A novel meningococcal outer membrane vesicle vaccine with constitutive expression of FetA: A phase I clinical trial

Objectives

Outer membrane vesicle (OMV) vaccines are used against outbreaks of capsular group B Neisseria meningitidis (MenB) caused by strains expressing particular PorA outer membrane proteins (OMPs). Ferric enterobactin receptor (FetA) is another variable OMP that induces type-specific bactericidal antibodies, and the combination of judiciously chosen PorA and FetA variants in vaccine formulations is a potential approach to broaden protection of such vaccines.

Methods

The OMV vaccine MenPF-1 was generated by genetically modifying N. meningitidis strain 44/76 to constitutively express FetA. Three doses of 25 μg or 50 μg of MenPF-1 were delivered intra-muscularly to 52 healthy adults.

Results

MenPF-1 was safe and well tolerated. Immunogenicity was measured by serum bactericidal assay (SBA) against wild-type and isogenic mutant strains. After 3 doses, the proportion of volunteers with SBA titres ≥1:4 (the putative protective titre) was 98% for the wild-type strain, and 77% for the strain 44/76 FetA_onPorA_off compared to 51% in the strain 44/76 FetA_offPorA_off, demonstrating that vaccination with MenPF-1 simultaneously induced FetA and PorA bactericidal antibodies.

Conclusion

This study provides a proof-of-concept for generating bactericidal antibodies against FetA after OMV vaccination in humans. Prevalence-based choice of PorA and FetA types can be used to formulate a vaccine for broad protection against MenB disease.

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MenB OMV vaccines' efficacy is strain-restricted by the variable antigen PorA.

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FetA is another variable antigen, but has iron-dependent expression.

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The combination of only a few PorA and FetA can induce broad-protection.

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A mutated OMV was created containing one PorA and one FetA.

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FetA induces bactericidal antibody response in addition to the PorA response in a Phase I trial.

Expression, refolding and crystallization of the OpcA invasin from Neisseria meningitidis

OpcA is an integral outer membrane from the Gram-negative pathogen Neisseria meningitidis that plays a role in adhesion of meningococci to host cells. The protein was overexpressed in Escherichia coli in an insoluble form and a procedure developed for refolding by rapid dilution from denaturant into detergent solution. The refolded material was identical to native OpcA isolated from meningococci, as judged by overall molecular weight, migration on SDS-PAGE and reaction against monoclonal antibodies. Both native and recombinant OpcA crystallized under similar conditions to give an orthorhombic crystal form (P2(1)2(1)2), with unit-cell parameters a = 96.9, b = 46.3, c = 74.0 A. Complete data sets of reflections were collected from native and refolded OpcA to 2.0 A resolution.

Analysis of the PilQ secretin from Neisseria meningitidis by transmission electron microscopy reveals a dodecameric quaternary structure

PilQ is a member of the secretin family of outer membrane proteins and is specifically involved in secretion of type IV pili in Neisseria meningitidis, Neisseria gonorrhoeae, and Pseudomonas aeruginosa. The quaternary structure of PilQ from N. meningitidis was analyzed by transmission electron microscopy by using a negative stain. Single particle averaging was carried out with a total data set of 650 individual particles, which produced a projection map generated from 296 particles at an estimated resolution of 2.6 nm. Oligomeric PilQ adopts a donut-like structure with an external ring that is 16.5 nm in diameter surrounding a central cavity that is 6.5 nm in diameter. Self-rotation and power spectrum analysis demonstrated the presence of 12-fold rotational symmetry, showing that PilQ is organized as a ring of 12 identical subunits. A model of the type IV meningococcal pilus fiber, based on the X-ray crystal structure of the N. gonorrhoeae pilin subunit, fitted neatly into the cavity, demonstrating how PilQ could serve as a channel for the growing pilus fiber.

