The reaction of aldolase with 2-methylmaleic anhydride

Polypeptide-chain stoicheiometry and lipoic acid content of the pyruvate dehydrogenase complex of Escherichia coli

The pyruvate dehydrogenase multienzyme complex was isolated from Escherichia coli grown in the presence of [35S]sulphate. The three component enzymes were separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and the molar ratios of the three polypeptide chains were determined by measurement of the radioactivity in each band. The chain ratio of lipoamide dehydrogenase to lipoate acetyltransferase approached unity, but there was a molar excess of chains of the pyruvate decarboxylase component. The 35S-labelled complex was also used in a new determination of the total lipoic acid content. It was found that each polypeptide chain of the lipoate acetyltransferase component appears to bear at least three lipoyl groups.

Synthesis of chloromethyl ketone derivatives of fatty acids. Their use as specific inhibitors of acetoacetyl-coenzyme A thiolase, cholesterol biosynthesis and fatty acid synthesis

A general route for the synthesis of chloromethyl ketone derivatives of fatty acids is described. 5-Chloro-4-oxopentanoic acid, 7-chloro-6-oxoheptanoic acid, 9-chloro-8-oxononanoic acid and 11-chloro-10-oxoundecanoic acid were synthesized by this method and tested as covalent inhibitors of pig heart acetoacetyl-CoA thiolase. The K1 decreased by approx. 20-fold for each pair of methylenes added to the chain length, showing that the initial stage in inhibitor binding occurs at a non-polar region of the protein. This region is probably located at the enzyme active site, since inhibition was prevented by acetoacetyl-CoA or acetyl-CoA but not by CoA. The site of modification by chloromethyl ketone derivatives of fatty acids is restricted to a thiol group, since inactivation of the enzyme was prevented by reversible thiomethylation of the active-site thiol. In contrast, an amino-directed reagent, citraconic anhydride, still inactivated the enzyme, even when the active-site thiol was protected. Evidence that the enzyme thiol was particularly reactive came from studies on the pH-dependence of the alkylation reaction and thiol-competition experiments. Inhibition of the enzyme proceeded suprisingly well at acidic pH values and a 10(5) molar excess of external thiol over active-site thiol was required to prevent inhibition by 0.3 mM-9-chloro-8-oxononanoic acid. In addition to inhibiting isolated acetoacetyl-CoA thiolase, in hepatocytes the chloromethyl ketone derivatives of fatty acids also inhibited chloresterol synthesis, which uses this enzyme as an early step in the biosynthetic pathway. In isolated cells, the chloromethyl ketone derivatives of fatty acids were considerably less specific in their inhibitory action compared with 3-acetylenic derivatives of fatty acids, which act as suicide inhibitors of acetoacetyl-CoA thiolase. However, 9-chloro-8-oxononanoic acid was also an effective inhibitor of both hepatic cholesterol and fatty acid synthesis in mice in vivo, whereas the acetylenic fatty acid derivative, dec-3-ynoic acid, was completely ineffective. The effective inhibitory dose of 9-chloro-8-oxononanoic acid (2.5-5 mg/kg) was substantially lower than the estimated LD50 for the inhibitor (100 mg/kg).

Extraction and fingerprint analysis of simian virus 40 large and small T-antigens

A study of simian virus 40 (SV40) T-antigens isolated from productively infected CV1 cells using a variety of different extraction procedures showed that under some conditions the highest molecular weight form of T-Ag (large-T) isolated comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with large-T from SV40-transformed H65-90B cells. Other faster-migrating forms of large-T are probably generated during the extraction procedure by a protease which is active at low pH, and such forms are probably experimental artifacts. After extraction under conditions which minimize proteolytic degradation of large-T, a further form of T-antigen was isolated; this has an apparent molecular weight in the range 15,000 to 20,000 and is referred to as small-t. Fingerprint analysis of [35S]methionine-labeled SV40 proteins showed that small-t has 10 to 12 methionine peptides whereas large-T has 15 to 18 methionine peptides. All but two of the methionine tryptic peptides present in small-t are also present in large-T. The fingerprint data also showed that T-antigens have no peptides in common with SV40 VP1. Experiments using reagents which inhibit posttranslational cleavage of encephalomyocarditis virus polyproteins showed that these reagents do not affect the synthesis of small-t and suggest that it is not made by proteolytic cleavage of large-T in vivo. An alternative model, which proposes that large-T and small-t are synthesized independently, is discussed in terms of the fingerprint data and the number of methionine tryptic peptides predicted from the primary sequence of SV40 DNA.

