Cleavage of beta-lactone ring by serine protease. Mechanistic implications

Both enantiomers of 3-benzyl-2-oxetanone (1) were found to be slowly hydrolyzed substrates of alpha-chymotrypsin having k(cat) values of 0.134+/-0.008 and 0.105+/-0.004 min(-1) for (R)-1 and (S)-1, respectively, revealing that alpha-CT is virtually unable to differentiate the enantiomers in the hydrolysis of 1. The initial step to form the acyl-enzyme intermediate by the attack of Ser-195 hydroxyl on the beta-lactone ring at the 2-position in the hydrolysis reaction may not be enzymatically driven, but the relief of high ring strain energy of beta-lactone may constitute a major driving force. The deacylation step is also attenuated, which is possibly due to the hydrogen bond that would be formed between the imidazole nitrogen of His-57 and the hydroxyl group generated during the acylation in the case of (R)-1, but in the alpha-CT catalyzed hydrolysis of (S)-1 the imidazole nitrogen may form a hydrogen bond with the ester carbonyl oxygen.     

Enzyme inhibition by dipeptides containing 2,3-methanophenylalanine, a sterically constrained amino acid

Both isomers of (E)-2,3-methanophenylalanine (^EPhe), a sterically restricted amino acid, were incorporated into peptides in order to examine their possible enzyme inhibitory activity. Both (2R,3S)- and (2S,3R)-^EPhe-Phe(or Leu)-OMe were found to inhibit effectively the hydrolysis of Ac-Tyr-OEt by chymotrypsin in a competitive manner. The ester groups of these dipeptides were quite resistant to chymotrypsin hydrolysis, and the ^EPhe-Phe peptide bond was also entirely stable. The inhibition constant (K_i) of the most potent dipeptide of H-(2R,3S)-^EPhe-Phe-OMe was 0.16 mM at 25°C. The inhibitory action of Phe-containing peptides was found to depend on the configuration of the Phe residue. The electrophilic nature of the cyclopropane ring which is conjugated with both the phenyl ring and the ester carbonyl group appears to be relevant to the inhibitory activity. Fully irreversible inactivation of chymotrypsin was achieved by its incubation with H-(2R,3S)-^EPhe-Leu-OMe. An enzyme carboxylate group is thought to be responsible for nucleophilic attack on the cyclopropane ring leading to irreversible inactivation.     

Generation of a thermostable and denaturant-resistant peptide ligase

The construction and characterization of a novel, thermostable, peptide ligase are described. Three amino acid substitutions were introduced into the secreted bacterial protease Streptomyces griseus protease B (SGPB). Mutations were chosen on the basis of two separate observations: (i) that a single substitution of the nucleophilic serine (S195A) created an enzyme with significant peptide-ligation activity, albeit greatly reduced stability [(2000) Chem. Biol. 7, 163], and (ii) that a pair of substitutions in the substrate-binding pocket (T213L and F228H) greatly increased the thermostability of the wild-type enzyme [(1996) J. Mol. Biol. 257, 233]. The triple mutant, named streptoligase, was found to catalyze peptide ligation (aminolysis of both a thiobenzyl ester and a p-nitroanilide-activated peptide) efficiently in nondenaturing and denaturing conditions including SDS (0.5% w/v) and guanidine hydrochloride (4.0 M). Moreover, streptoligase exhibited a half-live for unfolding of 16.3 min at 55 degrees C in the absence of stabilizing substrates. The fraction of the streptoligase-catalyzed reaction that gave coupled product with the acceptor peptide FAASR-NH(2) was greater for the p-nitroanilide donor (Sc-AAPF-pNA) than for the benzyl thioester substrate (Sc-AAPF-SBn). These observations are consistent with ligation proceeding through an acyl-enzyme intermediate involving histidine-57. In the case of the thioester donor the triple mutant promotes the direct attack of water on the thioester carbonyl carbon, in addition to hydrolysis occurring at the stage of the acyl-enzyme intermediate. The strategy of multiple point mutations outlined in this study may provide a general means of converting enzymes with chymotrypsin-like protein folds into peptide ligases.     