Methods for the Preparation and Crystallization of Fab Fragments in Complex with Peptide Antigens Derived from N. meningitidis Outer Membrane Proteins

An understanding of the molecular basis for the recognition of outer-membrane proteins (OMPs) by antibody is an important goal in the development of a more rational approach to vaccine design. X-ray crystallography has been outstandingly successful in delineating the detailed chemical interactions that are responsible for the high affinity and high selectivity of antibody-antigen interactions (1). Although a number of X-ray structures of OMPs have now been reported (e.g., 2), determination of the structure of a novel OMP is far from routine. Furthermore, it is more useful to know the structural basis for molecular recognition of a particular antigen by antibody than the structure of the cognate antigen alone. For this reason, work in this area has concentrated on studying the structures of antigen in complex with antibody Fab fragments. In practice, this requires the synthesis of a short peptide or oligosaccharide that binds to the antibody in question and then determination of the structure of the bound antigen by X-ray crystallography. Clearly, this has the limitation that only well-defined continuous epitopes can be studied in this way, but fortunately this is frequently the case with monoclonal antibodies (MAbs) that have been prepared against meningococcal OMPs. This type of approach has also been used to study the binding of the polysaccharide O-antigen from Shigella flexneri (3), and in principle it could be applied to meningococcal polysaccharide antigens if suitable small oligosaccharides could be synthesised in milligram quantities.

Equilibrium and kinetic studies of substrate binding to 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase from Escherichia coli

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) catalyzes the pyrophosphorylation of 6-hydroxymethyl-7,8-dihydropterin (HMDP) by ATP to form 6-hydroxymethyl-7,8-dihydropterin pyrophosphate, an intermediate in the pathway for folic acid biosynthesis. The enzyme has been identified as a potential target for antimicrobial drugs. Equilibrium binding studies showed that Escherichia coli HPPK-bound ATP or the nonhydrolyzable ATP analogue alpha, beta-methyleneadenosine triphosphate (AMPCPP) with high affinity. The fluorescent ATP analogue 2'(3')-O-(N-methylanthraniloyl) adenosine 5'-triphosphate (MANT-ATP) exhibited a substantial fluorescence enhancement upon binding to HPPK, with an equilibrium dissociation constant comparable with that for ATP (10.4 and 4.5 micrometer, respectively). The apoenzyme did not bind the second substrate HMDP, however, unless AMPCPP was present, suggesting that the enzyme binds ATP first, followed by HMDP. Equilibrium titration of HPPK into HMDP and AMPCPP showed an enhancement of fluorescence from the pterin ring of the substrate, and a dissociation constant of 36 nm was deduced for HMDP binding to the HPPK.AMPCPP binary complex. Stopped flow fluorimetry measurements showed that the rate constants for the binding of MANT-ATP and AMPCPP to HPPK were relatively slow (3.9 x 10(5) and 1.05 x 10(5) m(-1) s(-1), respectively) compared with the on rate for binding of HMDP to the HPPK.AMPCPP binary complex. The significance of these results with respect to the crystal structures of HPPK is discussed.

Overall rotational diffusion and internal mobility in domain II of protein G from Streptococcus determined from 15N relaxation data

The backbone dynamics and overall tumbling of protein G have been investigated using 15N relaxation. Comparison of measured R2/R1 relaxation rate ratios with known three-dimensional coordinates of the protein show that the rotational diffusion tensor is significantly asymmetric, exhibiting a prolate axial symmetry. Extensive Monte Carlo simulations have been used to estimate the uncertainty due to experimental error in the relaxation rates to be D(parallel)/D(perpendicular) = 1.68 +/- 0.08, while the dispersion in the NMR ensemble leads to a variation of D(parallel)/D(perpendicular) = 1.65 +/- 0.03. Incorporation of this tensorial description into a Lipari-Szabo type analysis of internal motion has allowed us to accurately describe the local dynamics of the molecule. This analysis differs from an earlier study where the overall rotational diffusion was described by a spherical top. In this previous analysis, exchange parameters were fitted to many of the residues in the alpha helix. This was interpreted as reflecting a small motion of the alpha helix with respect to the beta sheet. We propose that the differential relaxation properties of this helix compared to the beta sheet are due to the near-orthogonality of the NH vectors in the two structural motifs with respect to the unique axis of the diffusion tensor. Our analysis shows that when anisotropic rotational diffusion is taken into account NH vectors in these structural motifs appear to be equally rigid. This study underlines the importance of a correct description of the rotational diffusion tensor if internal motion is to be accurately investigated.

