Crystal versus solution structures of enzymes: NMR spectroscopy of a crystalline serine protease.

The hydrogen-bonding status of His57 in the catalytic triad (Asp-His-Ser) of serine protease has important mechanistic implications for this class of enzymes. Recent nitrogen-15 nuclear magnetic resonance (NMR) studies of alpha-lytic protease find His57 and Ser195 to be strongly hydrogen-bonded, a result that conflicts with the corresponding crystallographic studies, thereby suggesting that the crystal and solution structures may differ. This discrepancy is addressed and resolved in a nitrogen-15 NMR study of the enzyme in the crystalline state. The results show that the His-Ser and Asp-His interactions are identical in crystals and solutions, but that in crystals His57 titrates with a pKa of 7.9, nearly one pKa unit higher than in solution. This elevated pKa accounts for the absence of the His-Ser hydrogen bond in previous x-ray studies.

A low-barrier hydrogen bond in the catalytic triad of serine proteases? Theory versus experiment.

Cleland and Kreevoy recently advanced the idea that a special type of hydrogen bond (H-bond), termed a low-barrier hydrogen bond (LBHB), may account for the "missing" transition state stabilization underlying the catalytic power of many enzymes, and Frey et al. have proposed that the H-bond between aspartic acid 102 and histidine 57 in the catalytic triad of serine proteases is an example of a catalytically important LBHB. Experimental facts are here considered regarding the aspartic acid-histidine and cis-urocanic H-bonds that are inconsistent with fundamental tenets of the LBHB hypothesis. The inconsistencies between theory and experiment in these paradigm systems cast doubt on the existence of LBHBs, as currently defined, within enzyme active sites.

Potentiation of the immune response in HIV-1+ individuals.

T cells from HIV-1+ individuals have a defect in mounting an antigen specific response. HIV-1 Tat has been implicated as the causative agent of this immunosuppression. We have previously shown that HIV-1 Tat inhibits antigen specific proliferation of normal T cells in vitro by binding to the accessory molecule CD26, a dipeptidase expressed on the surface of activated T cells. We now demonstrate that the defective in vitro recall antigen response in HIV-1 infected individuals can be restored by the addition of soluble CD26, probably by serving as a decoy receptor for HIV-1 Tat. The restored response is comparable to that of an HIV-1- individual, suggesting that early in HIV infection there is a block in the memory cell response, rather than deletion of these cells.

Lower serum fibroblast activation protein shows promise in the exclusion of clinically significant liver fibrosis due to non-alcoholic fatty liver disease in diabetes and obesity.

UNLABELLED: Non-alcoholic fatty liver disease (NAFLD) is common in diabetes and obesity but few have clinically significant liver fibrosis. Improved risk-assessment is needed as the commonly used clinical-risk algorithm, the NAFLD fibrosis score (NFS), is often inconclusive. AIMS: To determine whether circulating fibroblast activation protein (cFAP), which is elevated in cirrhosis, has value in excluding significant fibrosis, particularly combined with NFS. METHODS: cFAP was measured in 106 with type 2 diabetes who had transient elastography (Cohort 1) and 146 with morbid obesity who had liver biopsy (Cohort 2). RESULTS: In Cohort 1, cFAP (per SD) independently associated with median liver stiffness (LSM) ≥ 10.3 kPa with OR of 2.0 (95% CI 1.2-3.4), p=0.006. There was 0.12 OR (95% CI 0.03-0.61) of LSM ≥ 10.3 kPa for those in the lowest compared with the highest FAP tertile (p=0.010). FAP levels below 730 pmol AMC/min/mL had 95% NPV for LSM ≥ 10.3 kPa and reclassified 41% of 64 subjects from NFS 'indeterminate-risk' to 'low-risk'. In Cohort 2, cFAP (per SD), associated with 1.7 fold (95% CI 1.1-2.8) increased odds of significant fibrosis (F ≥ 2), p=0.021, and low cFAP reclassified 49% of 73 subjects from 'indeterminate-risk' to 'low-risk'. CONCLUSIONS: Lower cFAP, when combined with NFS, may have clinical utility in excluding significant fibrosis in diabetes and obesity.

Structure-activity relationships of boronic acid inhibitors of dipeptidyl peptidase IV. 1. Variation of the P2 position of Xaa-boroPro dipeptides.

