The specificity of carboxypeptidase Y may be altered by changing the hydrophobicity of the S'1 binding pocket.

The S'1 binding pocket of carboxypeptidase Y is hydrophobic, spacious, and open to solvent, and the enzyme exhibits a preference for hydrophobic P'1 amino acid residues. Leu272 and Ser297, situated at the rim of the pocket, and Leu267, slightly further away, have been substituted by site-directed mutagenesis. The mutant enzymes have been characterized kinetically with respect to their P'1 substrate preferences using the substrate series FA-Ala-Xaa-OH (Xaa = Leu, Glu, Lys, or Arg) and FA-Phe-Xaa-OH (Xaa = Ala, Val, or Leu). The results reveal that hydrophobic P'1 residues bind in the vicinity of residue 272 while positively charged P'1 residues interact with Ser297. Introduction of Asp or Glu at position 267 greatly reduced the activity toward hydrophobic P'1 residues (Leu) and increased the activity two- to three-fold for the hydrolysis of substrates with Lys or Arg in P'1. Negatively charged substituents at position 272 reduced the activity toward hydrophobic P'1 residues even more, but without increasing the activity toward positively charged P'1 residues. The mutant enzyme L267D + L272D was found to have a preference for substrates with C-terminal basic amino acid residues. The opposite situation, where the positively charged Lys or Arg were introduced at one of the positions 267, 272, or 297, did not increase the rather low activity toward substrates with Glu in the P'1 position but greatly reduced the activity toward substrates with C-terminal Lys or Arg due to electrostatic repulsion. The characterized mutant enzymes exhibit various specificities, which may be useful in C-terminal amino acid sequence determinations.     

Protein engineering of the high-alkaline serine protease PB92 from Bacillus alcalophilus: functional and structural consequences of mutation at the S4 substrate binding pocket.

Serine endoproteases such as trypsins and subtilisins are known to have an extended substrate binding region that interacts with residues P6 to P3' of a substrate. In order to investigate the structural and functional effects of replacing residues at the S4 substrate binding pocket, the serine protease from the alkalophilic Bacillus strain PB92, which shows homology with the subtilisins, was mutated at positions 102 and 126-128. Substitution of Val102 by Trp results in a 12-fold increase in activity towards succinyl-L-Ala-L-Ala-L-Pro-L-Phe-p-nitroanilide (sAAPFpNA). An X-ray structure analysis of the V102W mutant shows that the Trp side chain occupies a hydrophobic pocket at the surface of the molecule leaving a narrow crevice for the P4 residue of a substrate. Better binding of sAAPFpNA by the mutant compared with the wild type protein as indicated by the kinetic data might be due to the hydrophobic interaction of Ala P4 of the substrate with the introduced Trp102 side chain. The observed difference in binding of sAAPFpNA by protease PB92 and thermitase, both of which possess a Trp at position 102, is probably related to the amino acid substitutions at positions 105 and 126 (in the protease PB92 numbering). Kinetic data for the variants obtained by random mutation of residues Ser126, Pro127 and Ser128 reveal that the activity towards sAAPFpNA increases when a hydrophobic residue is introduced at position 126.(ABSTRACT TRUNCATED AT 250 WORDS)     

A site-directed mutagenesis study on the role of isoleucine-23 of human epidermal growth factor in the receptor binding.

The isoleucine-23 residue of human epidermal growth factor (hEGF) was substituted by a variety of amino acid residues and the receptor-binding activities of variant hEGFs were determined by the use of human KB cell. Tight receptor binding was found of variants with hydrophobic amino acid residues in position 23. The size of the isoleucine residue was nearly optimum for the receptor binding as compared with other hydrophobic residues. The structure analysis by two-dimensional nuclear magnetic resonance spectroscopy showed that the substitution at position 23 only slightly affected the tertiary structure of hEGF. These indicate that the side chain of isoleucine residue in position 23, which is exposed on the protein surface, directly binds to a hydrophobic pocket of the receptor.     