Crystal structure of an Fab fragment in complex with a meningococcal serosubtype antigen and a protein G domain

Many pathogens present highly variable surface proteins to their host as a means of evading immune responses. The structure of a peptide antigen corresponding to the subtype P1.7 variant of the porin PorA from the human pathogen Neisseria meningitidis was determined by solution of the X-ray crystal structure of the ternary complex of the peptide (ANGGASGQVK) in complex with a Fab fragment and a domain from streptococcal protein G to 1.95 A resolution. The peptide adopted a beta-hairpin structure with a type I beta-turn between residues Gly4P and Gly7P, the conformation of the peptide being further stabilised by a pair of hydrogen bonds from the side-chain of Asn2P to main-chain atoms in Val9P. The antigen binding site within the Fab formed a distinct crevice lined by a high proportion of apolar amino acids. Recognition was supplemented by hydrogen bonds from heavy chain residues Thr50H, Asp95H, Leu97H and Tyr100H to main-chain and side-chain atoms in the peptide. Complementarity-determining region (CDR) 3 of the heavy chain was responsible for approximately 50 % of the buried surface area formed by peptide-Fab binding, with the remainder made up from CDRs 1 and 3 of the light chain and CDRs 1 and 2 of the heavy chain. Knowledge of the structures of variable surface antigens such as PorA is an essential prerequisite to a molecular understanding of antigenic variation and its implications for vaccine design.

Dihydropteroate synthase from Streptococcus pneumoniae: characterization of substrate binding order and sulfonamide inhibition

Dihydropteroate synthase (DHPS) catalyses a key step in the biosynthesis of folic acid and is the target for inhibition by the sulphonamide class of antimicrobial agents. Here we describe a study of the enzymatic mechanism and sulphonamide inhibition of DHPS from the pathogen Streptococcus pneumoniae. Equilibrium binding assays showed that binding of the substrate para-aminobenzoic acid (pABA) to DHPS was absolutely dependent on the presence of pyrophosphate, which acts as an analogue of the second substrate 6-hydroxymethyl-7, 8-dihydropterin pyrophosphate (DHPPP). The product of the reaction, dihydropteroate, was also able to bind to DHPS. Sulphonamides were capable of displacing pABA in a competitive manner, with equilibrium binding constants that were significantly higher than the equivalent Ki values deduced from steady state kinetic measurements. These results indicate that the target for sulphonamide inhibition of S. pneumoniae DHPS is the enzyme-DHPPP binary complex, rather than the apoprotein form of the enzyme.

Structural and evolutionary inference from molecular variation in Neisseria porins

The porin proteins of the pathogenic Neisseria species, Neisseria gonorrhoeae and Neisseria meningitidis, are important as serotyping antigens, putative vaccine components, and for their proposed role in the intracellular colonization of humans. A three-dimensional structural homology model for Neisseria porins was generated from Escherichia coli porin structures and N. meningitidis PorA and PorB sequences. The Neisseria sequences were readily assembled into the 16-strand beta-barrel fold characteristic of porins, despite relatively low sequence identity with the Escherichia proteins. The model provided information on the spatial relationships of variable regions of peptide sequences in the PorA and PorB trimers and insights relevant to the use of these proteins in vaccines. The nucleotide sequences of the porin genes from a number of other Neisseria species were obtained by PCR direct sequencing and from GenBank. Alignment and analysis of all available Neisseria porin sequences by use of the structurally conserved regions derived from the PorA and PorB structural models resulted in the recovery of an improved phylogenetic signal. Phylogenetic analyses were consistent with an important role for horizontal genetic exchange in the emergence of different porin classes and confirmed the close evolutionary relationships of the porins from N. meningitidis, N. gonorrhoeae, Neisseria lactamica, and Neisseria polysaccharea. Only members of this group contained three conserved lysine residues which form a potential GTP binding site implicated in pathogenesis. The model placed these residues on the inside of the pore, in close proximity, consistent with their role in regulating pore function when inserted into host cells.