Isolation and characterization of the cyanogen bromide peptides from the B fragment of diphtheria toxin

Homogeneous fragment B, obtained through nicking of diphtheria toxin with insoluble trypsin, was cleaved with cyanogen bromide in 70% formic acid. After citraconylation, the cleavage products were separated by gel filtration on Sephadex G--75 and purified by gel filtration, ion-exchange and thin-layer or paper chromatography. Six CNBr peptides were characterized, the composition of which account for the total amino acid content of fragment B. Their apparent molecular weights are: CB 1, 12 000; CB 2, 14 000; CB 3, 8000; CB 4a, 2400; CB 4b, 2200; CB 5, 2200. CB 4a is the NH2--terminal peptide; it contains the cysteine residue of the disulfide bridge linking fragment B to fragment A. CB 3 is the COOH--terminal peptide; it bears the disulfide bridge of fragment B. Characterization of two CNBr--derived overlapping peptides provided the positioning of CB 4b and CB 2 and allowed an alignment of the CNBr peptides of fragment B to be proposed.

The subunit structure of the arom multienzyme complex of Neurospora crassa. Evidence from peptide 'maps' for the identity of the subunits

Evidence was obtained, from polyacrylamide-gel electrophoresis in the presence of urea and from peptide 'mapping' of specifically labelled cysteine-and methionine-containing peptides, that the two subunits of the arom multienzyme complex of Neurospora crassa are chemically very similar and possibly identical.

Folding domains and intramolecular ionic interactions of lysine residues in glyceraldehyde 3-phosphate dehydrogenase

1. Treatment with methyl acetimidate was used to probe the topography of several tetrameric glyceraldehyde 3-phosphate dehydrogenases, in particular the holoenzymes from rabbit muscle and Bacillus stearothermophilus. During the course of the reaction with the rabbit muscle enzyme, the number of amino groups fell rapidly from the starting value of 27 per subunit to a value of approx. five per subunit. This number could be lowered further to values between one and two per subunit by a second treatment with methyl acetimidate. The enzyme remained tetrameric throughout and retained 50% of its initial catalytic activity at the end of the experiment. 2. Use of methyl [1-14C]acetimidate and small-scale methods of protein chemistry showed that only one amino group per subunit, that of lysine-306, was completely unavailable for reaction with imido ester in the native enzyme. This results is consistent with the structure of the highly homologous glyceraldehyde 3-phosphate dehydrogenase of lobster muscle deduced from X-ray-crystallographic analysis, since lysine-306 can be seen to form an intrachain ion-pair with aspartic acid-241 in the hydrophobic environment of a subunit-subunit interface. 3. Several other amino groups in the rabbit muscle enzyme that reacted only slowly with the reagent were also identified chemically. These were found to be located entirely in the C-terminal half of the polypeptides chain, which comprises a folding domain associated with catalytic activity and subunit contact in the three-dimensional structure. Slow reaction of these 'surface' amino groups with methyl acetimidate is attributed to intramolecular ionic interactions of the amino groups with neighbouring side-chain carboxyl groups, a conclusion that is compatible with the reported three-dimensional structure and with the dependence of the reaction of ionic stength. 4. Very similar results were obtained with the enzymes from B. stearothermophilus and from ox muscle and ox liver, supporting the view that the ion-pair involving lysine-306 and aspartic acid-241 will be a common structural feature in glyceraldehyde-3-phosphate dehydrogenases. The B. stearothermophilus enzyme was fully active after modification. 5. No differences could be detected between the enzymes from ox muscle and ox liver, in accord with other evidence that points to the identify of these enzymes. 6. The pattern of slowly reacting amino groups in the enzyme from B. stearothermophilus, although similar to that of the mammalian enzymes, indicated one or two additional intramolecular ionic interactions of lysine residues that might contribute to the thermal stability of this enzyme.

