Structure and control of pyridoxal phosphate dependent allosteric threonine deaminase

BACKGROUND

Feedback inhibition of biosynthetic threonine deaminase (TD) from Escherichia coli provided one of the earliest examples of protein-based metabolic regulation. Isoleucine, the pathway end-product, and valine, the product of a parallel pathway, serve as allosteric inhibitor and activator, respectively. This enzyme is thus a useful model system for studying the structural basis of allosteric control mechanisms.

RESULTS

We report the crystal structure of TD at 2.8 A resolution. The tetramer has 222 symmetry, with C-terminal regulatory domains projecting out from a core of catalytic PLP-containing N-terminal domains. The subunits, and especially the regulatory domains, associate extensively to form dimers, which associate less extensively to form the tetramer. Within the dimer, each monomer twists approximately 150 degrees around a thin neck between the domains to place its catalytic domain adjacent to the regulatory domain of the other subunit.

CONCLUSIONS

The structure of TD and its comparison with related structures and other data lead to the tentative identification of the regulatory binding site and revealed several implications for the allosteric mechanism. This work prepares the way for detailed structure/function studies of the complex allosteric behaviour of this enzyme.

Stable expression and rapid purification of Escherichia coli GroEL and GroES chaperonins

An Escherichia coli expression vector pRE (P. Reddy, A. Peterkofsky, and K. McKenney, 1989, Nucleic Acids Res. 17, 10473-10488), originally developed for the cloning and expression of lethal genes, was used for cloning and hyperexpression of GroEL and GroES genes. Regulated gene expression is achieved in the pRE vector under the tight control of the lambda PL promoter. Upon induction of the promoter, stable expression of GroEL to about 60% of the total cell protein was observed. Similarly, stable expression of GroES to about 40% of the total cell protein was achieved. GroES was found to be a heat-stable protein while GroEL was not. Both GroE chaperonins were purified in a single chromatographic step with a yield of about 100 mg GroEL and 25 mg GroES per liter of E. coli culture. GroE chaperonins purified by the protocols described here were active in the renaturation of urea-denatured rhodanese.

Engineering the independent folding of the subtilisin BPN' prodomain: analysis of two-state folding versus protein stability

In complex with subtilisin BPN', the 77 amino acid prodomain folds into a stable compact structure comprising a four-stranded antiparallel beta-sheet and two three-turn alpha-helices. When isolated from subtilisin, the prodomain is 97% unfolded even under optimal folding conditions. Traditionally, to study stable proteins, denaturing cosolvents or temperatures are used to shift the equilibrium from folded to unfolded. Here we manipulate the folding equilibrium of the unstable prodomain by introducing stabilizing mutations generated by design. By sequentially introducing three stabilizing mutations into the prodomain we are able to shift the equilibrium for independent folding from 97% unfolded to 65% folded. Spectroscopic and thermodynamic analysis of the folding reaction was carried out to assess the effect of stability on two-state behavior and the denatured state. The denatured states of single and combination mutants are not discernably different in spite of a range of DeltaGunfolding from -2.1 to 0.4 kcal/mol. Conclusions about the nature of the denatured state of the prodomain are based on CD spectral data and calorimetric data. Two state folding is observed for a combination mutant of marginal stability (DeltaG = 0). Evidence for its two-state folding is based on the observed additivity of individual mutations to the overall DeltaGunfolding and the conformity of DeltaGunfolding vs T to two-state assumptions as embodied in the Gibbs-Helmholz equation. We believe our success in stabilizing the two-state folding reaction of the prodomain originates from the selection of mutations with improved ability to fold subtilisin rather than selection for increase in secondary structure content. The fact that a small number of mutations can stabilize the independent folding of the prodomain implies that most of the folding information already exists in the wild-type amino acid sequence in spite of the fact that the unfolded state predominates.