Use of streptococcal protein G in obtaining crystals of an antibody Fab fragment in complex with a meningococcal antigen

Crystals have been obtained of an antibody Fab fragment grown in the presence of a single domain from streptococcal protein G and a ten amino-acid peptide corresponding to the P1.7 serosubtype antigen from the human pathogen Neisseria meningitidis. Crystal trials using the Fab fragment and peptide antigen alone were unsuccessful, but the inclusion of a protein G domain provided an additional variable that generated suitable crystals. Crystals are in space group P21 with unit-cell parameters a = 43.60, b = 63.42, c = 89.63 A, beta = 98.58 degrees and a data set has been collected to 2.9 A resolution using synchrotron radiation. The inclusion of protein G is likely to be of general utility for the crystallization of Fab-antigen complexes.

A novel approach for the crystallization of soluble proteins using non-ionic surfactants

Crystallization trials using three polyoxyethylene surfactants as precipitating agents are described. Of the eight soluble proteins screened, five were successfully crystallized at the first attempt. These included lysozyme, catalase, ferritin, ribonuclease A and ubiquitin. Further work suggested that these surfactants could also be suitable for cryo-crystallographic analysis of crystals. At the concentrations used in the crystallization trials [10-40%(v/v)], they are capable of promoting the formation of non-crystalline glasses at cryogenic temperatures (77K). This would facilitate crystal mounting and allow the minimization of crystal irradiation damage. Results from this study also suggest that proteins remain stable at high concentrations of these surfactants [40%(w/v)] and over long time periods (>1 month). A number of membrane proteins were also screened for crystallization. These included photosystems I and II and light harvesting complexes I and II from spinach and bacteriorhodopsin from Halobacterium halobium++. The trial s were unsuccessful both in the absence and presence of heptane-1,2,3-triol and over a wide range of surfactant concentrations.

A simplified squalene epoxidase assay based on an HPLC separation and time-dependent UV/visible determination of squalene

A novel and highly simplified enzyme assay for squalene epoxidase (EC 1.14.99.7) has been developed. The assay relies on the UV/visible determination of squalene at 195 nm, as it elutes from an octadecylsilane HPLC column. An acetonitrile/water (95.5/0.5, v/v) mixture was found to provide an ideal mobile phase, into which aqueous enzyme reaction mixture aliquots could be injected. Squalene, the natural substrate for squalene epoxidase, may be quantitatively determined within the concentration range 0-30 microM, with a calibration curve exhibiting an r2 (where r2 is the square of the Pearson correlation coefficient r) of 0.995. The HPLC retention time for squalene was significantly longer (> 15 min) than that for any other component required to prepare an enzyme assay reaction mixture, so facilitating its identification and quantification. In this way HPLC was used to follow enzymic squalene consumption within aliquots taken over a 30-min period. Previously reported squalene epoxidase assays rely on the radiolabeling and subsequent monitoring of squalene as it is metabolized by the enzyme. A highly simplified enzyme assay for squalene epoxidase is therefore reported.

Steroid recognition by chloramphenicol acetyltransferase: engineering and structural analysis of a high affinity fusidic acid binding site

The antibiotic fusidic acid and certain closely related steroidal compounds are potent competitive inhibitors of the type I variant of chloramphenicol acetyltransferase (CATI). In the absence of crystallographic data for CATI, the structural determinants of steroid binding were identified by (1) construction in vitro of genes encoding chimaeric enzymes containing segments of CATI and the related type III variant (CATIII) and (2) site-directed mutagenesis of the gene encoding CATIII, followed by kinetic characterisation of the substituted variants. Replacement of four residues of CATIII (Gln92, Asn146, Tyr168 and Ile172) by their equivalents from CATI yields an enzyme variant that is susceptible to competitive inhibition by fusidate with respect to chloramphenicol (Ki = 5.4 microM). The structure of the complex of fusidate and the Q92C/N146F/Y168F/I172V variant, determined at 2.2 A resolution by X-ray crystallography, reveals the inhibitor bound deep within the chloramphenicol binding site and in close proximity to the side-chain of His195, an essential catalytic residue. The aromatic side-chain of Phe146 provides a critical hydrophobic surface which interacts with non-polar substituents of the steroid. The remaining three substitutions act in concert both to maintain the appropriate orientation of Phe 146 and via additional interactions with the bound inhibitor. The substitution of Gln92 by Cys eliminates a critical hydrogen bond interaction which constrains a surface loop (residues 137 to 142) of wild-type CATIII which must move in order for fusidate to bind to the enzyme. Only two hydrogen bonds are observed in the CAT-fusidate complex, involving the 3-alpha-hydroxyl of the A-ring and both hydroxyl of Tyr25 and NE2 of His195, both of which are also involved in hydrogen bonds with substrate in the CATIII-chloramphenicol complex. In the acetyl transfer reaction catalysed by CAT, NE2, of His195 serves as a general base in the abstraction of a proton from the 3-hydroxyl of chloramphenicol as the first chemical step in catalysis. The structure of the CAT-inhibitor complex suggests that deprotonation of the 3-alpha-hydroxyl of bound fusidate by this mechanism could produce an oxyanion nucleophile analogous to that seen with chloramphenicol, but one which is incorrectly positioned to attack the thioester carbonyl of acetyl-CoA, accounting for the observed failure of CAT to acetylate fusidate.

