relaxed (R) states of the enzyme which involves large quaternary and tertiary rearrangements. The mechanisms of the transmission of the regulatory signal to the active site (60 A away) and that of the cooperative transition are not known in detail, although a large number of single, double, and triple site-specific mutants and chimeric forms of ATCase have been obtained and kinetically characterized. A previous analysis of the very low-frequency normal modes of both the T and R state structures of ATCase identified some of the large-amplitude motions mediating the intertrimer elongation and rotation that occur during the cooperative transition (Thomas et al., J. Mol. Biol. 257:1070-1087, 1996; Thomas et al., J. Mol. Biol. 261:490-506, 1996). As a complement to that study, the deformation of the quaternary and tertiary structure of ATCase by normal modes below 5 cm(-1) is investigated in this article. The ability of the modes to reproduce the domain motions occurring during the transition is analyzed, with special attention to the interdomain closure in the catalytic chain, which has been shown to be critical for homotropic cooperativity. The calculations show a coupling between the quaternary motions and more localized motions involving specific residues. The particular dynamic behavior of these residues is examined in the light of biochemical results to obtain insights into their role in the transmission of the allosteric signal.

1.85-A resolution crystal structure of human ornithine transcarbamoylase complexed with N-phosphonacetyl-L-ornithine. Catalytic mechanism and correlation with inherited deficiency

The crystal structure of human ornithine transcarbamoylase complexed with the bisubstrate analog N-phosphonacetyl-L-ornithine has been solved at 1.85-A resolution by molecular replacement. Deleterious mutations produce clinical hyperammonia that, if untreated, results in neurological symptoms or death (ornithine transcarbamylase deficiency). The holoenzyme is trimeric, and as in other transcarbamoylases, each subunit contains an N-terminal domain that binds carbamoyl phosphate and a C-terminal domain that binds L-ornithine. The active site is located in the cleft between domains and contains additional residues from an adjacent subunit. Binding of N-phosphonacetyl-L-ornithine promotes domain closure. The resolution of the structure enables the role of active site residues in the catalytic mechanism to be critically examined. The side chain of Cys-303 is positioned so as to be able to interact with the delta-amino group of L-ornithine which attacks the carbonyl carbon of carbamoyl phosphate in the enzyme-catalyzed reaction. This sulfhydryl group forms a charge relay system with Asp-263 and the alpha-amino group of L-ornithine, instead of with His-302 and Glu-310, as previously proposed. In common with other ureotelic ornithine transcarbamoylases, the human enzyme lacks a loop of approximately 20 residues between helix H10 and beta-strand B10 which is present in prokaryotic ornithine transcarbamoylases but has a C-terminal extension of 10 residues that interacts with the body of the protein but is exposed. The sequence of this C-terminal extension is homologous to an interhelical loop found in several membrane proteins, including mitochondrial transport proteins, suggesting a possible mode of interaction with the inner mitochondrial membrane.

Threonine 82 in the regulatory chain is important for nucleotide affinity and for the allosteric stabilization of Escherichia coli aspartate transcarbamoylase

The three-dimensional structure of Escherichia coli aspartate transcarbamoylase complexed with the allosteric effector CTP, shows an interaction between the hydroxyl of Thr-82 in the regulatory chain (Thr-82r) with the gamma-phosphate of CTP (R.P. Kosman, J.E. Gouaux, W.N. Lipscomb, Crystal structure of CTP-ligated T state aspartate transcarbamoylase at 2.5 A resolution: implications for aspartate transcarbamoylase mutants and the mechanism of negative cooperativity, Proteins Struct. Funct. Genet. 15 (1993) 147-176). In order to determine whether the Thr-82r interaction with the gamma-phosphate of CTP is important for either binding of the nucleotide effectors or their function, site-specific mutagenesis was employed. The mutant enzyme in which Thr-82r was replaced by Ala had almost the identical maximal observed specific activity as the wild-type enzyme; however, the mutant enzyme had a significantly increased [Asp]0.5, the aspartate concentration at one-half the maximal observed specific activity, as well as slightly increased homotropic cooperativity. The mutant enzyme was also activated more by ATP and inhibited less by CTP as compared to the wild-type enzyme. In addition, the nucleotide concentration required for one-half maximal effect was increased approx. 3-fold as compared to the corresponding values for the wild-type enzyme. The maximal inhibition of the mutant enzyme, in the presence of UTP and CTP was similar to that observed for the wild-type enzyme; however, higher concentrations of the nucleotides were required to achieve this level of inhibition. The reduced affinity of CTP, UTP and ATP induced by the mutation indicates that the hydrogen bonding interaction between the gamma-phosphate of the nucleotide effector and the side-chain hydroxyl of Thr-82r is important for the binding of the nucleotide effectors to the allosteric site. Furthermore, this interaction is important for the discrimination between CTP and CDP. Finally, the greater homotropic cooperativity, greater [Asp]0.5, diminished CTP inhibition and greater ATP activation of the mutant enzyme correlates with the X-ray structure of the mutant enzyme which shows that the unligated enzyme is in an 'extreme' T-state. These findings add support to the theory that the global stabilization of the enzyme is critical for both the homotropic and heterotropic properties of aspartate transcarbamoylase.