Redesigning the quaternary structure of R67 dihydrofolate reductase. Creation of an active monomer from a tetrameric protein by quadruplication of the gene

R67 dihydrofolate reductase (DHFR) provides resistance to the antibacterial drug trimethoprim. This R-plasmid-encoded enzyme does not share any homology with chromosomal DHFR. A recent crystal structure of active, homotetrameric R67 DHFR (Narayana, N., Matthews, D. A., Howell, E. E., and Xuong, N.-H. (1995) Nat. Struct. Biol. 2, 1018-1025) indicates that a single active site pore traverses the length of the molecule. Since the center of the pore possesses exact 222 symmetry, site-directed mutagenesis of residues in the pore will produce four mutations/active site. To break this inevitable symmetry, four copies of the gene have been linked in frame to create an active monomer possessing the essential tertiary structure of native tetrameric R67 DHFR. The protein product, quadruple R67 DHFR, is 4 times the molecular mass of native R67 DHFR in SDS-polyacrylamide gel electrophoresis and is monomeric under nondenaturing conditions as measured by sedimentation equilibrium experiments. The catalytic activity of quadruple R67 DHFR is decreased only slightly when compared with native R67 DHFR. Folding of quadruple R67 DHFR is completely reversible at pH 5. However, at pH 8, folding is not fully reversible; this is likely due to a competition between productive intramolecular versus nonproductive intermolecular domain association. The production of a fully active, monomeric R67 DHFR variant will enable the design of more meaningful site-directed mutants where single substitutions per active site pore can be generated.

A mutational analysis of the binding of two different proteins to the same antibody

The crystal structures of the complexes between the anti-hen egg white lysozyme (HEL) antibody D1.3 and HEL and between D1.3 and the anti-D1.3 antibody E5.2 have shown that D1.3 contacts these two proteins through essentially the same set of combining site residues [Fields, B. A., Goldbaum, F. A., Ysern, X., Poljak, R. J., & Mariuzza, R. A. (1995) Nature 374, 739-742]. To probe the relative contribution of individual residues to complex stabilization, single alanine substitutions were introduced in the combining site of D1.3, and their effects on affinity for HEL and for E5.2 were measured using surface plasmon resonance detection, fluorescence quench titration, or sedimentation equilibrium. The energetics of the binding to HEL are dominated by only 3 of the 13 contact residues tested (delta Gmutant-delta Gwild type > 2.5 kcal/mol): VLW92, VHD100, and VHY101. These form a patch at the center of the interface and are surrounded by residues whose apparent contributions are much less pronounced ( < 1.5 kcal/mol). This contrasts with the interaction of D1.3 with E5.2 in which most the contact residues (11 of 15) were found to play a significant role in ligand binding ( > 1.5 kcal/mol). Furthermore, even though D1.3 contacts HEL and E5.2 in very similar ways, the functionally important residues of D1.3 are different for the two interactions, with only substitutions at D1.3 positions VH100 and VH101 greatly affecting binding to both ligands. Thus, the same protein may recognize different ligands in ways that are structurally similar yet energetically distinct.

Nucleotide binding-promoted conformational changes release a nonnative polypeptide from the Escherichia coli chaperonin GroEL

The Escherichia coli chaperonins GroEL and GroES facilitate the refolding of polypeptide chains in an ATP hydrolysis-dependent reaction. The elementary steps in the binding and release of polypeptide substrates to GroEL were investigated in surface plasmon resonance studies to measure the rates of binding and dissociation of a normative variant of subtilisin. The rate constants determined for GroEL association with and dissociation from this variant yielded a micromolar dissociation constant, in agreement with independent calorimetric estimates. The rate of GroEL dissociation from the nonnative chain was increased significantly in the presence of 5'-adenylylimidodiphosphate (AMP-PNP), ADP, and ATP, yielding maximal values between 0.04 and 0.22 s(-1). The sigmoidal dependence of the dissociation rate on the concentration of AMP-PNP and ADP indicated that polypeptide dissociation is limited by a concerted conformational change that occurs after nucleotide binding. The dependence of the rate of release on ATP exhibited two sigmoidal transitions attributable to nucleotide binding to the distal and proximal toroid of a GroEL-polypeptide chain complex. The addition of GroES resulted in a marked increase in the rate of nonnative polypeptide release from GroEL, indicating that the cochaperonin binds more rapidly than the dissociation of polypeptides. These data demonstrate the importance of nucleotide binding-promoted concerted conformational changes for the release of chains from GroEL, which correlate with the sigmoidal hydrolysis of ATP by the chaperonin. The implications of these findings are discussed in terms of a working hypothesis for a single cycle of chaperonin action.