Model for the complex between protein G and an antibody Fc fragment in solution

BACKGROUND

Streptococcal protein G and staphylococcal protein A are bacterial antibody-binding proteins, widely used as immunological tools, whose antibody-binding domains are structurally quite different. The binding of protein G to Fc fragments is competitive with respect to protein A, suggesting that the binding sites for protein A and protein G on Fc overlap, notwithstanding the fact that they lack sequence or structural similarity.

RESULTS

To resolve this issue, the residues involved in the interaction between an IgG-binding domain of protein G (domain II) and the Fc fragment of mouse IgG2a have been identified by use of 13C and 15N NMR. Binding of protein G domain II selectively perturbed resonances from residues between the CH2 and CH3 domains of Fc, whereas in domain II the residues affected are primarily those on the alpha-helix and the third strand of the beta-sheet. This information was used, together with the structures of the two uncomplexed proteins, to construct a model of the complex, using Monte Carlo minimization techniques. In this model, the alpha-helix of protein G lies in the same position as helix 1 of protein A in the crystal structure of the protein A:Fc complex, but its orientation differs from the latter by 180 degrees.

CONCLUSIONS

The interactions of the bacterial antibody-binding proteins with their 'target' immunoglobulins involve a very versatile set of protein-protein interactions. First, the IgG-binding domains of protein A and protein G have quite different three-dimensional structures, but bind to sites on the Fc fragment that overlap extensively. Secondly, protein G employs two quite different regions of its surface to bind to the Fab and Fc regions of IgG.

The third IgG-binding domain from streptococcal protein G. An analysis by X-ray crystallography of the structure alone and in a complex with Fab

Protein G is a cell surface-associated protein from Streptococcus that binds to IgG with high affinity. We have determined the X-ray crystal structures of the third IgG-binding domain (domain III) alone to a resolution of 1.1 A (final R-factor of 19.3%), and in complex with an Fab fragment to 2.6 A (final R-factor of 16.8%). The structure of domain III is similar to the lower-resolution crystal structures of protein G domains determined previously by other investigators, but shows some minor differences when compared with the equivalent NMR structures. Domain III binds to the immunoglobulin by formation of an antiparallel interaction between the second beta-strand in domain III and the last beta-strand in the CH 1 domain. There is also a minor site of interaction between the C-terminal end of the alpha-helix in protein G and the first beta-strand in the CH 1 domain. Previous studies by NMR on the interactions between protein G and IgG have concluded that different portions of the protein G domain are involved in binding to the Fab and Fc portions. The results presented here support these findings and permit a detailed analysis of the recognition of Fab by protein G; formation of the complex buries a large water-accessible area, of a magnitude comparable with that found in antibody/antigen interactions. The majority of hydrogen bonds between the two proteins involve main-chain atoms from the CH 1 domain. The CH 1 domain residues that are in contact with protein G are shown to be highly conserved in alignments of mouse and human gamma chain amino acid sequences. We conclude that the binding site for protein G on Fab is relatively invariant across different species and gamma chain subclasses, providing an explanation for the widespread recognition of Fab fragments from mouse and human antibodies by protein G. The solution of the crystal structures of domain III alone and bound to Fab has demonstrated that there is no major structural change apparent in either protein on formation of the complex.