Temperature effects on the allosteric responses of native and chimeric aspartate transcarbamoylases

Although structurally very similar, the aspartate transcarbamoylases (ATCase) of Serratia marcescens and Escherichia coli have distinct allosteric regulatory patterns. It has been reported that a S. marcescens chimera, SM : rS5'ec, in which five divergent residues (r93 to r97) of the regulatory polypeptide were replaced with their Escherichia coli counterparts, possessed E. coli-like regulatory characteristics. The reverse chimera EC:rS5'sm, in which the same five residues of E. coli have been replaced with their S. marcescens counterpart, lost both heterotrophic and homotropic responses. These results indicate that the r93-r97 region is critical in defining the ATCase allosteric character. Molecular modeling of the regulatory polypeptides has suggested that the replacement of the S5' beta-strand resulted in disruption of the allosteric-zinc interface. However, the structure-function relationship could be indirect, and the disruption of the interface could influence allostery by altering the global energy of the enzyme. Studies of the temperature-sensitivity of the CTP response demonstrate that it is possible to convert CTP inhibition of the SM:rS5'ec chimera at high temperature to activation below 10 degreesC. Nonetheless, the temperature response of the native S. marcescens ATCase suggests a strong entropic effect that counteracts the CTP activation. Therefore, it is suggested that the entropy component of the coupling free energy plays a significant role in the determination of both the nature and magnitude of the allosteric effect in ATCase.

A single mutation in the regulatory chain of Escherichia coli aspartate transcarbamoylase results in an extreme T-state structure

Kinetic analysis of a mutant version of Escherichia coli aspartate transcarbamoylase in which Thr82 in the regulatory chain (Thr82r) was replaced by Ala results in a shift in the T <==> R equilibrium towards the T-state. In order to understand the structural determinants of this T-state stabilization, the X-ray structure of the unliganded Thr82r-->Ala enzyme was determined at 2. 6 A resolution and refined to a crystallographic residual of 0.175. The structure of the mutant r1 regulatory chain is more similar to that of the r6 regulatory chain than observed for the wild-type enzyme, resulting in a more symmetric structure. Furthermore, the structural changes in the mutant enzyme appears to occur only in the r1 chain, while the r6 chain is almost identical in structure to that of the r6 chain of the wild-type enzyme. The structure of the mutant enzyme exhibits alterations in the subunit interfaces between the regulatory and catalytic chains, as well as in the interface between the allosteric and zinc domains within the regulatory chain. Moreover, the regulatory dimers are rotated around their respective 2-fold axes approximately 1 degrees beyond the rotation which occurs in the wild-type T-state enzyme. The structural analysis indicates that the enzyme is an "extreme" T-state, in which a larger rotation of the regulatory dimers is required for the T to R transition compared to the wild-type enzyme. This extreme T-state structure correlates well with the kinetic parameters determined for the mutant enzyme, showing a stabilized T-state. Furthermore, the structural analysis of the mutant enzyme suggests that replacement of Thr82r with Ala alters the local conformation of the nucleotide binding pocket and therefore offers a plausible explanation for the reduced affinity of the enzyme for nucleotides.