Caspase-3 binds diverse P4 residues in peptides as revealed by crystallography and structural modeling

Caspase-3 recognition of various P4 residues in its numerous protein substrates was investigated by crystallography, kinetics, and calculations on model complexes. Asp is the most frequent P4 residue in peptide substrates, although a wide variety of P4 residues are found in the cellular proteins cleaved by caspase-3. The binding of peptidic inhibitors with hydrophobic P4 residues, or no P4 residue, is illustrated by crystal structures of caspase-3 complexes with Ac-IEPD-Cho, Ac-WEHD-Cho, Ac-YVAD-Cho, and Boc-D(OMe)-Fmk at resolutions of 1.9–2.6 Å. The P4 residues formed favorable hydrophobic interactions in two separate hydrophobic regions of the binding site. The side chains of P4 Ile and Tyr form hydrophobic interactions with caspase-3 residues Trp206 and Trp214 within a non-polar pocket of the S4 subsite, while P4 Trp interacts with Phe250 and Phe252 that can also form the S5 subsite. These interactions of hydrophobic P4 residues are distinct from those for polar P4 Asp, which indicates the adaptability of caspase-3 for binding diverse P4 residues. The predicted trends in peptide binding from molecular models had high correlation with experimental values for peptide inhibitors. Analysis of structural models for the binding of 20 different amino acids at P4 in the aldehyde peptide Ac-XEVD-Cho suggested that the majority of hydrophilic P4 residues interact with Phe250, while hydrophobic residues interact with Trp206, Phe250, and Trp214. Overall, the S4 pocket of caspase-3 exhibits flexible adaptation for different residues and the new structures and models, especially for hydrophobic P4 residues, will be helpful for the design of caspase-3 based drugs.     

Caspase-8 specificity probed at subsite S(4): crystal structure of the caspase-8-Z-DEVD-cho complex

Caspase-8 is an initiator enzyme in the Fas-mediated pathway of which the downstream executioner caspase-3 is a physiological target. Caspases are cysteine proteases that are specific for substrates with an aspartic acid residue at the P(1) position and have an optimal recognition motif that incorporates four amino acid residues N-terminal to the cleavage site. Caspase-8 has been classified as a group III caspase member because it shows a preference for a small hydrophobic residue at the P(4) substrate position. We report the X-ray crystallographic structure of caspase-8 in complex with benzyloxycarbonyl-Asp-Glu-Val-Asp-aldehyde (Z-DEVD), a specific group II caspase inhibitor. The structure shows that the inhibitor interacts favourably with the enzyme in subsite S(4). Kinetic data reveal that Z-DEVD (K(i) 2 nM) is an almost equally potent inhibitor of caspase-8 as the specific group III inhibitor Boc-IETD-aldehyde (K(i) 1 nM). In view of this finding, the original classification of caspases into three specificity groups needs to be modified, at least for caspase-8, which tolerates small hydrophobic residues as well as the acidic residue Asp in subsite S(4). We propose that the subsite S(3) must be considered as an important specificity-determining factor.     

Comparative molecular model building of two serine proteinases from cytotoxic T lymphocytes

Two genes that are expressed when precursor cytotoxic T lymphocytes are transformed to T killer cells have been cloned and sequenced. The derived amino acid sequences, coding for cytotoxic cell protease 1 (CCP1) and Hannuka factor (HF) are highly homologous to members of the serine proteinase family. Comparative molecular model building using the known three-dimensional structures and the derived amino acid sequences of the lymphocyte enzymes has provided useful structural information, especially in predicting the conformations of the substrate binding sites. In applying this modelling procedure, we used the X-ray structures of four serine proteinases to provide a structurally based sequence alignment: alpha-chymotrypsin (CHT), bovine trypsin (BT), Streptomyces griseus trypsin (SGT), and rat mast cell protease 2 (RMCP2). The root mean square differences in alpha-carbon atom positions among these four structures when compared in a pairwise fashion range from 0.79 to 0.97 A for structurally equivalent residues. The sequences of the two lymphocyte enzymes were then aligned to these proteinases using chemical criteria and the superimposed X-ray structures as guides. The alignment showed that the sequence of CCP1 was most similar to RMCP2, whereas HF has regions of homology with both RMCP2 and BT. With RMCP2 as a template for CCP1 and the two enzymes RMCP2 and BT as templates for HF, the molecular models were constructed. Intramolecular steric clashes that resulted from the replacement of amino acid side chains of the templates by the aligned residues of CCP1 and HF were relieved by adjustment of the side chain conformational angles in an interactive computer graphics device. This process was followed by energy minimization of the enzyme model to optimize the stereochemical geometry and to relieve any remaining unacceptably close nonbonded contacts. The resulting model of CCP1 has an arginine residue at position 226 in the specificity pocket, thereby predicting a substrate preference for P1 aspartate or glutamate residues. The model also predicts favorable binding for a small hydrophobic residue at the P2 position of the substrate. The primary specificity pocket of HF resembles that of BT and therefore predicts a lysine or arginine preference for the P1 residue. The arginine at position 99 in the model of HF suggests a preference for aspartate or glutamate side chains in the P2 position of the substrate. Both CCP1 and HF have a free cysteine in the segment of polypeptide 88 to 93.(ABSTRACT TRUNCATED AT 400 WORDS)     

Kinetic and modeling studies of S3-S3' subsites of HIV proteinases.