Intramolecular ionic interactions of lysine residues and a possible folding domain in fructose diphosphate aldolase

1. Treatment with methyl acetimidate was used to probe the topography of the tetrameric fructose 1,6-diphosphate aldolase from ox liver. A single treatment with imido ester in the presence or absence of 20mM-fructose 1,6-diphosphate caused the number of amino groups in the enzyme to fall to approx. 30% of the starting number (assumed to be 30 per subunit). The catalytic activity of the aldolase modified in the presence of fructose 1,6-diphosphate was unaffected, whereas that of the enzyme modified in the absence of substrate fell by about 20%. 2. Use of methyl [1-14C]acetimidate and small-scale methods of protein chemistry showed that the amino group of lysine-27 (the numbering is that of the highly homologous rabbit muscle enzyme) is essentially unavailable for amidination in the native enzyme and is therefore predicted to be buried in a hydrophobic environment, probably in the form of an ion-pair with a negatively charged side-chain carboxyl group. All the other lysine residues that reacted poorly with methyl acetimidate in the native enzyme (a total of 7) were found to be within the primary structure bounded by lysine-107 and lysine-227. An important member of this group of lysine residues displaying aberrant reactivity is lysine-227, which is known to form an imine with the substrate as part of the catalytic mechanism of the enzyme. 3. The results of the amidination experiments can be correlated in an interesting way with previous studies of thiol-group modification in the aldolases. Taken together, and arguing in part by analogy with the results of identical experiments with glyceraldehyde 3-phosphate dehydrogenases where the three-dimensional structure is known [Lambert & Perham (1977) Biochem. 4. 161. 49-62], they suggest that the region of primary structure from residues 107-227 may form the whole or part of a three-dimensional structural feature, perhaps a folding domain. A three-dimensional structure deduced from X-ray-crystallographic analysis will be needed to interpret these findings more closely. 4. The amino groups of lysine residues are commonly thought to reside at the 'surface' of protein structures. The patterns of specific lysine residues in glyceraldehyde 3-phosphate dehydrogenases and in aldolases that have been found to react poorly with methyl acetimidate in the native enzymes can be attributed to intramolecular ionic interactions deep in hydrophobic pockets and at the protein 'surface'. Such ionic interactions may contribute significantly to the stability of a given protein.

The preparation of protected fragments of lysozyme for semisynthesis

This paper reports the development of methods for preparing tryptic fragments of hen's-egg lysozyme in an appropriate state of protection for use in the chemical synthesis of modified polypeptides. 1. We describe the cleavage of the disulphide bridges of the enzyme and the simulatneous protection of the liberated thiol groups by S-sulphonation. Lysozyme resisted the usual conditions for this reaction. We have confirmed the stability of the S-sulphonyl group to the conditions met in peptide synthesis. 2. We describe the reversible protection of the amino groups of the enzyme by reaction with various anhydrides of 1,2-dicarboxylic acids. We conclude that 2-methylmaleic anhydride and exo-cis-3,6-endoxo-delta4-tetrahydrophthalic anhydride are unsuitable for our purpose but that maleic anhydride can, in spite of certain drawbacks, be used. 3. We describe the tryptic cleavage of the thiol- and amino-protected protein and the separation of the fragments. 4. We describe the reversible protection of the carboxylic acid groups (including the specific deprotection of the alpha-carboxyl group), the imidazolyl group and the aloph-amino groups of the fragments. Several alternative groups have been evaluated for most of these purposes. The side-chain amides did not present any serious problem of libility, 5. We describe experiments on the stability of the side chain of tryptophan, both protected by formylation and unprotected, to the acid conditions needed for the deprotection of the other functional groups in the peptide. We conclude that protection of tryptophan is unnecessary. We suggest that most of the methods described are of general application in peptide semisynthesis by fragment condensation. An Appendix is included to which points 6-ll appertain...

