Substrate specificity of carboxypeptidase from Watermelon.

The substrate specificity of carboxypeptidase (F-II) purified from watermelon for various synthetic peptides and esters was examined kinetically. The enzyme showed a broad substrate specificity against various carbobenzoxy- and benzyl-dipeptides. Peptides containing glycine or proline were hydrolyzed slowly by the enzyme. Peptides containing hydrophobic amino acids were hydrolyzed rapidly. The presence of hydrophobic amino acid residues, not only at the C-terminal position but also at the second position and probably the third position from the C-terminal resulted in an increase in the rate of hydrolysis. Inhibition studies with diisopropyl flurophosphate and diastereomers of carbobenzoxy-Phe-Ala demonstrated that the peptidase and esterase activities of the enzyme are both catalyzed by the same site of the enzyme molecule, but the binding sites for peptides and esters seem not to be the same. The enzyme also had amidase activity, which was optimal at pH 7.0.     

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.     

Enzyme-substrate interactions in the hydrolysis of peptide substrates by thermitase, subtilisin BPN', and proteinase K

Peptide substrates of the general structure acetyl-Alan (n = 2-5), acetyl-Pro-Ala-Pro-Phe-Alan-NH2 (n = 0-3), and acetyl-Pro-Ala-Pro-Phe-AA-NH2 (AA = various amino acids) were synthesized and used to investigate the enzyme-substrate interactions of the microbial serine proteases thermitase, subtilisin BPN', and proteinase K on the C-terminal side of the scissile bond. The elongation of the substrate peptide chain up to the second amino acid on the C-terminal side (P'2) enhances the hydrolysis rate of thermitase and subtilisin BPN', whereas for proteinase K an additional interaction with the third amino acid (P'3) is possible. The enzyme subsite S'1 specificity of the proteases investigated is very similar. With respect to kcat/Km values small amino acid residues such as Ala and Gly are favored in this position. Bulky residues such as Phe and Leu were hydrolyzed to a lower extent. Proline in P'1 abolishes the hydrolysis of the substrates. Enzyme-substrate interactions on the C-terminal side of the scissile bond appear to affect kcat more than Km for all three enzymes.     

Proteinase Enzyme System of Lactic Streptococci III. Substrate Specificity of Streptococcus lactis Intracellular Proteinase

The substrate specificity of an intracellular proteinase from Streptococcus lactis was investigated in an effort to understand the role of the enzyme in the cell. Peptides in which the N-terminal residue was glycine were not hydrolyzed by the enzyme (exceptions were glycyl-alanine, glycyl-aspartic acid, and glycyl-asparagine), but the peptide was hydrolyzed if the N-terminal residue was alanine. The enzyme also showed activity toward peptides containing aspartic acid or asparagine. Hydrolysis of only the peptide bonds of alanyl, aspartyl, or asparaginyl residues was confirmed by the action of the enzyme on oxidized bovine ribonuclease A- and B- chain insulin. The N-terminal residues of the peptide fragments liberated were identified. The enzyme attacked both substrates only at alanyl, aspartyl, and asparaginyl residues, releasing these as free amino acids. In addition to alanine, aspartic acid, and asparagine, certain other amino acids were liberated from ribonuclease A, but these were accounted for by the relation of their position to alanine, aspartic acid, and asparagine residues.     

A Novel Type of Substrate Specificity of Rice BAPAase (Benzoyl-L-arginine p-nitroanilide Hydrolase) with Mixed Endopeptidase and Carboxypeptidase

The substrate specificity of rice embryo benzoyl-L-argininep-nitroanilide hydrolase (BAPAase) was examined. No endopeptidaseactivity toward protein substrates was detectable. Small peptides(less than 8 residues) and amide, ester substrates, however,were hydrolyzed very well at the carboxyl side of the lysineor arginine residue. No other peptide bond was hydrolyzed. TheN-terminal arginine of the substrates was released very slowly.Peptides with lysine or arginine penultimate to the C-terminalposition were hydrolyzed well and released an amino acid. Theoxidized insulin B chain (30 residues) was cleaved very slowlyat the C-terminal Lys-Ala bond, whereas an Arg-Gly bond at aninner position was not cleaved. The hydrolytic rate increasedafter the chain length was shortened by chymotryptic digestion.These results show that the rice embryo BAPAase is a novel enzymewhich has mixed endopeptidase-carboxypeptidase activity towardthe Arg-X and Lys-X bonds of small peptides, a characteristicintermediate between trypsin and serine carboxypeptidase. Thisenzyme may act in the breakdown of small peptides that havephysiological functions. (Received May 26, 1984; Accepted August 29, 1984)     