Why does threonine, and not serine, function as the active site nucleophile in proteasomes?

Proteasomes belong to the N-terminal nucleophile group of amidases and function through a novel proteolytic mechanism, in which the hydroxyl group of the N-terminal threonines is the catalytic nucleophile. However, it is unclear why threonine has been conserved in all proteasomal active sites, because its replacement by a serine in proteasomes from the archaeon Thermoplasma acidophilum (T1S mutant) does not alter the rates of hydrolysis of Suc-LLVY-amc (Seemüller, E., Lupas, A., Stock, D., Lowe, J., Huber, R., and Baumeister, W. (1995) Science 268, 579-582) and other standard peptide amide substrates. However, we found that true peptide bonds in decapeptide libraries were cleaved by the T1S mutant 10-fold slower than by wild type (wt) proteasomes. In degrading proteins, the T1S proteasome was 3.5- to 6-fold slower than the wt, and this difference increased when proteolysis was stimulated using the proteasome-activating nucleotidase (PAN) ATPase complex. With mutant proteasomes, peptide bond cleavage appeared to be rate-limiting in protein breakdown, unlike with wt. Surprisingly, a peptide ester was hydrolyzed by both particles much faster than the corresponding amide, and the T1S mutant cleaved it faster than the wt. Moreover, the T1S mutant was inactivated by the ester inhibitor clasto-lactacystin-beta-lactone severalfold faster than the wt, but reacted with nonester irreversible inhibitors at similar rates. T1A and T1C mutants were completely inactive in all these assays. Thus, proteasomes lack additional active sites, and the N-terminal threonine evolved because it allows more efficient protein breakdown than serine.     

Clostridium histolyticum collagenase: development of new thio ester, fluorogenic, and depsipeptide substrates and new inhibitors

A new series of thio ester, depsipeptide, and peptide substrates have been synthesized for the bacterial enzyme Clostridium histolyticum collagenase. The hydrolysis of the depsipeptide substrate was followed on a pH stat, and thio ester hydrolysis was measured by inclusion of the chromogenic thiol reagent 4,4'-dithiopyridine in the assay mixture. The best thio ester substrate, Boc-Abz-Gly-Pro-Leu-SCH2CO-Pro-Nba, had a kcat/KM of 63 000 M-1 s-1, while several shorter thio ester sequences were inactive as substrates. In general, the peptide analogues of all the reactive thio ester substrates were shown to be hydrolyzed 5-10 times faster by collagenase. In one case (Z-Gly-Pro-Leu-Gly-Pro-NH2) where a comparison was made, the peptide substrate was respectively 8- and 106-fold more readily hydrolyzed than the corresponding thio ester and ester substrates. Cleavages of the two fluorescence-quench substrates Abz-Gly-Pro-Leu-Gly-Pro-Nba and Abz-Gly-Pro-Leu-SCH2CO-Pro-Nba could be easily followed fluorogenically since a 5-10-fold increase in fluorescence occurred upon hydrolysis. The fluorescent peptide substrate is the best synthetic substrate known for C. histolyticum collagenase with a kcat/KM value of 490 000 M-1 s-1. A series of new reversible inhibitors were developed by the attachment of zinc ligating groups (hydroxamic acid, carboxymethyl, and thiol) to various peptide sequences specific for C. histolyticum collagenase. The shorter peptides designed to bind to either the P3-P1 or P1'-P3' subsites were poor to moderate inhibitors. The thiol HSCH2CH2CO-Pro-Nba had the lowest K1 (0.02 mM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Origin of rate-acceleration in ester hydrolysis with metalloprotease mimics