Primary and tertiary structure of the principal human adenylate kinase

1. Human adenylate kinase (isoenzyme AK-1-1) from skeletal muscle is a single polypeptide chain of 194 amino-acid residues with an acetylmethionine at the N-terminus and a lysine at the C-terminus. 2. The primary structure of the enzyme was determined: Ac-Met-Glu-Glu-Lys-Leu-Lys-Lys-Thr-Lys-Ile-Ile-Phe-Val-Val-Gly-Gly-Pro-Gly-Ser-Gly-Lys-Gly-Thr-Gln-Cys-Glu-Lys-Ile-Val-Gln-Lys-Tyr-Gly-Tyr-Thr-His-Leu-Ser-Thr-Gly-Asp-Leu-Leu-Arg-Ser-Glu-Val-Ser-Ser-Gly-Ser-Ala-Arg-Gly-Lys-Lys-Leu-Ser-Glu-Ile-Met-Glu-Lys-Gly-Gln-Leu-Val-Pro-Leu-Glu-Thr-Val-Leu-Asp-Met-Leu-Arg-Asp-Ala-Met-Val-Ala-Lys-Val-Asn-Thr-Ser-Lys-Gly-Phe-Leu-Ile-Asp-Gly-Tyr-Pro-Arg-Glu-Val-Gln-Gln-Gly-Glu-Glu-Phe-Glu-Arg-Arg-Ile-Gly-Gln-Pro-Thr-Leu-Leu-Leu-Tyr-Val-Asp-Ala-Gly-Pro-Glu-Thr-Met-Thr-Arg-Arg-Leu-Leu-Lys-Arg-Gly-Glu-Thr-Ser-Gly-Arg-Val-Asp-Asn-Glu-Glu-Thr-Ile-Lys-Lys-Arg-Leu-Glu-Thr-Tyr-Tyr-Lys-Ala-Thr-Glu-Pro-Val-Ile-Ala-Phe-Tyr-Glu-Lys-Arg-Gly-Ile-Val-Arg-Lys-Val-Asn-Ala-Glu-Gly-Ser-Val-Asp-Glu-Val-Phe-Ser-Gln-Val-Cys-Thr-His-Leu-Asp-Ala-Leu-Lys. 3. When the primary structure of the human enzyme was fitted to the electron density map of porcine adenylate kinase, all nine amino acids which are different in the homologous enzymes from pig and man were located on the surface of the molecule. 4. Precession photographs of crystalline human and of crystalline porcine adenylate kinase corroborated the result that the polypeptide chains of the two enzymes are folded in a closely related manner. 5. The structure of human adenylate kinase incorporates the so-called nucleotide-binding domain which is present in a wide variety of proteins in nature. Some implications of this phenomenom for the molecular biology and the molecular pharmacology of man are discussed.

Rabbit antibody light chains: selective breeding narrows variability in framework and complementarity-determining residues

The amino acid sequence of 5 light (L) chain (b4) variable (Vl) regions and the partial sequence of VL (kappa) regions from 12 anti-streptococcal group A-varant polysaccharide (Av-CHO) and 2 anti-streptococcal group C polysaccharide (C-CHO) antibodies was determined. These sequences contain 70 invariant positions as opposed to 50 invariant positions in other rabbit VL regions. Variability within the framework residues lacks randomness, and parent offspring relationship or otherwise close familial relationship is apparent in several instances. Variability in the complementarity-determining regions is reduced by 2.3-5.5-fold in comparison with other rabbit L-chains with several identical first and third hypervariable regions. Residue positions 50-56, known to mark the second hypervariable region in human kappa-chains, are not hypervariable in L-chains from Av-CHO rabbit antibodies. Considering the 67 rabbit L-chain sequences, completely or partially known today, for counting the number of V region germ line genes, it is concluded that the species rabbit has at least 27 VL germ line genes available.