A series of prolineboronic acid (boroPro) containing dipeptides were synthesized and assayed for their ability to inhibit the serine protease dipeptidyl peptidase IV (DPPIV). Inhibitory activity, which requires the (R)-stereoisomer of boroPro in the P1 position, appears to tolerate a variety of L-amino acids in the P2 position. Substitution at the P2 position which is not tolerated include the D-amino acids, alpha,alpha-disubstituted amino acids, and glycine. Specificity against DPPII and proline specific endopeptidase is reported. A correlation between the ability to inhibit DPPIV in cell culture and in the human mixed lymphocyte reaction is demonstrated. A synthesis of prolineboronic acid is reported as well as conditions for generating the fully unprotected boronic acid dipeptides in either their cyclic or acyclic forms.

Confirmation of the assignment of the low-field proton resonance of serine proteases by using specifically nitrogen-15 labeled enzyme.

Proton NMR spectra of serine proteases in 1H2O solutions typically show a single resonance at very low magnetic field--i.e., 14-18 ppm from dimethylsilylapentanesulfonate. This resonance has been assigned to the proton hydrogen bonded between aspartic acid-102 and histidine-57 (chymotrypsin numbering system) of the "charge-relay system" or catalytic triad of serine proteases [Robillard, G. & Shulman, R. G. (1972) J. Mol. Biol. 71, 507-511]. Since then, there have been a number of reports that have cast doubt on its correctness. In the present work we have tested this assignment using alpha-lytic protease (EC 3.4.21.12, Myxobacter alpha-lytic proteinase), a bacterial serine protease homologous to elastase, which is specifically labeled with nitrogen-15 at N delta 1 of its single histidine residue. The low-field region of the proton spectra of this labeled enzyme shows a single resonance having the properties reported [Robillard, G. & Shulman, R. G. (1974) J. Mol. Biol. 86, 519-540], which, in addition, exhibits spin-spin splitting to the nitrogen-15 label. The observation of this 15N delta 1-H coupling makes the assignment of this resonance to the charge-relay proton unequivocal.

15N NMR spectroscopy of hydrogen-bonding interactions in the active site of serine proteases: evidence for a moving histidine mechanism.

Nitrogen-15 NMR spectroscopy has been used to study the hydrogen-bonding interactions involving the histidyl residue in the catalytic triad of alpha-lytic protease in the resting enzyme and in the transition-state or tetrahedral intermediate analogue complexes formed with phenylmethanesulfonyl fluoride and diisopropyl fluorophosphate. The 15N shifts indicate that a strong hydrogen bond links the active site histidine and serine residues in the resting enzyme in solution. This result is at odds with interpretations of the X-ray diffraction data of alpha-lytic protease and of other serine proteases, which indicate that the serine and histidine residues are too far apart and not properly aligned for the formation of a hydrogen bond. In addition, the nitrogen-15 shifts demonstrate that protonation of the histidine imidazole ring at low pH in the transition-state or tetrahedral intermediate analogue complexes formed with phenylmethanesulfonyl fluoride and diisopropyl fluorophosphate triggers the disruption of the aspartate-histidine hydrogen bond. These results suggest a catalytic mechanism involving directed movement of the imidazole ring of the active site histidyl residue.

Separation of L-Pro-DL-boroPro into its component diastereomers and kinetic analysis of their inhibition of dipeptidyl peptidase IV. A new method for the analysis of slow, tight-binding inhibition.

The potent dipeptidyl peptidase IV (DP IV) inhibitor [1-(2-pyrrolidinylcarbonyl)-2-pyrrolidinyl]boronic acid (L-Pro-DL-boroPro) [Flentke, G. R., Munoz, E., Huber, B. T., Plaut, A. G., Kettner, C. A., & Bachovchin, W. W. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1556-1559] was fractionated into its component L-L and L-D diastereomers by C18 HPLC, and the binding of the purified diastereomers to DP IV was analyzed. Inhibition kinetics confirms that the L-L diastereomer is a potent inhibitor of DP IV, having a Ki of 16 pM. The L-D isomer binds at least 1000-fold more weakly than the L-L, if it binds at all, as the approximately 200-fold weaker inhibition observed for the purified L-D isomer is shown here to be due entirely to the presence of a small amount (0.59%) of the L-L diastereomer contaminating the L-D preparation. The instability of Pro-boroPro, together with its very high affinity for DP IV and the time dependence of the inhibition, makes a rigorous kinetic analysis of its binding to DP IV difficult. Here we have developed a method which takes advantage of the slow rate at which the inhibitor dissociates from the enzyme. The method involves preincubating the enzyme and the inhibitor without substrate and then assaying the free enzyme by the addition of substrate and following its hydrolysis for a period of time which is short relative to the dissociation rate of the inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