Mimics of carboxypeptidase A, a prototypical metalloprotease, were synthesized by linking macrocyclicpolyamines to the primary side of beta-cyclodextrin followed by complexing with Zn(II). These enzyme mimics exhibit saturation kinetics in hydrolysis of p-nitrophenyl acetate (PNPA) and enhance the rate of hydrolysis reaction by almost 300-fold. The effective molarities (EM) of the mimics range from 0.2 to 1.9 M. Origin of the rate acceleration was examined: the reactivity of Zn(II) complexes of [12]aneN3 [12]aneN4, and [14]aneN4 for hydrolyzing PNPA increases with increase in basicity of the zinc bound hydroxides [Zn(II)-OH], yielding a linear Br?nsted plot. Free hydroxide fits well on this plot. A similar plot was obtained with the enzyme mimics. The Br?nsted relationships indicate that the Zn(II)-OH in the catalytic systems hydrolyzes the ester by direct nucleophilic attack on the ester carbonyl of cyclodextrin-bound but not Zn(II)-coordinated PNPA.     

Regioselective enzymatic aminoacylation of lobucavir to give an intermediate for lobucavir prodrug

Synthesis of lobucavir prodrug, L-valine, [(1S,2R,3R)-3-(2-amino-1,6-dihydro-6-oxo-9H-purin-9-yl)-2-(hydroxymethyl)cyclobutyl]methyl ester monohydrochloride (BMS 233866), requires regioselective coupling of one of the two hydroxyl groups of lobucavir (BMS 180194) with valine. Either hydroxyl group of lobucavir could be selectively aminoacylated with valine by using enzymatic reactions. N-[(Phenylmethoxy)carbonyl]-L-valine, [(1R,2R,4S)-2-(2-amino-6-oxo-1H-purin-9-yl)-4-(hydroxymethyl)cyclobutyl]methyl ester (3, 82.5% yield), was obtained by selective hydrolysis of N,N′-bis[(phenylmethoxy)carbonyl]bis[L-valine], O,O′-[(1S,2R,3R)-3-(2-amino-6-oxo-1H-purin-9-yl)cyclobuta-1,2-diyl]methyl ester (1) with lipase M, and L-valine, [(1R,2R,4S)-2-(2-amino-1,6-dihydro-6-oxo-9H-purin-9-yl)-4-(hydroxymethyl)cyclobutyl]methyl ester monohydrochloride (4, 87% yield) was obtained by hydrolysis of bis[L-valine], O,O′-[(1S,2R,3R)-3-(2-amino-6-oxo-1H-purin-9-yl)cyclobuta-1,2-diyl]methyl ester, dihydrochloride (2), with lipase from Candida cylindracea. The final intermediate for lobucavir prodrug, N-[(phenylmethoxy)carbonyl]-L-valine, [(1S,2R,4R)-3-(2-amino-6-oxo-1H-purin-9-yl)-2-(hydroxymethyl)cyclobutyl]methyl ester (5), could be obtained by transesterification of lobucavir using ChiroCLEC™ BL (61% yield), or more selectively by using immobilized lipase from Pseudomonas cepacia (84% yield).     

Effects of secondary interactions on the kinetics of peptide and peptide ester hydrolysis by tissue kallikrein and trypsin