Determination of the primary structure of a mouse IgG2a immunoglobulin. Amino-acid sequence of the H4 cyanogen-bromide fragment

The complete amino acid sequence of CNBr fragment H4 of the murine immunoglobulin MOPC 173 (IgG2a,chi) has been determined, thus completing the sequence determination of the entire heavy chain. The H4 fragment contains 150 residues, and extends from residue 105 to residue 254 of the heavy chain, which appears thus to be composed of 447 amino acids residues. This fragment contains the end of the V region, the switch peptide, the CH1 domain, the hinge region and the beginning of CH2. Sequence comparisons suggest that the CH1 domain is highly conserved in evolution, and allows the definition of two additional isotypic-specific regions.

Amino-acid sequence of the peptide segment liberated during activation of prochymosin (prorennin)

By conversion of prochymosin into active chymosin and N-terminal segment of 42 amino acid residues is liberated. In one activation experiment this segment was recovered in two peptides; in a second experiment the activation segment was cleaved into three peptides. The primary structures of the peptides have been determined. Overlaps between these peptides and between the activation segment and the active enzyme have been obtained from peptides produced by tryptic digestion of denatured prochymosin. Comparison of the amino acid sequences of the activation segments from bovine prochymosin, bovine pepsinogen and porcine pepsinogen shows considerable homology.

Dissociation of bovine 6S procarboxypeptidase A by reversible condensation with 2,3-dimethyl maleic anhydride: application to the partial characterization of subunit III

Bovine 6S procarboxypeptidase A can be dissociated into its three subunits by acylation with dimethyl maleic anhydride. The deacylated subunits are obtained in a largely native form due to instability of the bonds to dimethyl maleate at pH values near neutrality. The seven first residues of subunit III are identical to residues 18-24 of bovine chymotrypsinogen B and very similar with the same residues in bovine chymotrypsinogen A and C (subunit II) and also in proelastase A of the African lungfish. Therefore, this subunit is likely to be a chymotrypsinogen or proelastase-A-like zymogen which has lost the ability to be activated on account of a deletion of the N-terminal residues from half-cystine 1 to valine 17. Like other pancreatic zymogens, subunit III appears to possess a weakly functional active site.

Dissociation of small-intestinal sucrase - isomaltase complex into enzymatically active subunits

1. The sucrase - isomaltase complex from rabbit small intestine dissociated into its subunits upon reaction with citraconic anhydride. They can recombine after deacylation under mild acidic conditions. 2. When citraconylated, the subunits could be separated and isolated in a catalytically active form. 3. The previously reported procedure for separation of the subunits by alkaline treatment at pH 9.6 is apparently not due to contaminating degradative enzymes (possibly still present at undetectable levels in the isolated sucrase - isomaltase complex) but to the action of alkali.

Proteins selectively released from water-extracted human erythrocyte membranes upon citraconylation or maleylation. Electrophoretic analysis and chromatographic fractionation

1. Citraconylation or maleylation (carboxylation) of water-extracted human erythrocyte membranes (membrane residue) solubilized up to 20% (w/w) of the membrane residue protein, but less than 2% of the phospholipids. Amino acid analysis and polyacrylamide gel electrophoresis in the presence of dodecylsulfate showed that the solubilization was selective. 2. Polyacrylamide gel electrophoresis in the absence of detergent separated the solubilized acylated proteins into four well-separated major zones and two or three minor zones. Two of the major zones contained proteins of high molecular weight (spectrin components) and the other two contained components of intermediate molecular weight. 3. The solubilized components of intermediate molecular weight could be partially purified by hydroxyapatite chromatography. 4. Upon incubation at pH 5 and 36 degrees C of the unfractionated proteins solubilized by citraconylation some components became specifically degraded or dissociated into subunits.