Role of allosteric: zinc interdomain region of the regulatory subunit in the allosteric regulation of aspartate transcarbamoylase from Escherichia coli

The hydrophobic interface between the allosteric and the zinc domains of the regulatory subunit of aspartate transcarbamoylase has previously been implicated in the heterotropic ATP activation of the enzyme. The present work shows that this interface also affects CTP and CTP-UTP inhibition and proposes a structural explanation for the effects. Mutant enzymes derived from nonselective mutagenesis of residues r101-r106 (residues that contribute part of the interface) displayed a variety of homotropic and heterotropic effects. The cooperative behavior of the enzymes was affected, as indicated by reduced aspartate S0.5 values and apparent Hill coefficient values for V106L, V106L/N105S, and I103F/R102C. In addition, both ATP activation and CTP inhibition were significantly reduced and CTP+UTP synergistic inhibition was decreased in these mutants. The D104G mutant enzyme was subject to inhibition by CTP andCTP+UTP, but was not activated by ATP. Finally, the I103T mutant enzyme had an increased S0.5 value of 11.5 mM and displayed altered effector responses: ATP acted as an inhibitor, and the CTP+UTP synergistic inhibition was reduced. Most of these allosteric variations can be explained in terms of perturbations to the "tongue and groove" hydrophobic interface between the allosteric and the zinc domains and a consequent impact on a second interface ("reg1:cat4") between regulatory and catalytic subunits.

Binding of allosteric effectors to ribonucleotide reductase protein R1: reduction of active-site cysteines promotes substrate binding

BACKGROUND

Ribonucleotide reductase (RNR) is an essential enzyme in DNA synthesis, catalyzing all de novo synthesis of deoxyribonucleotides. The enzyme comprises two dimers, termed R1 and R2, and contains the redox active cysteine residues, Cys462 and Cys225. The reduction of ribonucleotides to deoxyribonucleotides involves the transfer of free radicals. The pathway for the radical has previously been suggested from crystallographic results, and is supported by site-directed mutagenesis studies. Most RNRs are allosterically regulated through two different nucleotide-binding sites: one site controls general activity and the other controls substrate specificity. Our aim has been to crystallographically demonstrate substrate binding and to locate the two effector-binding sites.

RESULTS

We report here the first crystal structure of RNR R1 in a reduced form. The structure shows that upon reduction of the redox active cysteines, the sulfur atom of Cys462 becomes deeply buried. The more accessible Cys225 moves to the former position of Cys462 making room for the substrate. In addition, the structures of R1 in complexes with effector, effector analog and effector plus substrate provide information about these binding sites. The substrate GDP binds in a cleft between two domains with its beta-phosphate bound to the N termini of two helices; the ribose forms hydrogen bonds to conserved residues. Binding of dTTP at the allosteric substrate specificity site stabilizes three loops close to the dimer interface and the active site, whereas the general allosteric binding site is positioned far from the active site.

CONCLUSIONS

Binding of substrate at the active site of the enzyme is structurally regulated in two ways: binding of the correct substrate is regulated by the binding of allosteric effectors and binding of the actual substrate occurs primarily when the active-site cysteines are reduced. One of the loops stabilized upon binding of dTTP participates in the formation of the substrate-binding site through direct interaction with the nucleotide base. The general allosteric effector site, located far from the active site, appears to regulate subunit interactions within the holoenzyme.

Aspartate transcarbamylase from the deep-sea hyperthermophilic archaeon Pyrococcus abyssi: genetic organization, structure, and expression in Escherichia coli

The genes coding for aspartate transcarbamylase (ATCase) in the deep-sea hyperthermophilic archaeon Pyrococcus abyssi were cloned by complementation of a pyrB Escherichia coli mutant. The sequence revealed the existence of a pyrBI operon, coding for a catalytic chain and a regulatory chain, as in Enterobacteriaceae. Comparison of primary sequences of the polypeptides encoded by the pyrB and pyrI genes with those of homologous eubacterial and eukaryotic chains showed a high degree of conservation of the residues which in E. coli ATCase are involved in catalysis and allosteric regulation. The regulatory chain shows more-extensive divergence with respect to that of E. coli and other Enterobacteriaceae than the catalytic chain. Several substitutions suggest the existence in P. abyssi ATCase of additional hydrophobic interactions and ionic bonds which are probably involved in protein stabilization at high temperatures. The catalytic chain presents a secondary structure similar to that of the E. coli enzyme. Modeling of the tridimensional structure of this chain provides a folding close to that of the E. coli protein in spite of several significant differences. Conservation of numerous pairs of residues involved in the interfaces between different chains or subunits in E. coli ATCase suggests that the P. abyssi enzyme has a quaternary structure similar to that of the E. coli enzyme. P. abyssi ATCase expressed in transgenic E. coli cells exhibited reduced cooperativity for aspartate binding and sensitivity to allosteric effectors, as well as a decreased thermostability and barostability, suggesting that in P. abyssi cells this enzyme is further stabilized through its association with other cellular components.