The effect of water activity on the association constant and the enthalpy of reaction between lysozyme and the specific antibodies D1.3 and D44.1

The reactions of lysozyme with the specific monoclonal antibody D1.3, its Fv fragment and a mutant of the Fv, were studied under conditions of reduced water activity through the addition of the cosolutes glycerol, ethanol, dioxane and methanol. Titration calorimetry, BIAcoreTM and ultracentrifugal analyses were used to determine enthalpy of reactions and affinity constants. There was a decrease in the values of the enthalpies of reactions as well as in the association constants which was proportional to the decrease in water activity. These results are consistent with a structural model in which water molecules bound to the antigen and the antibody are conserved upon complex formation and provide bonds which are important for the stability of the complex. In contrast, the reaction of lysozyme with the specific monoclonal antibody D44.1, or its Fab, showed the inverse effect: a small increase in the value of the association constant with decreasing water molarities. This is in agreement with a model in which binding of antigen to antibody D44.1 is accompanied by the release of a very small number of water molecules.

Superantigen binding to a T cell receptor beta chain of known three-dimensional structure

The three-dimensional structure of an unglycosylated T cell antigen receptor (TCR) beta chain has recently been determined to 1.7 A resolution. To investigate whether this soluble beta chain (murine V beta 8.2J beta 2.1C beta 1) retains superantigen (SAG)-binding activity, we measured its affinity for various bacterial SAGs in the absence of MHC class II molecules. Dissociation constants (KDs) were determined using two independent techniques: surface plasmon resonance detection and sedimentation equilibrium. Specific binding was demonstrated to staphylococcal enterotoxins (SEs) B, C1, C2, and C3 and to streptococcal pyrogenic exotoxin A (SPEA), consistent with the known proliferative effects of these SAGs on T cells expressing V beta 8.2. In contrast, SEA, which does not stimulate V beta 8.2-bearing cells, does not bind the recombinant beta chain. Binding of the beta chain to SAGs was characterized by extremely fast dissociation rates (> 0.1 s-1), similar to those reported for certain leukocyte adhesion molecules. Whereas the beta chain bound SEC1, 2, and 3 with KDs of 0.9-2.5 microM, the corresponding value for SEB was approximately 140 microM. The much weaker binding to SEB than to SEC1, 2, or 3 was surprising, especially since SEB was found to actually be 3- to 10-fold more effective, on a molar basis, than the other toxins in stimulating the parental T cell hybridoma. We interpret these results in terms of the ability of SEC to activate T cells independently of MHC, in contrast to SEB. We have also measured SE binding to the glycosylated form of the beta chain and found that carbohydrate apparently does not contribute to recognition, even though the N-linked glycosylation sites at V beta 8.2 residues Asn24 and Asn74 are at or near the putative SAG-binding site. This result, along with the structural basis for the V beta specificity of SEs, are discussed in relation to the crystal structure of the unglycosylated beta chain.

A monomeric variant of GroEL binds nucleotides but is inactive as a molecular chaperone