Analysis of hydrogen bonding in enzyme-substrate complexes of chloramphenicol acetyltransferase by infrared spectroscopy and site-directed mutagenesis

Chloramphenicol acetyltransferase (CAT) reversibly transfers an acetyl group between CoA and the 3-hydroxyl of either chloramphenicol (Cm) or 1-acetylchloramphenicol (1AcCm). The products of the forward reactions, 3-acetylchloramphenicol (3-AcCm) and 1,3-diacetylchloramphenicol (1,3Ac2-Cm), are the substrates for the reverse reaction. The role of the 3-acetyl carbonyl group in the binding of the substrates 3AcCm and 1,3Ac2Cm to CAT has been investigated using infrared spectroscopy. Comparison of difference spectra (3-[12C = O]acetyl- minus 3-[13C = O]acetyl-) obtained for the binary complexes of 3AcCm with wild-type CAT, and with a variant wherein serine-148 is replaced by alanine (S148A), reveals a large (9 cm-1) down frequency shift for the 3-acetyl carbonyl stretch in the wild-type complex, indicative of a hydrogen bond between this carbonyl and the hydroxyl group of Ser-148. The carbonyl bandwidth in the wild-type complex is reduced by 33% compared to that for the complex with S148A, indicating restriction of carbonyl mobility and dispersion in the former, an observation consistent with the proposed hydrogen bond between the ester carbonyl and the hydroxyl of Ser-148. Repetition of the experiment using 1,3Ac2Cm as the ligand reveals a frequency shift of only 3 cm-1 between wild-type and S148A complexes, indicating only a small change in the strength of carbonyl interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the solution structures of domains II and III of protein G from Streptococcus by 1H nuclear magnetic resonance

We have used 1H nuclear magnetic resonance spectroscopy to determine the solution structures of two small (61 and 64 residue) immunoglobulin G (IgG)-binding domains from protein G, a cell-surface protein from Streptococcus strain G148. The two domains differ in sequence by four amino acid substitutions, and differ in their affinity for some subclasses of IgG. The structure of domain II was determined using a total of 478 distance restraints, 31 phi and 9 chi 1 dihedral angle restraints; that of domain III was determined using a total of 445 distance restraints, 31 phi and 9 chi 1 dihedral angle restraints. A protocol which involved distance geometry, simulated annealing and restrained molecular dynamics was used to determine ensembles of 40 structures consistent with these restraints. The structures are found to consist of an alpha-helix packed against a four-stranded antiparallel-parallel-antiparallel beta-sheet. The structures of the two domains are compared to each other and to the reported structure of a similar domain from a protein G from a different strain of Streptococcus. We conclude that the difference in affinity of domains II and III for IgG is due to local changes in amino acid side-chains, rather than a more extensive change in conformation, suggesting that one or more of the residues which differ between them are directly involved in interaction with IgG.

Crystal structure of a streptococcal protein G domain bound to an Fab fragment

Protein G is a cell-surface protein from Streptococcus which binds to IgG molecules from a wide range of species with an affinity comparable to that of antigen. The high affinity of protein G for the Fab portion of IgG poses a particular challenge in molecular recognition, given the variability of heavy chain subclass, light chain type and complementarity-determining regions. Here we report the crystal structure of a complex between a protein G domain and an immunoglobulin Fab fragment. An outer beta-strand in the protein G domain forms an antiparallel interaction with the last beta-strand in the constant heavy chain domain of the immunoglobulin, thus extending the beta-sheet into the protein G. The interaction between secondary structural elements in Fab and protein G provides an ingenious solution to the problem of maintaining a high affinity for many different IgG molecules. The structure also contrasts with Fab-antigen complexes, in which all contacts with antigen are mediated by the variable regions of the antibody, and to our knowledge provides the first details of interaction of the constant regions of Fab with another protein.

Crystallization and preliminary X-ray analysis of the complex between a mouse Fab fragment and a single IgG-binding domain from streptococcal protein G

The Fab fragment of a mouse immunoglobulin G1, complexed with a single IgG-binding domain from streptococcal protein G, has been crystallized in a form suitable for analysis by X-ray diffraction. The needle-shaped crystals were grown from polyethylene glycol 4000 using vapour diffusion methods and diffract to 2.3 A resolution. The space group is P2(1)2(1)2(1) (a = 64.5 A, b = 70.5 A and c = 120.1 A), with one Fab-protein G domain complex in the asymmetric unit. Solution of the three-dimensional structure of the complex will permit a detailed analysis of the molecular interactions between protein G and the Fab portion of IgG.