Cleavage specificity of cucumisin, a serine protease, with synthetic substrates

The substrate specificity of a plant serine protease, cucumisin (EC 3.4.21.25), was studied by the use of synthetic oligopeptides and peptidyl-pNA substrates. Since P1'-Ser, Ala, and Gly substrates were hydrolyzed rapidly, cucumisin appears to prefer a small side chain at the P1' position of the oligopeptide substrate. The k(cat)/Km for the hydrolysis of P1-Leu, Ala, Phe, and Glu substrates demonstrated that they were preferentially cleaved over P1-Lys, diaminopropionic acid (Dap), Gly, Val, and Pro substrates. From the digestion of peptidyl-pNAs, the specificity of the protease was determined to be broad, but the preferential cleavage sites were hydrophobic amino acid residues at the P1 position.     

Bacterial beta-peptidyl aminopeptidases with unique substrate specificities for beta-oligopeptides and mixed beta,alpha-oligopeptides

We previously discovered that BapA, a bacterial beta-peptidyl aminopeptidase, is able to hydrolyze two otherwise metabolically inert beta-peptides [Geueke B, Namoto K, Seebach D and Kohler H-PE (2005) J Bacteriol 187, 5910-5917]. Here, we describe the purification and characterization of two distinct bacterial beta-peptidyl aminopeptidases that originated from different environmental isolates. Both bapA genes encode a preprotein with a signal sequence and were flanked by ORFs that code for enzymes with similar predicted functions. To form the active enzymes, which had an (alphabeta)(4) quaternary structure, the preproteins needed to be cleaved into two subunits. The two beta-peptidyl aminopeptidases had 86% amino acid sequence identity, hydrolyzed a variety of beta-peptides and mixed beta/alpha-peptides, and exhibited unique substrate specificities. The prerequisite for peptides being accepted as substrates was the presence of a beta-amino acid at the N-terminus; peptide substrates with an N-terminal alpha-amino acid were not hydrolyzed at all. Both enzymes cleaved the peptide bond between the N-terminal beta-amino acid and the amino acid at the second position of tripeptidic substrates of the general structure H-betahXaa-Ile-betahTyr-OH according to the following preferences with regard to the side chain of the N-terminal beta-amino acid: aliphatic and aromatic > OH-containing > hydrogen, basic and polar. Experiments with the tripeptides H-d-betahVal-Ile-betahTyr-OH and H-betahVal-Ile-betahTyr-OH demonstrated that the two BapA enzymes preferred the peptide with the l-configuration of the N-terminal beta-homovaline residue as a substrate.     

The aglycone specificity-determining sites are different in 2, 4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA)-glucosidase (Maize beta -glucosidase) and dhurrinase (Sorghum beta -glucosidase)

The maize beta-glucosidase isozyme Glu1 hydrolyzes a broad spectrum of substrates in addition to its natural substrate DIMBOAGlc (2-O-beta-d-glucopyranosyl-4-hydroxy-7-methoxy-1,4-benzoxazin-3-on e), whereas the sorghum beta-glucosidase isozyme Dhr1 hydrolyzes exclusively its natural substrate dhurrin (p-hydroxy-(S)-mandelonitrile-beta-d-glucose). To study the mechanism of substrate specificity further, eight chimeric beta-glucosidases were constructed by replacing peptide sequences within the C-terminal region of Glu1 with the homologous peptide sequences of Dhr1 or vice versa, where the two enzymes differ by 4 to 22 amino acid substitutions, depending on the length of the swapped regions. Five Glu1/Dhr1 chimeras hydrolyzed substrates that are hydrolyzed by both parental enzymes, including dhurrin, which is not hydrolyzed by Glu1. In contrast, three Dhr1/Glu1 chimeras hydrolyzed only dhurrin but with lower catalytic efficiency than Dhr1. Additional domain-swapping within the C-terminal domain of Glu1 showed that replacing the peptide (466)FAGFTERY(473) of Glu1 with the homologous peptide (462)SSGYTERF(469) of Dhr1 or replacing the peptide (481)NNNCTRYMKE(490) in Glu1 with the homologous peptide (477)ENGCERTMKR(486) of Dhr1 was sufficient to confer to Glu1 the ability to hydrolyze dhurrin. Data from various reciprocal chimeras, sequence comparisons, and homology modeling suggest that the Dhr1-specific Ser-462-Ser-463 and Phe-469 play a key role in dhurrin hydrolysis. Similar data suggest that DIMBOAGlc hydrolysis determinants are not located within the extreme 47-amino acid-long C-terminal domain of Glu1.     