Kinetic analysis and modeling studies of HIV-1 and HIV-2 proteinases were carried out using the oligopeptide substrate [formula: see text] and its analogs containing single amino acid substitutions in P3-P3' positions. The two proteinases acted similarly on the substrates except those having certain hydrophobic amino acids at P2, P1, P2', and P3' positions (Ala, Leu, Met, Phe). Various amino acids seemed to be acceptable at P3 and P3' positions, while the P2 and P2' positions seemed to be more restrictive. Polar uncharged residues resulted in relatively good binding at P3 and P2 positions, while at P2' and P3' positions they gave very high Km values, indicating substantial differences in the respective S and S' subsites of the enzyme. Lys prevented substrate hydrolysis at any of the P2-P2' positions. The large differences for subsite preference at P2 and P2' positions seem to be at least partially due to the different internal interactions of P2 residue with P1', and P2' residue with P1. As expected on the basis of amino acid frequency in the naturally occurring cleavage sites, hydrophobic residues at P1 position resulted in cleavable peptides, while polar and beta-branched amino acids prevented hydrolysis. On the other hand, changing the P1' Pro to other amino acids prevented substrate hydrolysis, even if the substituted amino acid had produced a good substrate in other oligopeptides representing naturally occurring cleavage sites. The results suggest that the subsite specificity of the HIV proteinases may strongly depend on the sequence context of the substrate.     

Crystal structures of branched-chain amino acid aminotransferase complexed with glutamate and glutarate: true reaction intermediate and double substrate recognition of the enzyme

Branched-chain amino acid aminotransferase (BCAT), which has pyridoxal 5'-phosphate as a cofactor, is a key enzyme in the biosynthetic pathway of hydrophobic amino acids (leucine, isoleucine, and valine). The enzyme reversibly catalyzes the transfer of the amino group of a hydrophobic amino acid to 2-oxoglutarate to form a 2-oxo acid and glutamate. Therefore, the active site of BCAT should have a mechanism to enable recognition of an acidic amino acid as well as a hydrophobic amino acid (double substrate recognition). The three-dimensional structures of Escherichia coli BCAT (eBCAT) in complex with the acidic substrate (glutamate) and the acidic substrate analogue (glutarate) have been determined by X-ray diffraction at 1.82 and 2.15 A resolution, respectively. The enzyme is a homo hexamer, with the polypeptide chain of the subunit folded into small and large domains, and an interdomain loop. The eBCAT in complex with the natural substrate, glutamate, was assigned as a ketimine as the most probable form based upon absorption spectra of the crystal complex and the shape of the residual electron density corresponding to the cofactor-glutamate bond structure. Upon binding of an acidic substrate, the interdomain loop approaches the substrate to shield it from the solvent region, as observed in the complex with a hydrophobic substrate. Both the acidic and the hydrophobic side chains of the substrates are bound to almost the same position in the pocket of the enzyme and are identical in structure. The inner side of the pocket is mostly hydrophobic to accommodate the hydrophobic side chain but has four sites to coordinate with the gamma-carboxylate of glutamate. The mechanism for the double substrate recognition observed in eBCAT is in contrast to those in aromatic amino acid and histidinol-phosphate aminotransferases. In an aromatic amino acid aminotransferase, the acidic side chain is located at the same position as that for the aromatic side chain because of large-scale rearrangements of the hydrogen bond network. In the histidinol-phosphate aminotransferase, the acidic and basic side chains are located at different sites and interact with different residues of the disordered loop.     