The primary structure of the toxin Laticauda semifasciata III, a weak and reversibly acting neurotoxin from the venom of a sea snake, Laticauda semifasciata

A weak and reversibly acting neurotoxic protein of Laticauda semifasciata venom, Laticauda semifasciata III (component LsIII), was sequenced. Component LsIII consists of 66 amino acid residues and has five disulphide bridges, one of which was located between residues 26 and 30. The weak and reversible neurotoxicity of component LsIII is discussed in relation to its structure, which falls between those of the neuro- and cardiotoxins of sea snakes and Elapidae snakes isolated and sequenced so far.

Amino acid sequence of the N-terminal 139 residues of light chain derived from a homogeneous rabbit antibody

The amino acid sequence of the N-terminal 139 residues of the L (light) chain derived from a homogeneous rabbit antibody (designated BS-1) to type III pneumococci was determined. A combination of methods involving tryptic cleavage restricted to the 2 arginine residues of the molecule and mild acid hydrolysis of a labile peptide bond between the V (variable) and C (constant) regions of the L chain (Fraser et al., 1972) allowed the isolation of two large peptides comprising the entire V region (residues 1-109); these peptides were suitable for automated Edman degradation. The complete sequence analysis of the V region was carried out with only 4mumol of L chain. This material was homogeneous, although minor variant sequences, if present at the 10% value, would not have been detected. The L chain contains 3 intrachain disulphide bridges, whose pairing was established by diagonal electrophoresis: there is one V-region bridge between positions 23 and 88 and one C-region bridge between positions 134 and 194; the third one connects V and C domains between positions 80 and 171. When compared with the basic sequence of human kappa chains, rabbit L chain BS-1 appears to be more similar to the V(KI) prototype sequence than to V(KII) or V(KIII) sequences, where V(KI), V(KII) and V(KIII) represent subgroups I, II and III respectively of V regions of kappa light chains. The V regions of rabbit heavy and light chains are homologous to each other. The presence of two clusters of 3 glycine residues in positions 94-96 and 99-101 respectively is remarkable. Residues 94-96 may be related to antibody complementarity whereas residues 99-101 function probably as a pivot permitting the combining region of the L chain to make optimal contact with the antigenic determinant (Wu & Kabat, 1970).

Comparison of the amino acid sequences of the variable regions of light chains derived from two homogeneous rabbit anti-pneumococcal antibodies

The amino acid sequence of the N-terminal 139 residues of the L (light) chain derived from a homogeneous rabbit antibody to type III pneumococci was determined. This L chain, designated BS-5, exhibits a greater degree of homology with the basic sequence of human kappa chains of subgroup I (72%) than with subgroups II and III. L-chain BS-5 differs from another L chain (BS-1), also derived from an antibody to type III pneumococci (Jaton, 1974), by eight amino acid residues, even though the chains are identical within the N-terminal 30 residues. Six of these eight substitutions are located within the three hypervariable sections of the variable half: Asn/Ser in position 31, Glu/Ala in position 55, Asx/Thr, Thr/Gly, Thr/Gly and Val/Tyr in positions 92, 94, 96 and 97 respectively. The two anti-pneumococcal L chains BS-1 and BS-5 are much more similar to each other than to an anti-azobenzoate L chain (Appella et al., 1973), from which they differ by 30 and 29 residues respectively. Of these interchanges 13-15 are confined to the three hypervariable sections, and 11 occur within the N-terminal 27 positions. The three chains have an identical sequence from residue 98 to residue 139, except for a possible inversion of two residues in positions 130-131 of the anti-azobenzoate chain.