Kinetic constants for the hydrolysis by porcine tissue beta-kallikrein B and by bovine trypsin of a number of peptides related to the sequence of kininogen (also one containing a P2 glycine residue instead of phenylalanine) and of a series of corresponding arginyl peptide esters with various apolar P2 residues have been determined under strictly comparative conditions. kcat and kcat/Km values for the hydrolysis of the Arg-Ser bonds of the peptides by trypsin are conspicuously high. kcat for the best of the peptide substrates, Ac-Phe-Arg-Ser-Val-NH2, even reaches kcat for the corresponding methyl ester, indicating rate-limiting deacylation also in the hydrolysis of a peptide bond by this enzyme. kcat/Km for the hydrolysis of the peptide esters with different nonpolar L-amino acids in P2 is remarkably constant (range 1.7), as it is for the pair of the above pentapeptides with P2 glycine or phenylalanine. kcat for the ester substrates varies fivefold, however, being greatest for the P2 glycine compounds. Obviously, an increased potential of a P2 residue for interactions with the enzyme lowers the rate of deacylation. In contrast to results obtained with chymotrypsin and pancreatic elastase, trypsin is well able to tolerate a P3 proline residue. In the hydrolysis of peptide esters, tissue kallikrein is definitely superior to trypsin. Conversely, peptide bonds are hydrolyzed less efficiently by tissue kallikrein and the acylation reaction is rate-limiting. The influence of the length of peptide substrates is similar in both enzymes and indicates an extension of the substrate recognition site from subsite S3 to at least S'3 of tissue kallikrein and the importance of a hydrogen bond between the P3 carbonyl group and Gly-216 of the enzymes. Tissue kallikrein also tolerates a P3 proline residue well. In sharp contrast to the behaviour of trypsin is the very strong influence of the P2 residue in tissue-kallikrein-catalyzed reactions. kcat/Km varies 75-fold in the series of the dipeptide esters with nonpolar L-amino acid residues in P2, a P2 glycine residue furnishing the worst and phenylalanine the best substrate, whereas this exchange in the pentapeptides changes kcat/Km as much as 730-fold. This behaviour, together with the high value of kcat/Km for Ac-Phe-Arg-OMe of 3.75 X 10(7) M-1 s-1, suggests rate-limiting binding (k1) in the hydrolysis of the best ester substrates.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hydrolysis of novel diacylglycerol analogs and phorbol diesters by serum lipase

Rat serum, active in the hydrolysis of the tumor-promoting phorbol diester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was examined with regard to lipid interferences of [3H]TPA hydrolysis and enzyme substrate specificity. The enzymatic hydrolysis of TPA could be enhanced 8-fold, over crude serum, by using a lipid-free acetone powder of rat serum. Addition of lipid to the lipid-free acetone powder produced potent inhibition of TPA hydrolysis. The inclusion of multilamallar liposomes resulted in similar inhibition, and isolation of liposomes by high-speed centrifugation showed that 95% of the radiolabeled TPA was associated with the fatty pellet. Substrate specificity studies demonstrated that the serum activity hydrolyzes the long-chain ester of TPA and the long-chain primary acyl group of diacylglycerols. TPA was hydrolyzed at approximately twice the rate of dioleoylglycerol; however, the most reactive substrates were those synthetic analogs of diacylglycerol containing a short-chain ester group at the sn-2 position. Palmitic acid was liberated from [1-14C]palmitoyl-2-acetyl-sn-glycerol and [1-14C]palmitoyl-2-butyryl-sn-glycerol at 120- and 33-times the rate of TPA hydrolysis, respectively. Lipase resistant 1-hexadecyl-2-[3H]acetylglycerol was also used as substrate, but the sn-2 ester moiety showed poor lability. The diacylglycerol analogs are new lipase substrates and, in view of their similarities to the fatty acyl portion of TPA, it is thought that these compounds could serve as protein kinase C activators.     

The excess GTP hydrolyzed during mistranslation is expended at the stage of EF-Tu-promoted binding of non-cognate aminoacyl-tRNA

The system of translation of Sepharose-bound poly(U) in which all ribosomes are active in peptide elongation was used to determine the stoichiometry of GTP hydrolysis at the stage of EF-Tu-promoted aminoacyl-tRNA binding. The ratio of GTP hydrolyzed at this stage per peptide bond was assayed during codon-specific elongation (polyphenylalanine synthesis) and misreading (polyleucine synthesis). It was demonstrated directly that the excess GTP hydrolyzed during misreading [(1984) FEBS Letters 178, 283-287] is expended at the stage of Ef-Tu-promoted binding of non-cognate aminoacyl-tRNA.     