Porcine spleen cathepsin H hydrolyzes oligopeptides solely by aminopeptidase activity

Cathepsin H purified from porcine spleens was studied for its specificity against various peptide and denatured protein substrates. The enzyme degraded all peptide substrates exclusively by an aminopeptidase activity. The enzyme preferentially released NH2-terminal amino acid residues with large hydrophobic (Phe, Trp, Leu, and Tyr) or basic (Arg and Lys) side chains. Amino acids containing small or polar side chains were not released. Peptides with a proline in the NH2-terminal or penultimate positions were not hydrolyzed either. Large polypeptides such as reduced and carboxymethylated soybean trypsin inhibitor and aldolase were not degraded. These results indicate that cathepsin H is an exopeptidase but not an endopeptidase. We propose that the biological role of this enzyme is the degradation of tissue proteins in lysosomes by its aminopeptidase activity.     

Neprilysin carboxydipeptidase specificity studies and improvement in its detection with fluorescence energy transfer peptides

We examined the substrate specificity of the carboxydipeptidase activity of neprilysin (NEP) using fluorescence resonance energy transfer (FRET) peptides containing ortho-aminobenzoyl (Abz) and 2,4-dinitrophenyl (Dnp) as a donor/acceptor pair. Two peptide series with general sequences Abz-RXFK(Dnp)-OH and Abz-XRFK(Dnp)-OH (X denotes the position of the altered amino acid) were synthesized to study P1 (cleavage at the X-F bond) and P2 (cleavage at R-F bond) specificity, respectively. In these peptides a Phe residue was fixed in P1' to fulfill the well-known NEP S1' site requirement for a hydrophobic amino acid. In addition, we explored NEP capability to hydrolyze bradykinin (RPPGFSPFR) and its fluorescent derivative Abz-RPPGFSPFRQ-EDDnp (EDDnp=2,4-dinitrophenyl ethylenediamine). The enzyme acts upon bradykinin mainly as a carboxydipeptidase, preferentially cleaving Pro-Phe over the Gly-Phe bond in a 9:1 ratio, whereas Abz-RPPGFSPFRQ-EDDnp was hydrolyzed at the same bonds but at an inverted proportion of 1:9. The results show very efficient interaction of the substrates' C-terminal free carboxyl group with site S2' of NEP, confirming the enzyme's preference to act as carboxydipeptidase at substrates with a free carboxyl-terminus. Using data gathered from our study, we developed sensitive and selective NEP substrates that permit continuous measurement of the enzyme activity, even in crude tissue extracts.     

Purification and characterization of two serine carboxypeptidases from Aspergillus niger and their use in C-terminal sequencing of proteins and peptide synthesis.

A procedure was developed to prepare in large amounts two carboxypeptidases, CPD-I and CPD-II, from Aspergillus niger. They were each shown to be serine proteases and single-chain monomers with molecular masses of ca. 81 kDa and containing 22% carbohydrates. Amino acid analysis, carbohydrate determination, and N-terminal sequencing (20 to 25 residues) were performed on each enzyme. CPD-I showed sequence homologies with malt carboxypeptidase II, while the N terminus of CPD-II was different from that of any known serine carboxypeptidase. Like carboxypeptidase Y from Saccharomyces cerevisiae and carboxypeptidase III from malt, CPD-II contained a free sulfhydryl group that could play a role in catalysis. Both A. niger enzymes had pH optima of about 4 and were unstable above pH 7. Their specificities for substrate positions P1 and P'1 were characterized by use of, as substrates, a series of N-blocked amino acid esters and dipeptides. Both enzymes were specific for Arg, Lys, and Phe in P1. CPD-I preferred hydrophobic residues in P'1, while CPD-II was highly specific for Arg and Lys in this position. Each displayed an original specificity when P1 and P'1 were considered together. The specificities were also studied by analyzing the time course of the release of amino acids from eight different peptides of various lengths. CPD-I and CPD-II appeared to be quite suitable for C-terminal sequence studies as well as for the synthesis of peptide bonds. The latter was studied with two peptide esters as aminolysis substrates and a series of amino acid amides as nucleophiles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human leukocyte cathepsin G. Subsite mapping with 4-nitroanilides, chemical modification, and effect of possible cofactors