The heat shock protein GroEL from Escherichia coli is a tetradecameric oligomer that facilitates the refolding of nonnative polypeptides in an ATP-hydrolysis dependent reaction. A mutant in GroEL was prepared in which lysine 3 was substituted with glutamate, which destabilizes the oligomeric structure of GroEL (Horovitz, A., Bochkareva, E.S., and Girshovich, A.S. (1993) J. Biol. Chem. 268, 9957-9959). The highly expressed and purified GroELK3E was judged to be monomeric by sedimentation equilibrium, yielding a molecular weight of 54,500, despite a weak tendency of the mutant to reversibly form higher order aggregates above 4 mg ml-1. The monomeric variant appears to be folded based on the far UV circular dichroism spectrum, which shows significant alpha-helical content, but with slight differences in conformation relative to wild-type GroEL. The increase in exposed hydrophobic surface of the monomer was probed with the dye 4,4'-bis-1-anilino-3-naphthalenesulfonate (bis-ANS). The fluorescence of bis-ANS increases approximately 150-fold in the presence of the mutant, and about 4 mol of bis-ANS bind per mol of monomer, with a binding constant of 1.6 microM. Adenosine nucleotide binding to monomeric GroELK3E resulted in considerable quenching of bis-ANS fluorescence, correlating with significant structural changes as seen in the far UV circular dichroism, and permitted the measurement of binding isotherms for ATP and ADP. Hyperbolic ATP binding isotherms yield a dissociation constant of 82 microM, about 4-fold weaker than the K0.5 for ATP seen in steady-state kinetics assays of the wild-type GroEL ATPase.A similar difference was seen for ADP binding. These results suggest that the mutation disrupts the native tetradecameric quaternary structure through conformational changes that may also weaken nucleotide binding. The monomeric mutant exhibited no chaperone activity as evidenced by a filure to inhibit or facilitate the refolding of chemically denatured enolase, an inability to refold denatured rhodanese above spontaneous levels, and a lack of binding to alpha-casein, a competitor in many chaperonin-promoted refolding reactions. Thus, the formation of assembly incompetent monomers by the lysine 3 to glutamate mutation results in a dramatic decrease in the affinity for nonnative polypeptide chains and suggests that the oligomeric nature of GroEL is crucial for its molecular chaperone function.

Monomer-heptamer equilibrium of the Escherichia coli chaperonin GroES

In an effort to clarify the role of GroES in chaperonin-facilitated protein folding, a plasmid-encoding expression system for GroES incorporating a histidine-tagged, thrombin-cleavable, N-terminal sequence was constructed. This approach facilitated the rapid purification of native-like, histidine-cleaved GroES (HC-GroES). The addition of NaSCN to purification buffers to mildly promote subunit dissociation enabled the complete separation of chromosomally encoded, wild-type GroES chains from recombinant chains, allowing the production of homogeneous mutant variants of GroES. A substitution of histidine-7 to tryptophan in GroES was used to demonstrate the concentration-dependent modulation of the heptameric quaternary structure of the chaperonin. Fluorescence and light scattering studies of this mutant suggest that GroES heptamers dissociate to monomers upon dilution with half-times of 2-4 min. Sedimentation equilibrium experiments using either wild-type or HC-GroES can best be described by a monomer--heptamer equilibrium, yielding dissociation constants of 1 x 10(-38) M6 for native GroES and 2 x 10(-32) M6 for HC-GroES. These results are supported by subunit exchange experiments using mixtures of native or HC-GroES and GroES containing the complete N-terminal histidine tail. Native polyacrylamide gel electrophoresis demonstrates that these mixtures form an eight-membered hybrid set within minutes. The studies described here suggest a dynamic equilibrium for the quaternary structure of GroES, which may be an important feature for its role in GroEL-mediated protein folding reactions.

An expanded two-state model accounts for homotropic cooperativity in biosynthetic threonine deaminase from Escherichia coli