Nucleophile specificity of subtilisin in an organic solvent with low water content: investigation via acyl transfer reactions

Nucleophile specificity of subtilisin (subtilopeptidase A) was studied via acyl transfer reactions in acetonitrile containing piperidine and 10 vol% of water. Ac-Tyr-OEt was used as acyl donor and a series of amino acid derivatives, di- and tripeptides of the general structure Xaa-Gly, Gly-Xaa, Gly-Gly-Xaa (Xaa represents all natural L-amino acids except cysteine) were used as nucleophiles. The nucleophilic efficiencies of these peptides were characterized by the values of the apparent partition constants, p(app), determined from the HPLC analysis of the reactions. The order of preference for the P'(1) position was estimated to be: Gly > hydrophilic, positively charged > hydrophobic, aromatic > negatively charged > Leu > Pro side chain. For the P'(2) position the order of preference was: Gly > hydrophilic, charged > hydrophobic, aromatic > Pro side chain. The values of p(app) for Gly-Gly-Xaa tripeptides cover a range of only two orders of magnitude, with lower nucleophile efficiency for those with hydrophobic amino acid residues in the P'(3) position. The dipeptide with Pro in P'(1) did not react at all, but a tripeptide having Pro in P'(3) was a very good nucleophile. The negatively charged amino acid residues in the P'(1) position result in very weak nucleophilic behavior, whereas the peptides with Asp or Glu in P'(2) and P'(3) are well accepted. Generally, peptides of the Gly-Xaa or Gly-Gly-Xaa series were better nucleophiles than peptides of the Xaa-Gly series. The length of the peptide chain or amidation of alpha-carboxyl function had no influence on nucleophilic behavior. No significant difference in nucleophile specificity between subtilopeptidase A and nagarse was observed. (c) 1996 John Wiley & Sons, Inc.     

Proline-specific endopeptidases from microbial sources: isolation of an enzyme from a Xanthomonas sp.

An extensive screening among microorganisms for the presence of post-proline-specific endopeptidase activity was performed. This activity was found among ordinary bacteria from soil samples but not among fungi and actinomycetes. This result is in contrast to the previous notion that this activity is confined to the genus Flavobacterium. A proline endopeptidase was isolated from a Xanthomonas sp. and characterized with respect to physicochemical and enzymatic properties. The enzyme is composed of a single peptide chain with a molecular weight of 75,000. The isoelectric point is 6.2. It is inhibited by diisopropylfluorophosphate and may therefore be classified as a serine endopeptidase. The activity profile is bell shaped with an optimum at pH 7.5. By using synthetic peptide substrates and intramolecular fluorescence quenching it was possible to study the influence of substrate structure on the rate of hydrolysis. The enzyme specifically hydrolyzed Pro-X peptide bonds. With Glu at position X, low rates of hydrolysis were observed; otherwise the enzyme exhibited little preference for particular amino acid residues at position X. A similar substrate preference was observed with respect to the amino acid residue preceding the prolyl residue in the substrate. The enzyme required a minimum of two amino acid residues toward the N terminus from the scissile bond, but further elongation of the peptide chain by up to six amino acid residues caused only a threefold increase in the rate of hydrolysis. Attempts to cleave at the prolyl residues in oxidized RNase failed, indicating that the enzyme does not hydrolyze long peptides, a peculiar property it shares with other proline-specific endopeptidases.     

Structural basis for the unusual specificity of Escherichia coli aminopeptidase N

Aminopeptidase N from Escherichia coli is a M1 class aminopeptidase with the active-site region related to that of thermolysin. The enzyme has unusual specificity, cleaving adjacent to the large, nonpolar amino acids Phe and Tyr but also cleaving next to the polar residues Lys and Arg. To try to understand the structural basis for this pattern of hydrolysis, the structure of the enzyme was determined in complex with the amino acids L-arginine, L-lysine, L-phenylalanine, L-tryptophan, and L-tyrosine. These amino acids all bind with their backbone atoms close to the active-site zinc ion and their side chain occupying the S1 subsite. This subsite is in the form of a cylinder, about 10 A in cross-section and 12 A in length. The bottom of the cylinder includes the zinc ion and a number of polar side chains that make multiple hydrogen-bonding and other interactions with the alpha-amino group and the alpha-carboxylate of the bound amino acid. The walls of the S1 cylinder are hydrophobic and accommodate the nonpolar or largely nonpolar side chains of Phe and Tyr. The top of the cylinder is polar in character and includes bound water molecules. The epsilon-amino group of the bound lysine side chain and the guanidinium group of arginine both make multiple hydrogen bonds to this part of the S1 site. At the same time, the hydrocarbon part of the lysine and arginine side chains is accommodated within the nonpolar walls of the S1 cylinder. This combination of hydrophobic and hydrophilic binding surfaces explains the ability of ePepN to cleave Lys, Arg, Phe, and Tyr. Another favored substrate has Ala at the P1 position. The short, nonpolar side chain of this residue can clearly be bound within the hydrophobic part of the S1 cylinder, but the reason for its facile hydrolysis remains uncertain.     