The extended substrate binding site of cathepsin G from human leukocytes has been mapped by using a series of peptide 4-nitroanilide substrates. The enzyme has a significant preference for substrates with a P1 Phe over those with the other aromatic amino acids Tyr and Trp. The S2 subsite was mapped with the substrates Suc-Phe-AA-Phe-NA where AA was 13 of the 20 amino acid residues commonly found in proteins. The best residues were Pro and Met. The S3 subsite was mapped with the sequence Suc-AA-Pro-Phe-NA by using 14 different amino acid residues for AA. The two best residues were the isosteric Val and Thr. No significant improvement in reactivity was obtained by extending the substrate to include seven different P4 residues. The kinetic parameters for cathepsin G are significantly slower than those for many other serine proteases. Changes in the reaction conditions and addition of possible cofactors or ligands were in general found to have little effect on the enzymatic activity, while chemical modifications and proteolysis destroyed the activity of cathepsin G. Cathepsin G hydrolyzed peptides containing model desmosine residues and prefers the hydrophobic picolinoyllysine derivative over lysine by substantial margins at both the S4 and S2 subsites but will not tolerate it at S3. Substrates with sequences related to the cathepsin G cleavage site in angiotensin I and angiotensinogen, and the reactive site of alpha 1-antichymotrypsin, were hydrolyzed effectively by enzyme, but with unexceptional rates. Our results indicate that the natural substrate(s) and function(s) of cathepsin G still remain to be discovered.     

An amino acid at position 142 in nitrilase from Rhodococcus rhodochrous ATCC 33278 determines the substrate specificity for aliphatic and aromatic nitriles

Nitrilase from Rhodococcus rhodochrous ATCC 33278 hydrolyses both aliphatic and aromatic nitriles. Replacing Tyr-142 in the wild-type enzyme with the aromatic amino acid phenylalanine did not alter specificity for either substrate. However, the mutants containing non-polar aliphatic amino acids (alanine, valine and leucine) at position 142 were specific only for aromatic substrates such as benzonitrile, m-tolunitrile and 2-cyanopyridine, and not for aliphatic substrates. These results suggest that the hydrolysis of substrates probably involves the conjugated pi-electron system of the aromatic ring of substrate or Tyr-142 as an electron acceptor. Moreover, the mutants containing charged amino acids such as aspartate, glutamate, arginine and asparagine at position 142 displayed no activity towards any nitrile, possibly owing to the disruption of hydrophobic interactions with substrates. Thus aromaticity of substrate or amino acid at position 142 in R. rhodochrous nitrilase is required for enzyme activity.     

Purification and characterization of two serine carboxypeptidases from Aspergillus niger and their use in C-terminal sequencing of proteins and peptide synthesis.

A procedure was developed to prepare in large amounts two carboxypeptidases, CPD-I and CPD-II, from Aspergillus niger. They were each shown to be serine proteases and single-chain monomers with molecular masses of ca. 81 kDa and containing 22% carbohydrates. Amino acid analysis, carbohydrate determination, and N-terminal sequencing (20 to 25 residues) were performed on each enzyme. CPD-I showed sequence homologies with malt carboxypeptidase II, while the N terminus of CPD-II was different from that of any known serine carboxypeptidase. Like carboxypeptidase Y from Saccharomyces cerevisiae and carboxypeptidase III from malt, CPD-II contained a free sulfhydryl group that could play a role in catalysis. Both A. niger enzymes had pH optima of about 4 and were unstable above pH 7. Their specificities for substrate positions P1 and P'1 were characterized by use of, as substrates, a series of N-blocked amino acid esters and dipeptides. Both enzymes were specific for Arg, Lys, and Phe in P1. CPD-I preferred hydrophobic residues in P'1, while CPD-II was highly specific for Arg and Lys in this position. Each displayed an original specificity when P1 and P'1 were considered together. The specificities were also studied by analyzing the time course of the release of amino acids from eight different peptides of various lengths. CPD-I and CPD-II appeared to be quite suitable for C-terminal sequence studies as well as for the synthesis of peptide bonds. The latter was studied with two peptide esters as aminolysis substrates and a series of amino acid amides as nucleophiles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biosynthesis of peptide neurotransmitters: studies on the formation of peptide amides