Conversion of the allosteric regulatory patterns of aspartate transcarbamoylase by exchange of a single beta-strand between diverged regulatory chains

Although structurally very similar, the aspartate transcarbamoylases (ATCase) of Serratia marcescens and Escherichia coli differ in both regulatory and catalytic characteristics. Most notably, CTP stimulates the catalytic activity of the S. marcescens ATCase and CTP/UTP inhibitory synergism has been lost. These allosteric characteristics contradict the traditional logic developed from the E. coli enzyme in which CTP and UTP function together as end products of the pyrimidine pathway to allosterically control the catalytic activity. In this study, five divergent residues (r93-r97) of the regulatory polypeptide of the S. marcescens enzyme have been replaced with their E. coli counterparts. These residues correspond to the S5' beta-strand of the allosteric effector binding domain at the junction of the allosteric and zinc domains of the regulatory polypeptide. In spite of the fact that the chimeric ATCase (SM:rS5'ec) retained 455 out of 460 amino acids of the S. marcescens enzyme, it possessed characteristics similar to those of the E. coli enzyme: (1) the [Asp]0.5 decreased from 40 to 5 mM; (2) ATP activation of the enzyme was greatly reduced; (3) CTP was converted from a strong activator to a strong inhibitor; and (4) the synergistic inhibition by CTP and UTP was restored. The S5' beta-strand is located at the outer surface of a five-stranded beta-sheet of the allosteric domain, providing a potential structural mechanism defining the allostery of this enzyme.

Identification and analysis of long-range electrostatic effects in proteins by computer modeling:aspartate transcarbamylase

While ion pairs are readily identified in crystal structures, longer range electrostatic interactions cannot be identified from the three-dimensional structure alone. These interactions are likely to be important in large, multisubunit proteins that are regulated by allosteric interactions. In this paper, we show that these interactions are readily detected by electrostatic modeling, using, as an example, unliganded Escherichia coli aspartate transcarbamylase, a widely studied allosteric enzyme with 12 subunits and a molecular weight of 310 kD. The Born, dipolar, and site-site interaction terms of the free energy of protonation of the 810 titratable sites in the holoenzyme were calculated using the multigrid solution of the nonlinear Poisson-Boltzmann equation. Calculated titration curves are in good agreement with experimental titration curves, and the structural asymmetry observed in the crystal structure is readily apparent in the calculated free energies and pK1/2 values. Most of the residues with pK1/2 values that differ substantially from those of model compounds are buried in the low dielectric medium of the protein, particularly at the intersubunit interfaces. The dependence of the site-site interaction free energies on distance is complex, with a steep dependence at distances less than 5 A and a more shallow dependence at longer distances. Interactions over distances of 6 to 15 A require a bridging residue and are often not apparent in the structure. The network of interactions between ionizable groups extends across and between subunits and provides a potential mechanism for transmitting long-range structural effects and allosteric signals.

Threading a database of protein cores

We present an analysis of 10 blind predictions prepared for a recent conference, "Critical Assessment of Techniques for Protein Structure Prediction." The sequences of these proteins are not detectably similar to those of any protein in the structure database then available, but we attempted, by a threading method, to recognize similarity to known domain folds. Four of the 10 proteins, as we subsequently learned, do indeed show significant similarity to then-known structures. For 2 of these proteins the predictions were accurate, in the sense that a similar structure was at or near the top of the list of threading scores, and the threading alignment agreed well with the corresponding structural alignment. For the best predicted model mean alignment error relative to the optimal structural alignment was 2.7 residues, arising entirely from small "register shifts" of strands or helices. In the analysis we attempt to identify factors responsible for these successes and failures. Since our threading method does not use gap penalties, we may readily distinguish between errors arising from our prior definition of the "cores" of known structures and errors arising from inherent limitations in the threading potential. It would appear from the results that successful substructure recognition depends most critically on accurate definition of the "fold" of a database protein. This definition must correctly delineate substructures that are, and are not, likely to be conserved during protein evolution.