The linkage between substrate and regulatory effector binding to separate sites on allosteric enzymes results in shifts in their sigmoidal kinetics to regulate metabolism. Control of branched chain amino acid biosynthesis in Escherichia coli occurs in part through shifts in the sigmoidal dependence of alpha-ketobutyrate production promoted by isoleucine and valine binding to biosynthetic threonine deaminase. The structural similarity of threonine, valine, and isoleucine have given rise to suggestions that there may be competition among different ligands for the same sites on this tetrameric enzyme, resulting in a complex pattern of regulation. In an effort to provide a coherent interpretation of the cooperative association of ligands to the active sites and to the effector sites of threonine deaminase, binding studies using single amino acid variants were undertaken. A previously-isolated, feedback-resistant mutant identified in Salmonella typhimurium, ilvA219, has been cloned and sequenced. The phenotype is attributable to a single amino acid substitution in the regulatory domain of the enzyme in which leucine at position 447 is substituted with phenylalanine. The mutant exhibits hyperbolic saturation curves in both ligand binding and steady-state kinetics. These results, in addition to calorimetric and spectroscopic measurements of isoleucine and valine binding, indicate that the low affinity (T) state is destabilized in the mutant and that it exists predominantly in the high affinity (R) conformation in the absence of ligands, providing an explanation for its resistance to isoleucine. Chemical and spectroscopic analyses of another mutant, in which alanine has replaced an essential lysine at position 62 that forms a Schiff base with pyridoxal phosphate, indicate that the cofactor is complexed to exogenous threonine and is therefore unable to bind additional amino acids at the active sites. Isoleucine and valine binding to this inactive, active site-saturated enzyme revealed that it too was stabilized in the R state, yielding binding constants in excellent agreement with the leucine to phenylalanine mutant. The lysine to alanine mutant was further utilized to demonstrate that both threonine and 2-aminobutyrate bind with stronger affinity to the regulatory sites than to the active sites. A direct consequence of these results is that substrates and analogs have a synergistic effect on the allosteric transition since, in effect, they act as both homotropic and heterotropic effectors.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein motion and lock and key complementarity in antigen-antibody reactions

Antibodies possess a highly complementary combining site structure to that of their specific antigens. In many instances their reactions are driven by enthalpic factors including, at least in the case of the reaction of monoclonal antibody D1.3 with lysozyme, enthalpy of solvation. They require minor structural rearrangements, and their equilibrium association constants are relatively high (10(7)-10(11) M-1). By contrast, in an idiotope--anti-idiotope (antibody-antibody) reaction, which is entropically driven, the binding equilibrium constant is only 1.5 x 10(5) M-1 at 20 degrees C. This low value results from a slow association rate (10(3) M-1 s-1) due to a selection of conformational states that allow one of the interacting molecular surfaces (the idiotope on antibody D1.3) to become complementary to that of the anti-idiotopic antibody. Thus, antibody D1.3 reacts with two different macromolecules: with its specific antigen, hen egg lysozyme, and with a specific anti-idiotopic antibody. Complementarity with lysozyme is closer to a "lock and key" model and results in high affinity (2-4 x 10(8) M-1). That with the anti-idiotopic antibody involves conformational changes at its combining site and it results in a lower association constant (1.5 x 10(5) M-1). Thus, an "induced fit" mechanism may lead to a broadening of the binding specificity but with a resulting decrease in the intrinsic binding affinity which may weaken the physiological function of antibodies.

The hydrophobic nature of GroEL-substrate binding

The molecular chaperone GroEl from Escherichia coli is a member of the highly conserved Hsp60 family of proteins that facilitates protein folding. A central question regarding the mechanism of GroEL-assisted refolding of proteins concerns its broad substrate specificity. The nature of GroEL-polypeptide chain interaction was investigated by isothermal titration calorimetry using proteins that maintain a non-native conformation in neutral buffer solutions. A single molecule of an unfolded variant of subtilisin BPN' binds non-cooperatively to GroEL with micromolar affinity and a positive enthalpy change. Additional calorimetric titrations of this chain with GroEL show that the positive enthalpy change decreases with increasing temperature between 6 and 25 degrees C, yielding a delta CP of -0.85 kcal mol-1 degree-1. alpha-Casein similarly binds to GroEL with micromolar affinity and a positive enthalpy change in the range of 15-20 degrees C, yielding a delta CP of -0.44 kcal mol-1 degree-1. The negative heat capacity change provides strong evidence for the role of hydrophobic interactions as the driving force for the association of these substrates with the GroEL chaperonin.