A high proportion of peptide transmitters and peptide hormones terminate their peptide chain in a C-terminal amide group which is essential for their biological activity. The specificity of an enzyme that catalyses the formation of the amide was investigated with the aid of synthetic peptide substrates. With peptides containing l-amino acids the enzyme exhibited an essential requirement for glycine in the C-terminal position; amidation did not take place with peptides that had leucine, alanine, glutamic acid, lysine or N-methylglycine at the C-terminus and a peptide extended by the attachment of lysine to the C-terminal glycine did not act as a substrate. Amidation did occur with a peptide containing C-terminal D-alanine but no reaction was detected with peptides having C-terminal, D-serine or D-leucine. In tripeptides with a neutral amino acid in the penultimate position, amidation, took place readily but the reaction was slower when this position was occupied by an acidic or a basic residue. A series of overlapping peptides with C-terminal glycine, based on partial sequences of calcitonin, underwent amidation at similar rates, indicating that the amidating enzyme recognizes only a limited sequence at the C-terminus of its substrates. The results provide evidence that the amidating enzyme has a highly compact substrate binding site.     

Liver prenylated methylated protein methyl esterase is the same enzyme as Sus scrofa carboxylesterase

The C-terminal --COOH of prenylated proteins is methylated to --COOCH3. The --COOCH3 ester forms are hydrolyzed by prenylated methylated protein methyl esterase (PMPMEase) to the original acid forms. This is the only reversible step of the prenylation pathway. PMPMEase has not been purified and identified and is therefore understudied. Using a prenylated-L-cysteine methyl ester as substrate, PMPMEase was purified to apparent homogeneity from porcine liver supernatant. SDS-PAGE analysis revealed an apparent mass of 57 kDa. Proteomics analyses identified 17 peptides (242 amino acids). A Mascot database search revealed these as portions of the Sus scrofa carboxylesterase, a 62-kDa serine hydrolase with the C-terminal HAEL endoplasmic reticulum-retention signal. It is at least 71% identical to such mammalian carboxylesterases as human carboxylesterase 1 with affinities toward hydrophobic substrates and known to activate prodrugs, metabolize active drugs, as well as detoxify various substances such as cocaine and food-derived esters. The purified enzyme hydrolyzed benzoyl-Gly-farnesyl-L-cysteine methyl ester and hydrocinamoyl farnesyl-L-cysteine methyl ester with Michaelis-Menten constant (K(m)) values of 33 +/- 4 and 25 +/- 4 microM and V(max) values of 4.51 +/- 0.28 and 6.80 +/- 0.51 nmol/min/mg of protein, respectively. It was inhibited by organophosphates, chloromethyl ketones, ebelactone A and B, and phenylmethylsulfonyl fluoride.     

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.     