11B NMR spectroscopy of peptide boronic acid inhibitor complexes of alpha-lytic protease. Direct evidence for tetrahedral boron in both boron-histidine and boron-serine adduct complexes.

We have previously shown, using 15N and 1H NMR spectroscopy, that MeOSuc-Ala-Ala-Pro-boroPhe and certain other boronic acid inhibitors form boron-histidine adducts with alpha-lytic protease instead of transition-state-like tetrahedral boron-serine adducts as is generally supposed [Bachovchin, W. W., Wong, W. Y. L., Farr-Jones, S., Shenvi, A. B., & Kettner, C. (1988) Biochemistry 27, 7689-7697]. An X-ray crystallographic study of the MeOSuc-Ala-Ala-Pro-boroPhe complex with alpha-lytic protease [Bone, R., Frank, D., Kettner, C. A., & Agard, D. A. (1989) Biochemistry 28, 7600-7609] has confirmed the existence of the boron-histidine bond but has concluded that the boron atom is trigonal rather than tetrahedral. Here we report a 11B NMR study at 160.46 MHz of this histidine adduct complex and of two other complexes known to be serine adducts: alpha-lytic protease with MeOSuc-Ala-Ala-Pro-boroVal and chymotrypsin with MeOSucAla-Ala-Pro-boroPhe. The 11B NMR chemical shifts demonstrate that the boron atom is tetrahedral in both the histidine and serine adduct complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinlsq: a program for fitting kinetics data with numerically integrated rate equations and its application to the analysis of slow, tight-binding inhibition data.

Kinlsq, a Matlab-based computer program for the least-squares fitting of parameters to kinetics data described by numerically integrated rate equations, is described, and three applications to the analysis of enzyme kinetics data are given. The first application was to the analysis of a simple bimolecular enzyme plus inhibitor binding curve. The kinlsq fit to these data was essentially identical to that obtained with the corresponding analytically integrated rate equation, validating kinlsq. The second application was to the fit of a numerically integrated Michaelis-Menten model to the progress curve for dipeptidyl peptidase IV-catalyzed hydrolysis of Ala-Pro-p-nitroanilide as a demonstration of the analysis of steady-state enzyme kinetics data. The results obtained with kinlsq were compared with the results obtained by fitting this time course with the integrated Michaelis-Menten equation, and with the results obtained by fitting the (S,dP/dt) transform of the data with the Michaelis-Menten equation. The third application was to the analysis of the inhibition of chymotrypsin by the slow, tight-binding inhibitor MeOSuc-Ala-Ala-Pro-boroPhe, data not readily amenable to other methods of analysis. These applications demonstrate how kinlsq can be used to fit rate constants, equilibrium constants, steady-state constants, and the stoichiometric relationships between components.

Identification of serine and histidine adducts in complexes of trypsin and trypsinogen with peptide and nonpeptide boronic acid inhibitors by 1H NMR spectroscopy.

We have previously shown, in 15N NMR studies of the enzyme's active site histidine residue, that boronic acid inhibitors can form two distinct types of complexes with alpha-lytic protease. Inhibitors that are structural analogs of good alpha-lytic protease substrates form transition-state-like tetrahedral complexes with the active site serine whereas those that are not form complexes in which N epsilon 2 of the active site histidine is covalently bonded to the boron of the inhibitor. This study also demonstrated that the serine and histidine adduct complexes exhibit quite distinctive and characteristic low-field 1H NMR spectra [Bachovchin, W. W., Wong, W. Y. L., Farr-Jones, S., Shenvi, A. B., & Kettner, C. A. (1988) Biochemistry 27, 7689-7697]. Here we have used low-field 1H NMR diagnostically for a series of boronic acid inhibitor complexes of trypsin and trypsinogen. The results show that H-D-Val-Leu-boroArg and Ac-Gly-boroArg, analogs of good trypsin substrates, form transition-state-like serine adducts with trypsin, whereas the nonsubstrate analog inhibitors boric acid, methane boronic acid, butane boronic acid, and triethanolamine borate all form histidine adducts, thereby paralleling the previous results obtained with alpha-lytic protease. However, with trypsinogen, Ac-Gly-boroArg forms predominantly a histidine adduct while H-D-Val-Leu-boroArg forms both histidine and serine adducts, with the histidine adduct predominating below pH 8.0 and the serine adduct predominating above pH 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of action of adenosylcobalamin: 3-fluoro-1,2-propranediol as substrate for propanediol dehydrase--mechanistic implications.