Subunit interactions in hybrids of native, carboxypeptidase-treated and citraconylated rabbit muscle aldolase

1. The kinetic properties of hybrids of native (or carboxypeptidase-treated) and citraconylated rabbit muscle aldolase are compared with those of equivalent mixtures of the parental enzymes. 2. In the hybrids, the native subunits function slightly less well than in the homotetramer, but the citraconylated subunits have enhanced activity. 3. Subunits of carboxypeptidase-treated aldolase behave essentially as expected in a hybrid environment, but the citraconylated subunits do not show the same enhancement of activity found in the hybrids of native and citraconylated enzyme. The apparent affinity for fructose 1,6-diphosphate of the citraconylated subunits in hybrids of carboxypeptidase-treated and citraconylated aldolase is increased. 4. These results are interpreted in terms of a substrate-induced conformational difference between native and carboxypeptidase-treated aldolase. 5. This conformational change can take place within a single native subunit in the hybrids and does not require a similar conformational change to occur simultaneously in the other three subunits.

Kinetic and molecular properties of citraconyl-aldolase. The reversible denaturation and hybridization of the native and modified enzymes

1. The preparation of enzymically active N-citraconyl derivatives of fructose diphosphate aldolase from rabbit muscle is described. Reaction is restricted to amino groups and the derivatives are not very heterogeneous with respect to the number of substituents. 2. Linear double-reciprocal plots of enzyme velocity against substrate concentration are found up to about 15% blocking of amino groups. With more than 15% blocking, there is a marked downward curvature in the double-reciprocal plots at high substrate concentrations. 3. Over the range 0-25% blocking of amino groups the apparent V(max.) for fructose diphosphate falls to 10% that of the native enzyme, and the apparent K(m) rises from 1 to 400mum. 4. Various pieces of evidence suggest that citraconyl-aldolase is slightly distorted in structure compared with the native enzyme. However, the kinetic properties and tetrameric structure of citraconyl-aldolase can be completely recovered after denaturation in 4m-guanidine hydrochloride. 5. After removal of the citraconyl groups in acid conditions the kinetic and molecular properties of native enzyme are restored. 6. Hybrid forms of aldolase can be constructed containing native and citraconylated subunits and the suitability of these derivatives for the study of subunit interactions in the enzyme is discussed. 7. The kinetic properties of hybridized aldolase containing native and citraconylated subunits are not exactly those predicted from the kinetic properties of the two parental forms. This result is interpreted in terms of conformational changes induced in the native and modified subunits when both are present in a hybrid molecule, evidently as a result of interactions in the tetramer.

An amino acid sequence in the active site of lipoamide dehydrogenase from pig heart

1. The two cysteine residues forming the disulphide bridge that comprises part of the active site of lipoamide dehydrogenase from pig heart were specifically labelled with iodo[2-(14)C]acetic acid. 2. A tryptic peptide containing these carboxymethylcysteine residues was isolated from digests of reduced and S-carboxymethylated lipoamide dehydrogenase and its amino acid sequence of 23 residues was determined. 3. The sequence is highly homologous with a similar sequence containing the active-site disulphide bridge of lipoamide dehydrogenase derived from the 2-oxoglutarate dehydrogenase complex of Escherichia coli (Crookes strain) and it is probable that, as in the bacterial enzyme, the disulphide bridge forms an intrachain loop containing six residues. The results indicate that the bacterial and mammalian proteins have a common genetic origin. 4. Amino acid sequences containing six other unique carboxymethylcysteine residues were also partly determined. 5. The analysis of the primary structure thus far is consistent with the view that the enzyme (mol.wt. approx. 110000) is composed of two identical polypeptide chains.