Human liver alanine aminopeptidase. Inhibition by amino acids.

Human liver alanine aminopeptidase is inhibited by L-amino acids having hydrophobic side chains such as Phe, Tyr, Trp, Met, and Leu. Blocking of the amino group or the carboxyl group greatly reduces the inhibitory capacity of the amino acid. Kinetic studies demonstrate that inhibition of hydrolysis of the substrate L-Ala-beta-naphthylamide is of the noncompetitive type. Inhibition of the substrate L-Leu-L-Leu is of the mixed type. Inhibition of the substrate L-Ala-L-Ala-L-Ala is of the competitive type. These changes in the mechanism of inhibition are thought to be the result of the binding of the amino acid to the third residue binding site on the enzyme. This is the part of the active center to which the third residue from the amino end of a peptide substrate is normally bound. The inhibitor constants of several alanine oligopeptides are shown to decrease with increasing length through L-Ala-L-Ala-L-Ala-L-Ala, demonstrating that alanine aminopeptidase is a multisited enzyme with three and possibly four residue sites per active center. The inhibitor constant for Gly-Gly--Phe suggesting that indeed the third residue site preferentially binds large hydrophobic residues.     

Structural basis of plasticity in protein tyrosine phosphatase 1B substrate recognition

Protein tyrosine phosphatase 1B (PTP1B) displays a preference for peptides containing acidic as well as aromatic/aliphatic residues immediately NH(2)-terminal to phosphotyrosine. The structure of PTP1B bound with DADEpYL-NH(2) (EGFR(988)(-)(993)) offers a structural explanation for PTP1B's preference for acidic residues [Jia, Z., Barford, D., Flint, A. J., and Tonks, N. K. (1995) Science 268, 1754-1758]. We report here the crystal structures of PTP1B in complex with Ac-ELEFpYMDYE-NH(2) (PTP1B.Con) and Ac-DAD(Bpa)pYLIPQQG (PTP1B.Bpa) determined to 1.8 and 1.9 A resolution, respectively. A structural analysis of PTP1B.Con and PTP1B.Bpa shows how aromatic/aliphatic residues at the -1 and -3 positions of peptide substrates are accommodated by PTP1B. A comparison of the structures of PTP1B.Con and PTP1B.Bpa with that of PTP1B.EGFR(988)(-)(993) reveals the structural basis for the plasticity of PTP1B substrate recognition. PTP1B is able to bind phosphopeptides by utilizing common interactions involving the aromatic ring and phosphate moiety of phosphotyrosine itself, two conserved hydrogen bonds between the Asp48 carboxylate side chain and the main chain nitrogens of the pTyr and residue 1, and a third between the main chain nitrogen of Arg47 and the main chain carbonyl of residue -2. The ability of PTP1B to accommodate both acidic and hydrophobic residues immediately NH(2)-terminal to pTyr appears to be conferred upon PTP1B by a single residue, Arg47. Depending on the nature of the NH(2)-terminal amino acids, the side chain of Arg47 can adopt one of two different conformations, generating two sets of distinct peptide binding surfaces. When an acidic residue is positioned at position -1, a preference for a second acidic residue is also observed at position -2. However, when a large hydrophobic group occupies position -1, Arg47 adopts a new conformation so that it can participate in hydrophobic interactions with both positions -1 and -3.     

A novel approach to the characterization of substrate specificity in short/branched chain Acyl-CoA dehydrogenase