3-Fluro-1,2-propanediol has been found to be a substrate for propanediol dehydrase and has very similar binding and catalytic constants compared to the natural substrate. The only isolable products of the reaction are acrolein and inorganic fluoride; with 3-fluoro-3,3-dideuterio-1,2-propanediol as substrate, only 3,3-dideuterioacrolein is obtained. These results indicate that the primary product of the reaction is 3-fluoropropionaldehyde which spontaneously loses hydrogen fluoride to yield acrolein. The similar kinetic parameters for the fluorinated as compared to the normal substrate suggest that significant charge does not develop on the fluorinated or, by implication, the natural substrate during any rate-limiting steps of the reaction. These results support a radical, as contrasted to an ionic pathway for reactions involving adenosylcobalamin and diol dehydrase.

Mechanism of action of adenosylcobalamin: hydrogen transfer in the inactivation of diol dehydratase by glycerol.

We have investigated the kinetic characteristics of the inactivation of the adenosylcobalamin-dependent enzyme propanediol dehydratase by glycerol, (RS)-1,1-dideuterioglycerol, (R)-1,1-dideuterioglycerol, and perdeuterioglycerol in the presence of 1,2-propanediol and 1,1-dideuterio-1,2-propanediol. The results imply that hydrogen (or deuterium) attached to C-1 of 1,2-propanediol participates in the inactivation process and contributes to the expression of a kinetic isotope effect on the rate of inactivation. The mechanism for this inactivation must involve the cofactor as an intermediate hydrogen carrier, presumably in the form of 5'-deoxyadenosine. Moreover, a mechanism involving a rate-determining transfer of hydrogen from an intermediate containing three equivalent hydrogens quantitatively accounts for all of the results. When diol dehydratase holoenzyme is inactivated by [1-3H]glycerol, 5'-deoxyadenosine which is enriched in tritium by a factor of 2.1 over that in glycerol can be isolated from the reaction mixture.

Mechanism of action of adenosylcobalamin: glycerol and other substrate analogues as substrates and inactivators for propanediol dehydratase--kinetics, stereospecificity, and mechanism.

A number of vicinal diols were found to react with propanediol dehydratase, typically resulting in the conversion of enzyme-bound adenosylcobalamin to cob(II)alamin and formation of aldehyde or ketone derives from substrate. Moreover, all are capable of effecting the irreversible inactivation of the enzyme. The kinetics and mechanism of product formation and inactivation were investigated. Glycerol, found to be a very good substrate for diol dehydratase as well as a potent inactivator, atypically, did not induce cob(II)alamin formation to any detectable extent. With glycerol, the inactivation process was accompanied by conversion of enzyme-bound adenosylcobalamin to an alkyl or thiol cobalamin, probably by substitution of an amino acid chain near the active site for the 5'-deoxy-5'-adenosyl ligand on the cobalamin. The inactivation reaction with glycerol as the inactivator exhibits a deuterium isotope effect of 14, strongly implicating hydrogen transfer as an important step in the mechanism of inactivation. The isotope effect on the rate of product formation was found to be 8.0. Experiments with isotopically substituted glycerols indicate that diol dehydrase distinguishes between "R" and "S" binding conformations, the enzyme-(R)-glycerol complex being predominately responsible for the product-forming reaction, while the enzyme-(S)-glycerol complex results primarily in the activation reaction. Mechanistic implications are discussed. A method for removing enzyme-bound hydroxycobalamin that is nondestructive to the enzyme and a technique for measuring the binding constants of (R)- and (S)-1,2-propanediols are presented.

Hydrogen transfer in catalysis by adenosylcobalamin-dependent diol dehydratase.