Preparation of chiral derivatives of beta-Ala containing the alpha-phenylethyl group: useful starting materials for the asymmetric synthesis of beta-amino acids

The use of microwave (MW) irradiation for the condensation reaction between acetophenone and alpha-phenylethylamine to prepare (R,R)-bis[alpha-phenylethyl]amine results in significantly reduced reaction times relative to the use of conventional heating. In this protocol, a secondary amine, (R,R)-bis(alpha-phenylethyl)amine is treated with acryloyl chloride to afford conjugated amide N,N-bis[(R)-alpha-phenylethyl]prop-2-enamide, (R,R)-3. 1,4-Addition of alpha-phenylethylamine to unsaturated (R,R)-3 affords propanamide N,N-Bis[(R)-alpha-phenylethyl]-3-N-[(S)-alpha-phenylethyl]amino-propanamide, (R,R,S)-4, which can be alkylated with high diastereoselectivity to give derivative N,N-Bis[(R)-alpha-phenylethyl]-3-N'-[(S)-alpha-phenylethyl]amino-propanamide, (R,R,S,S)-5. Hydrogenolysis of (R,R,S,S)-5 catalyzed by palladium hydroxide and final hydrolysis (4 N HCl) resulting in the formation of (S)-alpha-benzyl-beta-alanine, (S)-7, is facilitated by MW irradiation. The use of MW irradiation in this step prevents racemization of the desired amino acid. The present protocol constitutes one of the simplest strategies for the asymmetric synthesis of biologically relevant alpha-substituted-beta-amino acids since it takes advantage of inexpensive, commercially available beta-Ala and either (R)- or (S)-alpha-phenylethylamine as chiral auxiliary. The required time for this protocol is approximately 90 h, which can be carried out in 5 d.     

Enantioselective hydrolysis of oxazepam 3-acetate by esterases in human and rat liver microsomes and rat brain S9 fraction

Rates of hydrolysis of racemic and enantiomeric oxazepam 3-acetates (OXA) by esterases in human and rat liver microsomes and rat brain S9 fraction were compared. When rac-OXA was the substrate, esterases in human and rat liver microsomes were highly enantioselective toward (R)-OXA. In contrast, esterases in rat brain S9 fraction were highly enantioselective toward (S)-OXA. Hydrolysis rates of rac-OXA were highly dependent on the amount of esterases used. At 0.05 mg protein equivalent of esterases and 150 nmol of rac-OXA per ml of incubation mixture, the (R)-OXA was hydrolyzed 3.6-fold and 18.5-fold faster than (S)-OXA by rat and human liver microsomes, respectively. The specific activities (nmol of OXA hydrolyzed/mg microsomal protein/min) of liver microsomes in the hydrolysis of enantiomerically pure (R)-OXA were approximately 120 (rat) and 1,980 (human), and in the hydrolysis of enantiomerically pure (S)-OXA were 4 (rat) and 7 (human), respectively. In the incubation of rac-OXA with rat brain S9 fraction, (S)-OXA was hydrolyzed approximately 6-fold faster than (R)-OXA. Results also indicated an enantiomeric interaction in the hydrolysis of rac-OXA by esterases in rat and human liver microsomes; the presence of (R)-OXA stimulated the hydrolysis of (S)-OXA, whereas the presence of (S)-OXA inhibited the hydrolysis of (R)-OXA. In rat brain S9 fraction, the presence of (R)-OXA inhibited the hydrolysis of (S)-OXA, whereas the presence of (S)-OXA appeared to have stimulated the hydrolysis of (R)-OXA.     

The electrophilic and leaving group phosphates in the catalytic mechanism of yeast pyrophosphatase

Binding of pyrophosphate or two phosphate molecules to the pyrophosphatase (PPase) active site occurs at two subsites, P1 and P2. Mutations at P2 subsite residues (Y93F and K56R) caused a much greater decrease in phosphate binding affinity of yeast PPase in the presence of Mn(2+) or Co(2+) than mutations at P1 subsite residues (R78K and K193R). Phosphate binding was estimated in these experiments from the inhibition of ATP hydrolysis at a sub-K(m) concentration of ATP. Tight phosphate binding required four Mn(2+) ions/active site. These data identify P2 as the high affinity subsite and P1 as the low affinity subsite, the difference in the affinities being at least 250-fold. The time course of five "isotopomers" of phosphate that have from zero to four (18)O during [(18)O]P(i)-[(16)O]H(2)O oxygen exchange indicated that the phosphate containing added water is released after the leaving group phosphate during pyrophosphate hydrolysis. These findings provide support for the structure-based mechanism in which pyrophosphate hydrolysis involves water attack on the phosphorus atom located at the P2 subsite of PPase.     