The reaction of protein amino groups with methyl 5-iodopyridine-2-carboximidate. A possible general method of preparing isomorphous heavy-atom derivatives of proteins

1. The synthesis of methyl 5-iodopyridine-2-carboximidate and its reaction with amino groups of model compounds and performic acid-oxidized insulin are described. The reagent was designed to introduce heavy atoms into specific sites in proteins. 2. Specific reaction with the amino groups of oxidized insulin can be achieved under reasonably mild conditions giving rise to the corresponding N-monosubstituted amidines. 3. The extent of reaction of this reagent with protein amino groups can be readily determined by difference spectroscopy. Modification of lysine residues inhibits tryptic cleavage at such residues, and this can be of assistance in establishing the site of modification in the primary structure. 4. Evidence is presented to show that methyl 5-iodopyridine-2-carboximidate can react specifically, at pH5.0, with the aromatic amino group of 3-amino-l-tyrosine; the final product of this reaction is a 2-arylbenzoxazole. 5. The use of this reagent as a general method for preparing heavy-atom isomorphous derivatives of proteins is discussed.

Purification and characterization of two fructose diphosphate aldolases from Escherichia coli (Crookes' strain)

Two fructose diphosphate aldolases (EC 4.1.2.13) were detected in extracts of Escherichia coli (Crookes' strain) grown on pyruvate or lactate. The two enzymes can be resolved by chromatography on DEAE-cellulose at pH7.5, or by gel filtration on Sephadex G-200, and both have been obtained in a pure state. One is a typical bacterial aldolase (class II) in that it is strongly inhibited by metal-chelating agents and is reactivated by bivalent metal ions, e.g. Ca(2+), Zn(2+). It is a dimer with a molecular weight of approx. 70000, and the K(m) value for fructose diphosphate is about 0.85mm. The other aldolase is not dependent on metal ions for its activity, but is inhibited by reduction with NaBH(4) in the presence of substrate. The K(m) value for fructose diphosphate is about 20mum (although the Lineweaver-Burk plot is not linear) and the enzyme is probably a tetramer with molecular weight approx. 140000. It has been crystallized. On the basis of these properties it is tentatively assigned to class I. The appearance of a class I aldolase in bacteria was unexpected, and its synthesis in E. coli is apparently favoured by conditions of gluconeogenesis. Only aldolase of class II was found in E. coli that had been grown on glucose. The significance of these results for the evolution of fructose diphosphate aldolases is briefly discussed.

The reactivity of functional groups as a probe for investigating the topography of tobacco mosaic virus. The use of mutants with additional lysine residues in the coat protein

Several mutants of tobacco mosaic virus that contain additional lysine residues as a result of mutations in the coat protein were investigated. Mutant E66 has a lysine residue replacing asparagine at position 140 when compared with the wild-type vulgare and this lysine residue reacts readily in the intact virus with methyl picolinimidate. Mutant B13a has two new lysine residues in the coat protein, replacing a glutamine at position 9 and an asparagine at position 33, whereas mutant B13b has the single replacement of glutamine by lysine at position 9. The lysine residue at position 9 in mutants B13a and B13b also reacts readily with methyl picolinimidate in the intact virus but the lysine at position 33 in mutant B13a did not react under these conditions. However, when the isolated coat protein from mutant B13a was treated with methyl picolinimidate, the lysine residue at position 33 did become modified, showing that the loss in reactivity of this residue towards the imidoester in the intact virus is a result of the assembly of the protein subunit into the virus structure. These results are compatible with and extend previous studies on the sero-logical properties of mutants of tobacco mosaic virus and illustrate the value of methyl picolinimidate as a reagent for probing the accessibility of amino groups in proteins. When intact tobacco mosaic virus (vulgare) was treated with p-iodobenzenesulphonyl chloride, no reaction with the lysine residues at positions 33 or 68 in the virus subunit could be detected but complete modification of tyrosine-139 was achieved. This result also extends previous studies with other reagents. The usefulness of the differential reactivity of the lysine residues in tobacco mosaic virus and its mutants as a means of attaching heavy-atom labels at chemically defined positions for subsequent X-ray-diffraction analysis and the implications of these experiments for deciphering the folding of the peptide chain in the virus subunit are discussed.