Allosteric regulation of biosynthetic threonine deaminase from Escherichia coli: effects of isoleucine and valine on active-site ligand binding and catalysis

The sigmoidal steady-state kinetics of biosynthetic threonine deaminase from Escherichia coli is allosterically controlled by isoleucine and valine, the end-products of branched-chain amino acid biosynthesis. A basis for the regulation of threonine deaminase by heterotropic effectors has been studied by capitalizing on the intrinsic fluorescence of the essential pyridoxal phosphate cofactor in this enzyme in kinetic and equilibrium binding studies with the substrate analog D-threonine. D-Threonine binds cooperatively to four sites on the free enzyme, with an average dissociation constant of 19.8 mM. However, in the presence of saturating valine, or isoleucine, the D-threonine binding isotherms are noncooperative and characterized by dissociation constants of 3.9 and 24.8 mM, respectively. The rate of association of D-threonine with threonine deaminase in the presence of the regulatory ligands was biphasic. Analysis of the data in terms of a two-step scheme whereby the internal aldimine Schiff base in the initial encounter complex undergoes transimination with D-threonine to form an external aldimine yielded estimates for overall binding constants that were in good agreement with those determined from equilibrium binding isotherms. These analyses indicate that the positive allosteric effector valine acts solely to alter the binding of D-threonine to the active sites of threonine deaminase by shifting the equilibrium between a low-affinity and high-affinity state, consistent with predictions from a simple two-state model. However, isoleucine has a compound effect on the enzyme. The negative allosteric ligand promotes decreases in the rate and equilibrium constants for encounter complex formation, consistent with its preferential binding to the low-affinity state of the enzyme. In addition, however, isoleucine promotes a decrease in the transimination rate and equilibrium constants. Since transimination is generally considered to be protein-catalyzed in pyridoxal phosphate requiring enzymes, it is proposed that isoleucine affects both binding and catalysis in threonine deaminase, which provides a possible explanation for the inadequacy of a simple two-state model to describe the allosteric regulation of this enzyme.

Cooperative binding of the feedback modifiers isoleucine and valine to biosynthetic threonine deaminase from Escherichia coli

Control of the regulatory enzyme threonine deaminase from Escherichia coli is achieved by isoleucine inhibition and valine activation. The mechanism by which these heterotropic effectors regulate the enzyme was investigated by measuring the binding of isoleucine and valine by spectroscopic, kinetic, calorimetric and equilibrium dialysis techniques. The addition of isoleucine or valine to threonine deaminase resulted in large changes in the intrinsic fluorescence of the two tryptophans per polypeptide chain. Slightly cooperative binding isotherms for isoleucine were obtained in potassium phosphate, pH 7.5, yielding an average dissociation constant of 4.91 microM, which was confirmed by equilibrium dialysis measurements. Valine binding was much more cooperative, and yielded an average dissociation constant of 122 microM. Titration calorimetry experiments indicated that cooperative heterotropic ligand binding was exothermic, and yielded a stoichiometry of four isoleucine bound per tetrameric enzyme, with an average enthalpy of -10.70 kcal/mol. Valine also bound to four sites per tetramer, with an average enthalpy of -7.45 kcal/mol. The effect of ligands on the fluorescence and circular dichroism spectra of the essential pyridoxal phosphate cofactor indicates that isoleucine and valine bind to effector sites that are distinct from the active sites in threonine deaminase. Shifts in the kinetic properties of threonine deaminase promoted by isoleucine and valine binding are to a first approximation consistent with analyses of effector binding isotherms in terms of a simple two-state model, and suggest that isoleucine regulates threonine deaminase by preferentially binding to the low activity T state, whereas valine binds preferentially to the high activity R state. Finally, analyses of heterotropic effector binding isotherms suggest that active site ligands may have significant affinity for the regulatory sites, which gives rise to underestimates for the allosteric equilibrium constants determined from substrate analog binding isotherms.