Synthesis of affinity ligands and radioactive probes for isolation and study of myo-inositol 1,4,5-trisphosphate binding proteins

To synthesize an affinity matrix for isolation of D-myo-inositol 1,4,5-trisphosphate binding proteins, racemic 3-cyclohexene-1-carboxaldehyde was oxidized and converted to a mixture of trans-3,4-di-hydroxycyclohexane-1-carboxylic acid methyl ester isomers, which was phosphorylated and separated into (+-)-(1R,3R,4R)- and (+-)-(1R,3S,4S)-trans-3,4-bis[(diphenoxyphosphoryl)oxy]cyclohex an e-1- carboxylic acid methyl esters. Each of these racemic compounds was hydrogenolyzed and reacted with ethylenediamine to give a monoamide, N-(2-aminoethyl)-bis(phosphonyloxy)cyclohexane-1-carboxamide, that was coupled to cyanogen bromide activated Sepharose 4B to provide the desired affinity matrices. The intermediate trans-3,4-bis[(diphenoxyphosphoryl)oxy]cyclohexane-1-carboxylic acid methyl ester was also reduced with lithium borotritide to give the (hydroxy[3H]methyl)cyclohexane derivative, which was phosphorylated and hydrogenolyzed to yield trans-3,4-bis(phosphonyloxy)-1-[(phosphonyloxy)[3H]methyl]cy clohexane, a radiolabeled analogue of inositol 1,4,5-trisphosphate. The carboxamide was also coupled to 4-azidosalicylic acid, and the product was iodinated to provide a 125I-radiolabeled photoactivatable cross-linking derivative of cyclohexanediol bisphosphate.     

Structural effects on enzymatic degradabilities for poly[(R)-3-hydroxybutyric acid] and its copolymers.

Poly[(R)-3-hydroxybutyric acid] and its copolymers were prepared by biosynthetic and chemosynthetic methods. The films of polyesters were prepared by both the solution-cast and melt-crystallized techniques. The enzymatic degradation of polyester films was carried out at 37 degrees C in an aqueous solution (pH 7.4) of PHB depolymerase from Alcaligenes faecalis. The rate of enzymatic erosion on the solution-cast films increased markedly with an increase in the fraction of second monomer units up to 10-20 mol% to reach a maximum value followed by a decrease in the erosion rate. Analysis of the water-soluble products liberated during the enzymatic degradation of polyester films showed the formation of a mixture of monomers and oligomers of (R)-3HB and hydroxyalkanoic acids units, suggesting that the active site of PHB depolymerase recognizes at least three monomeric units as substrate for the hydrolysis of ester bonds in a polymer chain. The rate of enzymatic erosion of melt-crystallized polyester films decreased with an increase in crystallinity. PHB depolymerase predominantly hydrolyzed the polymer chains in the amorphous phase and subsequently eroded crystalline phase. In addition, the enzymatic degradation of crystalline phase by PHB depolymerase progressed from the edges of crystalline lamellar stacks. The enzymatic erosion rate of crystalline phase in polyester films decreased with an increase in the lamellar thickness.     

Structural and biological effects of a beta2- or beta3-amino acid insertion in a peptide.