Role of the hydrophobic segment of diacylglycerol kinase epsilon

Diacylglycerol kinase epsilon (DGKepsilon) is unique among mammalian DGK isoforms in having a segment of hydrophobic amino acids. We have evaluated the contributions of this segment to the membrane interactions and functions of this protein. To test the role of the hydrophobic segment, we have compared the properties of DGKepsilon with those of a truncated form of the protein (DGKDeltaepsilon) lacking the 40 N-terminal amino acids, which includes the hydrophobic segment. The proteins were expressed in COS-7 cells from a gene for human DGKepsilon or from a gene for a truncated form (DGKDeltaepsilon), both of which had a FLAG tag at the amino terminus. Full-length FLAG-DGKepsilon and truncated FLAG-DGKDeltaepsilon were both more specific for 1-stearoyl-2-arachidonoyl-sn-glycerol than for 1,2-dioleoyl-sn-glycerol. 1-Stearoyl-2-linoleoyl-sn-glycerol exhibited intermediate specificity for both forms of the enzyme. The results show that the truncated form of the enzyme maintains substrate specificity for lipids with an arachidonoyl moiety present at the sn-2 position. The truncation increases the catalytic rate constant for all three substrates and may suggest a role in the negative regulation of this enzyme. A full-length DGKepsilon with a C-terminal His tag exhibited substrate specificity similar to that of the other two forms of the enzyme, indicating that the nature and position of the epitope tag did not strongly affect this property. Using an ultracentrifugation floatation assay, we showed that at neutral pH DGKDeltaepsilon is extracted with 1.5 M KCl while DGKepsilon remains essentially fully membrane bound. The full-length protein had a weak tendency to oligomerize in the presence of weak detergents. DGKepsilon was monomeric on SDS-PAGE but exhibited partial dimerization with low concentrations of perfluorooctanoic acid. The major conclusions of this work are that the hydrophobic domain of DGKepsilon does not contribute to substrate specificity but plays a role in permanently sequestering the enzyme to a membrane.     

Human complement proteins D, C2, and B. Active site mapping with peptide thioester substrates

The specificity and reactivity of complement serine proteases D, B, Bb, C2, and C2a were determined using a series of peptide thioester substrates. The rates of thioester hydrolysis were measured using assay mixtures containing the thiol reagent 4,4'-dithiodipyridine at pH 7.5. Each substrate contained a P1 arginine residue, and the effect of various groups and amino acids in the P2, P3, P4, and P5 positions was determined using kcat/Km values to compare reactivities. Among peptide thioesters corresponding to the activation site sequence in B, dipeptide thioesters containing a P2 lysine residue were the best substrates for D. Extending the chain to include a P3 or P4 amino acid resulted in loss of activity, and neither the tripeptide nor the tetrapeptide containing the cleavage sequence of B was hydrolyzed. Overall, D cleaved fewer substrates and was 2-3 orders of magnitude less reactive than C1s against some thioester substrates. C2 and fragment C2a had comparable reactivities and hydrolyzed peptides containing Leu-Ala-Arg and Leu-Gly-Arg, which have the same sequence as the cleavage sites of C3 and C5, respectively. The best substrates for C2 and C2a were Z-Gly-Leu-Ala-Arg-SBzl and Z-Leu-Gly-Leu-Ala-Arg-SBzl, respectively, where Bzl is benzyl. B was the least reactive among these complement enzymes. The best substrate for B was Z-Lys-Arg-SBzl with a kcat/Km value of 1370 M-1 s-1. The catalytic fragment of B, Bb, had higher activity toward these peptide thioester substrates. The best substrate for Bb was Z-Gly-Leu-Ala-Arg-SBzl with a kcat/Km similar to C2a and 10 times higher than the value for B. Both C2a and Bb were considerably more reactive against C3-like than C5-like substrates. Bovine trypsin hydrolyzed thioester substrates with kcat/Km approximately 10(3) higher than the complement enzymes. These thioester substrates for D, B, and C2 should be quite useful in kinetic and active site studies of the purified enzymes.     

Glutathione,S-substituted glutathiones,and leukotriene C4 as substrates for peptidylglycine alpha-amidating monooxygenase

The C-terminal alpha-amide moiety of most peptide hormones arises by the posttranslational cleavage of a glycine-extended precursor in a reaction catalyzed by bifunctional peptidylglycine alpha-amidating monooxygenase (PAM). Glutathione and the S-alkylated glutathiones have a C-terminal glycine and are, thus, potential substrates for PAM. The addition of PAM to glutathione, a series of S-alkylated glutathiones, and leukotriene C(4) results in the consumption of O(2) and the production of the corresponding amidated peptide and glyoxylate. This reaction proceeds in two steps with the intermediate formation of a C-terminal alpha-hydroxyglycine-extended peptide. Amidated glutathione (gammaGlu-Cys-amide) is a relatively poor substrate for glutathione S-transferase with a V/K value that is 1.3% of that for glutathione. Peptide substrates containing a penultimate hydrophobic or sulfur-containing amino acid exhibit the highest (V/K)(app) values for PAM-catalyzed amidation. The S-alkylated glutathiones incorporate both features in the penultimate position with S-decylglutathione having the highest (V/K)(app) of the substrates described in this report.