Rat and human short/branched chain acyl-CoA dehydrogenases exhibit key differences in substrate specificity despite an overall amino acid identity of 85% between them. Rat short/branched chain acyl-CoA dehydrogenases (SBCAD) are more active toward substrates with longer carbon side chains than human SBCAD, whereas the human enzyme utilizes substrates with longer primary carbon chains. The mechanism underlying this difference in substrate specificity was investigated with a novel surface plasmon resonance assay combined with absorbance and circular dichroism spectroscopy, and kinetics analysis of wild type SBCADs and mutants with altered amino acid residues in the substrate binding pocket. Results show that a relatively few amino acid residues are critical for determining the difference in substrate specificity seen between the human and rat enzymes and that alteration of these residues influences different portions of the enzyme mechanism. Molecular modeling of the SBCAD structure suggests that position 104 at the bottom of the substrate binding pocket is important in determining the length of the primary carbon chain that can be accommodated. Conformational changes caused by alteration of residues at positions 105 and 177 directly affect the rate of electron transfer in the dehydrogenation reactions, and are likely transmitted from the bottom of the substrate binding pocket to beta-sheet 3. Differences between the rat and human enzyme at positions 383, 222, and 220 alter substrate specificity without affecting substrate binding. Modeling predicts that these residues combine to determine the distance between the flavin ring of FAD and the catalytic base, without changing the opening of the substrate binding pocket.     

Structure and dynamics of the fatty acid binding cavity in apo rat intestinal fatty acid binding protein.

The structure and dynamics of the fatty acid binding cavity in I-FABP (rat intestinal fatty acid binding protein) were analyzed. In the crystal structure of apo I-FABP, the probe occupied cavity volume and surface are 539+/-8 A3 and 428 A2, respectively (1.4 A probe). A total of 31 residues contact the cavity with their side chains. The side-chain cavity surface is partitioned according to the residue type as follows: 36-39% hydrophobic, 21-25% hydrophilic, and 37-43% neutral or ambivalent. Thus, the cavity surface is neither like a typical protein interior core, nor is like a typical protein external surface. All hydrophilic residues that contact the cavity-with the exception of Asp74-are clustered on the one side of the cavity. The cavity appears to expand its hydrophobic surface upon fatty acid binding on the side opposite to this hydrophilic patch. In holo I-FABP the fatty acid chain interactions with the hydrophilic side chains are mediated by water molecules. Molecular dynamics (MD) simulation of fully solvated apo I-FABP showed global conformational changes of I-FABP, which resulted in a large, but seemingly transient, exposure of the cavity to the external solvent. The packing density of the side chains lining the cavity, studied by Voronoi volumes, showed the presence of two distinctive small hydrophobic cores. The MD simulation predicts significant structural perturbations of the cavity on the subnanosecond time scale, which are capable of facilitating exchange of I-FABP internal water.     

Mutagenesis of rat acyl-CoA synthetase 4 indicates amino acids that contribute to fatty acid binding

Although each of the five mammalian long-chain acyl-CoA synthetases (ACSL) can bind saturated and unsaturated fatty acids ranging from 12 to 22 carbons, ACSL4 prefers longer chain polyunsaturated fatty acids. In order to gain a better understanding of ACSL4 fatty acid binding, we based a mutagenesis approach on sequence alignments related to ttLC-FACS crystallized from Thermus thermophilus HB8. Four residues selected for mutagenesis corresponded to residues in ttLC-FACS that comprise the fatty acid binding pocket; the fifth residue aligned with a region thought to be involved in fatty acid selectivity of the Escherichia coli acyl-CoA synthetase, FadD. Changing an amino acid at the entry of the putative fatty acid binding pocket, G401L, resulted in an inactive enzyme. Mutating a residue near the pocket entry, L399M, did not significantly alter enzyme activity, but mutating a residue at the hydrophobic terminus of the pocket, S291Y, altered ACSL4's preference for 20:5 and 22:6 and increased its apparent K(m) for ATP. Mutating a site in a region previously identified as important for fatty acid binding also altered activation of 20:4 and 20:5. These studies suggested that the preference of ACSL4 for long-chain polyunsaturated fatty acids can be modified by altering specific amino acid residues.     

Enhanced bacteriolytic activity of hen egg-white lysozyme due to conversion of Trp62 to other aromatic amino acid residues

Tryptophan at the 62nd position (Trp62) of hen egg-white lysozyme is an amino acid residue whose action is essential for its enzymatic activity. Its indole ring may possibly come into direct contact with sugar residues of the substrate, and thus contribute significantly to substrate binding. For further elucidation of its role in catalytic processes, this amino acid was converted to other aromatic residues, such as Tyr, Phe, and His, by site-directed mutagenesis. All the mutations were found to enhance the bacteriolytic activity but to decrease the hydrolytic activity toward an artificial substrate, glycol chitin. Such a change in substrate preference appears remarkable considering the smaller size of the aromatic residue on the mutant enzyme at the 62nd position.     

Solution structure of a platelet receptor peptide bound to bovine alpha-thrombin.