Analysis of the binding of 1,3-diacetylchloramphenicol to chloramphenicol acetyltransferase by isotope-edited 1H NMR and site-directed mutagenesis

The binary complex of diacetylchloramphenicol and chloramphenicol acetyltransferase (CAT) has been studied by a combination of isotope-edited 1H NMR spectroscopy and site-directed mutagenesis. One-dimensional HMQC spectra of the complex between 1,3-[2-13C]diacetylchloramphenicol and the type III natural variant of CAT revealed the two methyl 1H signals arising from each 13C-labeled carbon atom in the acetyl groups of the bound ligand. Slow hydrolysis of the 3-acetyl group by the enzyme precluded further analysis of this binary complex. It was possible to slow down the rate of hydrolysis by use of the catalytically defective S148A mutant of CATIII (Lewendon et al., 1990); in the complex of diacetylchloramphenicol with S148A CATIII, the chemical shifts of the acetyl groups of the bound ligand were the same as in the wild-type complex. The acetyl signals were individually assigned by repeating the experiment using 1-[2-13C],3-[2-12C]diacetylchloramphenicol, where only one signal from the bound ligand was observed. A two-dimensional 1H, 1H NOESY experiment, with 13C(omega 2) half-filter, on the 1,3-[2-13C]diacetylchloramphenicol/S148A CATIII complex showed a number of intermolecular NOEs from each methyl group in the ligand to residues in the chloramphenicol binding site. The 3-acetyl group showed strong NOEs to two aromatic signals which were selected for assignment. The possibility that the NOEs originated from the aromatic protons of diacetylchloramphenicol itself was eliminated by assignment of the signals from enzyme-bound diacetylchloramphenicol and chloramphenicol using perdeuterated CATIII. Examination of the X-ray crystal structure of the chloramphenicol/CATIII binary complex indicated four plausible candidate aromatic residues: Y25, F33, F103, and F158.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential 1H NMR assignments and secondary structure of an IgG-binding domain from protein G

Protein G is a member of a class of cell surface bacterial proteins from Streptococcus that bind IgG with high affinity. A fragment of molecular mass 6988, which retains IgG-binding activity, has been generated by proteolytic digestion and analyzed by 1H NMR. Two-dimensional DQF-COSY, TOCSY, and NOESY spectra have been employed to assign the 1H NMR spectrum of the peptide. Elements of regular secondary structure have been identified by using nuclear Overhauser enhancement, coupling constant, and amide proton exchange data. The secondary structure consists of a central alpha-helix (Ala28-Val44), flanked by two portions of beta-sheet (Val5-Val26 and Asp45-Lys62). This is a fundamentally different arrangement of secondary structure from that of protein A, which is made up of three consecutive alpha-helices in free solution (Torigoe et al., 1990). We conclude that the molecular mechanisms underlying the association of protein A and protein G with IgG are different.

Identification of the C2-1H histidine NMR resonances in chloramphenicol acetyltransferase by a 13C-1H heteronuclear multiple quantum coherence method

Chloramphenicol acetyltransferase (CAT) was used to assess the feasibility of study of specific proton resonances in an enzyme of overall molecular mass 75,000, [ring 2-13C]Histidine was selectively incorporated into the type III chloramphenicol acetyltransferase (CATIII) using a histidine auxotroph of E. coli. Heteronuclear multiple and single quantum experiments were used to select the C2 protons in the histidyl imidazole ring. One- and two-dimensional spectra revealed six signals out of a total of seven histidine residues in CATIII. pH titration, chemical modification and ligand binding were used to demonstrate that the signal from H195, the histidine at the active site, is not among those observed. Nevertheless, this work demonstrates that selective isotopic enrichment and multiple quantum coherence techniques can be used to distinguish proton resonances in a protein of high molecular mass.