Molecular mechanics calculations on conformers of Ac-HGly-NHMe, Ac-beta2-HAla-NHMe and Ac-beta3-HAla-NHMe indicate that low-energy conformations of the beta-amino acids backbone, corresponding to gauche rotamers around the Calpha-Cbeta bond, may overlap canonical backbone conformers observed for alpha-amino acids. Therefore, Substance P (SP) was used as a model peptide to analyse the structural and biological consequences of the substitution of Phe7 and Phe8 by (R)-beta2-HPhe and of Gly9 by HGly (R)-beta2-HAla or (S)-beta3-HAla. [(R)-beta2-HAla9]SP has pharmacological potency similar to that of SP while [HGly9]SP and [(S)-beta3-HAla9]SP show a 30- to 50-fold decrease in biological activities. The three analogues modified at position 9 are more resistant to degradation by angiotensin converting enzyme than SP and [Ala9]SP. NMR analysis of these SP analogues suggest that a beta-amino acid insertion in position 9 does not affect the overall backbone conformation. Altogether these data suggest that [HGly9]SP, [(S)-beta3-HAla9]SP and [(R)-beta2-HAla9]SP could adopt backbone conformations similar to that of SP, [Ala9]SP and [Pro9]SP. In contrast, incorporation of beta2-HPhe in position 7 and 8 of SP led to peptides that are almost devoid of biological activity. Thus, a beta-amino acid could replace an alpha-amino acid within the sequence of a bioactive peptide provided that the additional methylene group does not cause steric hindrance and does not confine orientations of the side chain to regions of space different from those permitted in the alpha-amino acid.     

Metal-ion-catalyzed hydrolysis of monomethyl and dimethyl esters of 3-(2-imidazoleazo)phthalic acid: Bifunctional catalysis by carboxyl group and the metal ion in the hydrolysis of an alkyl ester

The Cu(II) or Ni(II) ion-catalyzed hydrolysis of methyl 2-carboxy-6-(2-imidazoleazo)benzoate (1) and the corresponding dimethyl ester (2) was studied kinetically at various pH values. For 2, the ester group located at the o position to the azo substiuent was hydrolyzed. From the rate data obtained at various metal concentrations, the values of k_cat and K_f were estimated at each pH value. For the Ni(II)-catalyzed hydrolysis of 1 at pH < 4, k_cat increases as pH is lowered, indicating bifunctional catalysis by the carboxyl group and the metal ion. For most of the reactions investigated under other conditions, the ester hydrolysis was subjected to sole catalysis by the metal ions. Detailed analysis of kinetic data obtained for these reactions indicated that the metal-ion catalysis involves the rate-determining breakdown of the tetrahedral intermediates formed by the addition of a water molecule or hydroxide ion. The bifunctional catalysis by the carboxyl group and Ni(II) ion can be considered as a model for carboxypeptidase A. The kinetic data indicate that the bifunctional catalysis proceeds through the nucleophilic attack of the carboxylate ion at the Ni(II)-coordinated carbonyl group.     

Enhanced cleavage of diaminopimelate-containing isopeptides by leucine aminopeptidase and matrix metalloproteinases in tumors: application to bioadhesive peptides.