NMR experiments were carried out to study the interaction of thrombin with a synthetic peptide, ESKATNATLDPR, derived from the newly-identified platelet receptor for thrombin [Vu, T.-K. H., Hung, D. T., Wheaton, V. I., & Coughlin, S. R. (1991) Cell 64, 1057-1068]. On the basis of the observation of the thrombin-induced line broadening and transferred NOEs, binding of the peptide was found to be located exclusively within residues LDPR of the proteolytic cleavage site LDPR/S essential for receptor activation by thrombin. Measurement of transferred NOEs and molecular modeling indicate that the side chain of the Asp(P3) residue may form a hydrogen bond with thrombin and, by doing so, it is brought near a positively-charged thrombin residue Arg(221A), thereby partially neutralizing the negative charge of an Asp residue at this site of protein substrates. The hydrophobic side chains of residues Leu(P4) and Pro(P2) reside on the same side of the peptide backbone as indicated by transferred NOEs and were found by modeling to fit into a hydrophobic cage around the thrombin active site. These results suggest that the interaction of thrombin with protein substrates such as prothrombin, protein C, protein S, the platelet receptor, and the A alpha- and B beta-chains of fibrinogen all follow the same canonical binding mode in that the substrate forms an antiparallel beta-strand with thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Solution structure of BmKK4, the first member of subfamily alpha-KTx 17 of scorpion toxins

BmKK4 is a 30 amino acid peptide purified from the venom of the Chinese scorpion Buthus martensi Karsch. It has been classified as the first member of scorpion toxin subfamily alpha-KTx 17. The 3D structure of BmKK4 in solution has been determined by 2D NMR spectroscopy. This toxin adopts a common alpha/beta-motif, but shows a distinctive local conformation. The most novel feature is that the regular arrangements of the side chains of the residues involved in the beta-sheet of BmKK4 are distorted by a classic beta-bulge structure, which involves two residues (Asp18 and Arg19) in the first strand opposite a single residue (Tyr26) in the second strand. The bulge produces two main changes in the structure of the antiparallel beta-sheet: (1) It disrupts the normal alteration of the side chain direction; the side chain of Asp18 turns over to form a salt bridge with that of Arg19. (2) It accentuates the twist of the sheet, and alters the direction of the antiparallel beta-sheet. The unusual structural feature of the toxin is attributed to the shorter peptide segment (Leu15-Arg19) between the third and fourth Cys residues and two unique residues (Asp18 and Arg19) at the position preceding the fourth Cys. In addition, the lower affinity of the peptide for the Kv channel is correlated to the structural features: residue Arg19 instead of a Lys residue at the critical position for binding and the salt bridge formed between residues Arg19 and Asp18.     

The molecular structure of hyperthermostable aromatic aminotransferase with novel substrate specificity from Pyrococcus horikoshii

Aromatic amino acid aminotransferase (ArATPh), which has a melting temperature of 120 degrees C, is one of the most thermostable aminotransferases yet to be discovered. The crystal structure of this aminotransferase from the hyperthermophilic archaeon Pyrococcus horikoshii was determined to a resolution of 2.1 A. ArATPh has a homodimer structure in which each subunit is composed of two domains, in a manner similar to other well characterized aminotransferases. By the least square fit after superposing on a mesophilic ArAT, the ArATPh molecule exhibits a large deviation of the main chain coordinates, three shortened alpha-helices, an elongated loop connecting two domains, and a long loop transformed from an alpha-helix, which are all factors that are likely to contribute to its hyperthermostability. The pyridine ring of the cofactor pyridoxal 5'-phosphate covalently binding to Lys(233) is stacked parallel to F121 on one side and interacts with the geminal dimethyl-CH/pi groups of Val(201) on the other side. This tight stacking against the pyridine ring probably contributes to the hyperthermostability of ArATPh. Compared with other ArATs, ArATPh has a novel substrate specificity, the order of preference being Tyr > Phe > Glu > Trp > His> Met > Leu > Asp > Asn. Its relatively weak activity against Asp is due to lack of an arginine residue corresponding to Arg(292)* (where the asterisk indicates that this is a residues supplied by the other subunit of the dimer) in pig cytosolic aspartate aminotransferase. The enzyme recognizes the aromatic substrate by hydrophobic interaction with aromatic rings (Phe(121) and Tyr(59)*) and probably recognizes acidic substrates by a hydrophilic interaction involving a hydrogen bond network with Thr(264)*.