The serine acetyltransferase from Escherichia coli. Over-expression, purification and preliminary crystallographic analysis

An expression vector has been constructed which increases the expression of serine acetyltransferase (SAT) from E. coli to 17% of the soluble cell protein. A novel purification procedure, using dye-affinity chromatography, allows purification of SAT to homogeneity. The enzyme has been crystallised from polyethylene glycol, in the presence of L-cysteine (an inhibitor of SAT). The crystals which diffract to beyond 3.0 A resolution are of the tetragonal spacegroup P4(1)2(1)2 (or P4(3)2(1)2) with cell dimensions a = b = 123 A, c = 79 A. Since ultracentrifugation and gel-filtration experiment indicate that purified SAT is a tetramer, there appears to be one-half tetramer in the asymmetric unit (Vm = 2.55 A3/Da).

L-carnitine acyltransferase in intact peroxisomes is inhibited by malonyl-CoA

Inhibition of the overt mitochondrial carnitine palmitoyltransferase by malonyl-CoA is important in the regulation of fatty acid oxidation. In the past, the contribution of peroxisomal carnitine acyltransferase activity to the generation of medium- and long-chain acylcarnitines in the cytoplasm has been ignored. On the basis of marker enzyme levels, we now estimate that peroxisomal palmitoyltransferase activity constitutes about 20% of the peroxisomal plus overt-mitochondrial pool in fed rat liver. When assayed in situ, both the palmitoyltransferase and decanoyltransferase activities of gradient-purified peroxisomes are sensitive to malonyl-CoA, with up to 90% inhibition reached at less than 10 microM-malonyl-CoA. Very similar results were obtained with intact gradient-purified mitochondria from the same livers. In addition, the acyl-CoA substrate chain-length specificity was identical in both the peroxisomes and the mitochondria, with a decanoyltransferase/palmitoyltransferase ratio of 2. Thus the overt carnitine acyltransferase activities in peroxisomes and mitochondria have the same properties. Further, the malonyl-CoA sensitivity of the peroxisomal activity is lost on solubilization, as has been observed for the overt mitochondrial enzyme. It is suggested that malonyl-CoA inhibition of the peroxisomal enzyme as well as of the mitochondrial enzyme is important for the regulation of mitochondrial fatty acid oxidation.

A case of carnitine palmitoyltransferase II deficiency in human skeletal muscle

A 20-year-old man was shown to have a deficiency of carnitine palmitoyltransferase (CPT) II in skeletal muscle. The evidence was: (i) there was no significant oxidation of [9,10-3H]-palmitate or of [1-14C]palmitate in mitochondrial fractions from fresh skeletal muscle from the patient; (ii) all the CPT activity in a homogenate of fresh muscle from the patient was overt (CPT I) with no increase in activity after the inner membrane was disrupted; (iii) all the CPT activity in the patient's muscle was inhibited by malonyl-CoA; and (iv) an immunoreactive peptide of 67 kDa corresponding to CPT II, present in mitochondria from controls, was absent in those from the patient.

Purification and properties of the soluble carnitine palmitoyltransferase from bovine liver mitochondria

A new carnitine palmitoyltransferase (CPT) was purified to homogeneity from bovine liver mitochondria which were 96% free of peroxisomal contamination, as judged by catalase and glutamate dehydrogenase activities. The enzyme is easily removed from mitochondria, without the use of detergent. It is monomeric (Mr 63,500), unlike other preparations of CPT from mitochondria, and is most active with myristoyl-CoA and palmitoyl-CoA. The Km values are between 0.8 and 4 microM for a range of substrates from hexanoyl-CoA to stearoyl-CoA; these are much lower than values reported for other purified CPT preparations. The Km for L-carnitine is 185 microM measured with palmitoyl-CoA, and does not vary greatly with the chain length. This is also lower than the values reported for other CPT preparations, but higher than those cited for the medium-chain transferases. Kinetic and inhibitor studies were consistent with a rapid-equilibrium random-order mechanism. 2-Bromopalmitoyl-CoA, which is an inhibitor of the outer CPT, inhibited the enzyme competitively with palmitoyl-CoA as the variable substrate, when added without preincubation. If the enzyme was preincubated with 2-bromopalmitoyl-CoA and carnitine, the activity did not reappear after gel filtration of the protein. The inhibitor was bound in a 1:1 stoichiometry per subunit of enzyme.