We prepared (2S,6S)-Z-Dpm(Z)(OMe) (4) by protease-mediated hydrolysis of (R,R/S,S)-Z-Dpm(Z)(OMe)-OMe (3), converted it to (2S,6S)-Dpm(Z)(OMe) (6) via PCI5 to an NCA intermediate and hydrolysis, protected the amino group with Boc to give (2S,6S)-Boc-Dpm(Z)(OMe) (7), which upon ammonolysis of the Me ester afforded (2S,6S)-Boc-Dpm(Z)(NH2) (8). Hydrogenolysis of 8 and protection with Fmoc gave (2S,6S)-Boc-Dpm(Fmoc)(NH2)(10). Using 10 and SPPS, we prepared three Dpm-containing peptides and their corresponding Lys peptides. Enzymatic studies with mLAP and cLAP showed that the Leu moiety in Ac-Gly-(2S,6S)-Dpm(Leu)(NH2)-Ala (14) was hydrolyzed 68-fold and >1000-fold more rapidly, respectively, than that in Ac-Gly-Lys(Leu)-Ala (12). The enhanced rate of Leu formation from 14 compared to 12 was also observed with homogenates of mouse C3 sarcomas. This homogenate also hydrolyzed Ac-Gly-(2S,6S)-Dpm(Ac-Gly-Pro-Gln-Gly-Leu)(NH2)-Ala (16) to Ac-Gly-(2S,6S)-Dpm(NH2)-Ala (13), Leu and Ac-Gly-Pro-Gln-Gly (17). This implies the side chain is cleaved first by endopeptidases, such as matrix metalloproteinases (MMPs), and then the remaining Leu is cleaved by LAP-like exopeptidases. The rate of liberation of 17 from 16 and the corresponding Lys isopeptide, Ac-Gly-Lys(Ac-Gly-Pro-Gln-Gly-Leu)-Ala (15), was not significantly different. The rate of formation of 13 was faster from 16 than Ac-Gly-Lys-Ala (11) was from 15. Thus, the entire isopeptide side chain can be removed by the cooperative action of LAP-like and MMP-like peptidases present in tumor tissue, which occurs faster in the Dpm peptide 16 than in the Lys peptide 15. The rate of formation of 13 from 16 by lung, liver, and intestine homogenates (from the same C3 tumor-bearing mice) was comparable to or higher than from the tumor homogenates, but the rate by blood was only 4% the value of the tumor homogenates. Analogs of a bioadhesive fragment from the laminin alpha1 chain were prepared by replacing the essential Lys with Dpm(NH2) (20) and Dpm(Leu)(NH2) (21). Both Dpm-containing peptides were active, although considerably weaker than the corresponding Lys peptides 18 and 19, in a cell attachment assay with human fibrosarcoma HT-1080 cells.     

Hydrolysis of triacylglycerol arachidonic and linoleic acid ester bonds by human pancreatic lipase and carboxyl ester lipase

The hydrolysis of polyenoic fatty acid ester bonds with pure human colipase-dependent lipase, with carboxyl ester lipase (CEL) and with these enzymes in combination was studied, using [3H]arachidonic- and [14C]linoleic acid-labelled rat chylomicrons as a model substrate. During the hydrolysis with colipase-dependent lipase, the amount of 3H appearing in 1,2-X-diacylglycerol (DG) markedly exceeded that of 14C. When CEL was added in addition this [3H]DG was efficiently hydrolyzed. CEL alone hydrolyzed the triacylglycerol (TG) at a low rate. The hydrolysis pattern with human duodenal content was similar to that seen with colipase-dependent lipase and CEL in combination. Increasing the concentration of taurodeoxycholate (TDC) and taurocholate (TC) or of TDC alone stimulated the hydrolysis of [3H]- and [14C]TG, but increased the accumulation of labelled DG that could act as substrate for CEL. It is suggested that very-long-chain polyenoic fatty acids of DG formed during the action of the colipase-dependent lipase on TG containing these fatty acids may be a physiological substrate for CEL.     

Purification and partial characterization of two feruloyl esterases from the anaerobic fungus Neocallimastix strain MC-2.

Two extracellular feruloyl esterases (FAE-I and FAE-II) produced by the anaerobic fungus Neocallimastix strain MC-2 which cleave ferulic acid from O-(5-O-[(E)-feruloyl]-alpha-L- arabinofuranosyl)-(1-->3)-O-beta-D-xylopyranosyl-(1-->4)-D-xylopyranose (FAXX) were purified. The molecular masses of FAE-I and FAE-II were 69 and 24 kDa, respectively, under both denaturing and nondenaturing conditions. Apparent Km and maximum rate of hydrolysis with FAXX were 31.9 microM and 2.9 mumol min-1 mg-1 for FAE-I and 9.6 microM and 11.4 mumol min-1 mg-1 for FAE-II. FAE-II was specific for FAXX, but FAE-I hydrolyzed FAXX and PAXX, the equivalent p-coumaroyl ester, at a maximum rate of metabolism ratio of 3:1.