Energetics of cooperative ligand binding to the active sites of biosynthetic threonine deaminase from Escherichia coli

The sigmoidal kinetics of alpha-ketobutyrate production catalyzed by threonine deaminase are shifted in the presence of the feedback inhibitor isoleucine and the activator valine to control carbon flow through branched-chain amino acid biosynthesis in Escherichia coli. As an initial effort toward developing a molecular mechanism for cooperativity and feedback inhibition in this enzyme, the binding of the substrate analogs 2-aminobutyrate and alanine were measured. Binding isotherms were determined in potassium phosphate at pH 7.5 by detection of an 8-10-fold increase in intrinsic fluorescence of the external aldimine Schiff base of these analogs with the essential pyridoxal phosphate cofactor of the enzyme. Both 2-aminobutyrate and alanine bind cooperatively to four sites on threonine deaminase, with an average dissociation constant of 12.7 and 43.8 mM, respectively. The feedback inhibitor isoleucine decreases the average affinity for the ligands and increases the degree of cooperativity. The activator valine decreases the degree of cooperativity, but gives rise to a slight increase in the average dissociation constant for 2-aminobutyrate and alanine, possibly due to a competitive effect with active site ligands. The temperature dependence of the average affinity of ligands for the active sites indicates that binding is entropically controlled, with average values for delta H0 of +6.0 kcal/mol for alanine and +4.87 kcal/mol for 2-aminobutyrate. Since none of the ligands under investigation had any effect on the tetrameric quaternary structure of the enzyme as judged by sedimentation equilibrium, an initial attempt to describe cooperative ligand association with threonine deaminase was undertaken by analysis of the binding isotherms in terms of a two-state model in which the enzyme equilibrates between two conformations, T and R, that differ in their affinity for ligands. This analysis suggests that cooperative binding to the enzyme is largely the result of the high preferential affinity of ligands for the R state. In addition, estimates for the allosteric equilibrium constant L range from 2 to 5, in the absence of modifiers, to 35 in the presence of isoleucine, and 0.9 in the presence of valine. To a first approximation, the data are consistent with isoleucine and valine regulating threonine deaminase by shifting the allosteric equilibrium between the different affinity forms.

Aromatic amine dehydrogenase, a second tryptophan tryptophylquinone enzyme

Aromatic amine dehydrogenase (AADH) catalyzes the oxidative deamination of aromatic amines including tyramine and dopamine. AADH is structurally similar to methylamine dehydrogenase (MADH) and possesses the same tryptophan tryptophylquinone (TTQ) prosthetic group. AADH exhibits an alpha 2 beta 2 structure with subunit molecular weights of 39,000 and 18,000 and with a quinone covalently attached to each beta subunit. Neither subunit cross-reacted immunologically with antibodies to the corresponding subunits of MADH, and the N-terminal amino acid sequence of the beta subunit of AADH exhibited no homology with the highly conserved beta subunits of MADH. The absorption spectra for the oxidized, semiquinone, and reduced forms of AADH have been characterized, and extinction coefficients for the absorption maxima of each redox form have been determined. These spectra are very similar to those for MADH, indicating the likelihood of a TTQ cofactor. This was verified by the near identity of the vibrational frequencies and intensities in the resonance Raman spectra for the oxidized forms of AADH and MADH. A stable semiquinone of AADH could be observed during a reductive titration with dithionite, whereas titration with tyramine proceeded directly from the oxidized to the reduced form. AADH was very stable against denaturation by heat and exposure to guanidine. The individual subunits could be separated by gel filtration after incubation in guanidine hydrochloride, and partial reconstitution of activity was observed on recombination of the subunits. Steady-state kinetic analysis of AADH yielded a Vmax of 17 mumol/min/mg and a Km for tyramine of 5.4 microM. Substrate inhibition by tyramine was observed. AADH was irreversibly inhibited by hydrazine, phenylhydrazine, hydroxylamine, semicarbazide, and aminoguanidine. Isonicotinic acid hydrazide (isoniazid) and isonicotinic acid 2-isopropyl hydrazide (iproniazid) were reversible noncompetitive inhibitors of AADH and exhibited K(i) values of 8 and 186 microM, respectively. The similarities and differences between AADH and other amine oxidizing enzymes are also discussed.