The molecular basis of ornithine transcarbamylase deficiency: modelling the human enzyme and the effects of mutations

Human ornithine transcarbamylase is a trimer with 46% amino acid sequence homology to the catalytic subunit of E coli aspartate transcarbamylase. Secondary structure predictions, distributions of hydrophilic and hydrophobic regions, and the pattern of conserved residues suggest that the three dimensional structures of the two proteins are likely to be similar. A three dimensional model of ornithine transcarbamylase was generated from the crystal structure of the catalytic subunit of E coli aspartate transcarbamylase in the holoenzyme, by aligning the sequences, building in gaps, and minimising the energy. The binding sites for carbamyl phosphate in both enzymes are similar and the ornithine binding site in ornithine transcarbamylase appears to be in the same location as the L-aspartate binding site in aspartate transcarbamylase, with negatively charged side chains replaced by positively charged residues. Mutations in the ornithine transcarbamylase gene found in patients with hyperammonaemia of the "neonatal type" are clustered in important structural or functional domains, either in the interior of the protein, at the active site, or at the interchain interface, while mutations found in patients with milder "late onset" disease are located primarily on the surface of the protein. The predicted effects of all known missense mutations and in frame deletions in the ornithine transcarbamylase gene on the structure and function of the mature enzyme are described.

Weakening of the interface between adjacent catalytic chains promotes domain closure in Escherichia coli aspartate transcarbamoylase

Aspartate transcarbamoylase from Escherichia coli is a dodecameric enzyme consisting of two trimeric catalytic subunits and three dimeric regulatory subunits. Asp-100, from one catalytic chain, is involved in stabilizing the C1-C2 interface by means of its interaction with Arg-65 from an adjacent catalytic chain. Replacement of Asp-100 by Ala has been shown previously to result in increases in the maximal specific activity, homotropic cooperativity, and the affinity for aspartate (Baker DP, Kantrowitz ER, 1993, Biochemistry 32:10150-10158). In order to determine whether these properties were due to promotion of domain closure induced by the weakening of the C1-C2 interface, we constructed a double mutant version of aspartate transcarbamoylase in which the Asp-100-->Ala mutation was introduced into the Glu-50-->Ala holoenzyme, a mutant in which domain closure is impaired. The Glu-50/Asp-100-->Ala enzyme is fourfold more active than the Glu-50-->Ala enzyme, and exhibits significant restoration of homotropic cooperativity with respect to aspartate. In addition, the Asp-100-->Ala mutation restores the ability of the Glu-50-->Ala enzyme to be activated by succinate and increases the affinity of the enzyme for the bisubstrate analogue N-(phosphonacetyl)-L-aspartate (PALA). At subsaturating concentrations of aspartate, the Glu-50/Asp-100-->Ala enzyme is activated more by ATP than the Glu-50-->Ala enzyme and is also inhibited more by CTP than either the wild-type or the Glu-50-->Ala enzyme. As opposed to the wild-type enzyme, the Glu-50/Asp-100-->Ala enzyme is activated by ATP and inhibited by CTP at saturating concentrations of aspartate. Structural analysis of the Glu-50/Asp-100-->Ala enzyme by solution X-ray scattering indicates that the double mutant exists in the same T quaternary structure as the wild-type enzyme in the absence of ligands and in the same R quaternary structure in the presence of saturating PALA. However, saturating concentrations of carbamoyl phosphate and succinate only convert a fraction of the Glu-50/Asp-100-->Ala enzyme population to the R quaternary structure, a behavior intermediate between that observed for the Glu-50-->Ala and wild-type enzymes. Solution X-ray scattering was also used to investigate the structural consequences of nucleotide binding to the Glu-50/Asp-100-->Ala enzyme.

The allosteric ligand site in the Vmax-type cooperative enzyme phosphoglycerate dehydrogenase

The crystal structure of the phosphoglycerate dehydrogenase from Escherichia coli is unique among dehydrogenases. It consists of three clearly separate domains connected by flexible hinges. The tetramer has approximate 222 symmetry with the principal contacts between the subunits forming between either the nucleotide binding domains or the regulatory domains. Two slightly different subunit conformations are present which vary only in the orientations of the domains. There is a hinge-like arrangement near the active site cleft and the serine effector site is provided by the regulatory domain of each of two subunits. Interdomain flexibility may play a key role in both catalysis and allosteric inhibition.