Studies [bachovchin, W. W., et al. (1978) Biochemistry 17, 2218] of the mechanism of inactivation of adenosylcobalamin-dependent diol dehydratase have led to the development of a general method to describe the kinetics of a reaction pathway containing a reservoir of mobile hydrogen. Analysis by this method of catalytic rate measurements for mixtures of 1,2-propanediol and 1,1-dideuterio-1,2-propanediol supports a mechanism involving an intermediate with three equivalent hydrogens, in which hydrogen transfer from this intermediate to product is the major rate-contributing step. Other results using tritium as a trace label [essenberg, M. K., et al. (1971) J. Am. Chem. Soc. 93, 1242] are considered in light of these deuterium isotope studies.

Kinetic properties of the binding of alpha-lytic protease to peptide boronic acids.

The kinetic parameters for peptide boronic acids in their interaction with alpha-lytic protease were determined and found to be similar to those of other serine proteases [Kettner, C., & Shenvi, A. B. (1984) J. Biol. Chem. 259, 15106-15114]. alpha-Lytic protease hydrolyzes substrates with either alanine or valine in the P1 site and has a preference for substrate with a P1 alanine. The most effective inhibitors are tri- and tetrapeptide analogues that have a -boroVal-OH residue in the P1 site. At pH 7.5, MeOSuc-Ala-Ala-Pro-boroVal-OH has a Ki of 6.4 nM and Boc-Ala-Pro-boroVal-OH has a Ki of 0.35 nM. Ac-boroVal-OH and Ac-Pro-boroVal-OH are 220,000- and 500-fold less effective, respectively, than the tetrapeptide analogue. The kinetic properties of the tri- and tetrapeptide analogues are consistent with the mechanism for slow-binding inhibition, E + I in equilibrium EI in equilibrium EI*, while the less effective inhibitors are simple competitive inhibitors. MeO-Suc-Ala-Ala-Pro-boroAla-OH is a simple competitive inhibitor with a Ki of 67 nM at pH 7.5. Other peptide boronic acids, which are analogues of nonsubstrates, are less effective than substrate analogues but still are effective competitive inhibitors. For example, MeOSuc-Ala-Ala-Pro-boroPhe-OH has a Ki of 0.54 microM although substrates with a phenylalanine in the P1 position are not hydrolyzed. Binding for boronic acid analogues of both substrate and nonsubstrate analogues is pH dependent with higher affinity near pH 7.5. Similar binding properties have been observed for pancreatic elastase. Both enzymes have almost identical requirements for an extended peptide inhibitor sequence in order to exhibit highly effective binding and slow-binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of HIV-1 Tat induced inhibition of antigen-specific T cell responsiveness.

HIV-1 Tat has been shown to have an inhibitory effect on the Ag-specific responsiveness of human peripheral T cells. We have previously demonstrated that this retroviral protein binds to and partially inhibits the enzymatic activity of dipeptidyl aminopeptidase type IV (DP IV), also known as CD26, which is expressed on a variety of mammalian tissue, including T lymphocytes. A number of studies have implicated a role for DP IV in the activation of T lymphocytes. By utilizing HIV-1 Tat, as well as ProboroPro, a potent and specific boronic acid analog inhibitor of DP IV, we show here that blocking DP IV partially inactivates Ag and anti-CD3-mediated T cell proliferation. Neither mitogen nor anti-CD2 mediated proliferation of T lymphocytes, however, is impaired by blocking DP IV. The target molecule for the inhibition induced by both compounds was confirmed by the finding that soluble DP IV neutralized the reduced Ag responsiveness. The Ag-specific inhibition could be overcome by the addition of exogenous IL-2, suggesting that blocking or inactivation of DP IV results in a state of anergy, probably by interfering with the delivery or amplification of a signal necessary for IL-2 production. This is further substantiated by the finding that costimulation of human PBMC via the CD28 molecule, which initiates a non-TCR-dependent signaling pathway, overcomes the reduced Ag responsiveness induced by Tat and ProboroPro. The fact that ProboroPro has no impact on stimulation of T cells with PMA and ionomycin implies that blocking DP IV is influencing events before the activation of protein kinase C and Ca2+ flux. These results suggest that DP IV is necessary for amplification of signals generated by the engagement of the TCR-CD3 complex by nominal Ag.