Conformational states of N-acylalanine dithio esters: correlation of resonance Raman spectra with structures

The conformational states of N-acylalanine dithio esters, involving rotational isomers about the RC(=O)NH--CH(CH3) and NHCH(CH3)--C(=S) bonds, are defined and compared to those of N-acylglycine dithio esters. The structure of N-(p-nitrobenzoyl)-DL-alanine ethyl dithio ester has been determined by X-ray crystallographic analysis; it is a B-type conformer with the amide N atom cis to the thiol sulfur. Raman and resonance Raman (RR) measurements on this compound and for the B conformers of solid N-benzoyl-DL-alanine ethyl dithio ester and N-(beta-phenylpropionyl)-DL-alanine ethyl dithio ester and its NHCH(CD3)C(=S) and NHCH(CH3)13C(=S) analogues are used to set up a library of RR data for alanine-based dithio esters in a B-conformer state. (Methyloxycarbonyl)-L-phenylalanyl-L-alanine ethyl dithio ester crystallizes in an A-like conformational state wherein the alanine N atom is nearly cis to the thiono S atom (C=S) [Varughese, K.I., Angus, R.H., Carey, P.R., Lee, H., & Storer, A.C. (1986) Can. J. Chem. 64, 1668-1673]. RR data for this solid material in its isotopically unsubstituted and CH(C-D3)C(=S) and CH(CH3)13C(=S) forms provide information on the RR signatures of alanine dithio esters in A-like conformations. RR spectra are compared for the solid compounds, for N-(p-nitrobenzoyl)-DL-alanine, N-(beta-phenylpropionyl)-DL-alanine, and (methyloxycarbonyl)-L-phenylalanyl-DL-alanine ethyl dithio esters, and for several 13C=S- and CD3-substituted analogues in CCl4 or aqueous solutions. The RR data demonstrate that the alanine-based dithio esters take up A, B, and C5 conformations in solution.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relaxed and perturbed substrate conformations in enzyme active sites: evidence from multichannel resonance raman spectra

A diode array based multichannel Raman spectrometer has made it possible to record complete, high quality, resonance Raman (RR) spectra of enzyme-substrate intermediates. The intermediates are dithioacylpapains in which the acyl group is either N-benzoylglycine or N-(beta-phenylpropionyl)glycine. RR data are reported for the unlabeled dithioacylpapains as well as for the intermediates labeled separately with ND, 15N, and 13C = S in the glycine residue. Comparison of the results for the dithioacylpapains with that of the corresponding labeled glycine ethyl dithioesters [Lee, H., Storer, A. C., & Carey, P. R. (1983) Biochemistry (preceding paper in this issue)] leads to the conclusion that for both substrates in the active site the dihedral angles in the glycine NH-C-C(= S) linkages assume an essentially relaxed type B conformation. Similarly, there is no evidence for distortion about the C(= O)-NH peptide bond which links the P1 and P2 sites on the substrate. However, for the N-benzoylglycine case there is evidence for some conformational distortion in the -S-C-C cysteine linkages. The present data favor a single homogeneous conformational population about the substrates' NH-C-C(= S) bonds in the native dithioacylpapains. However, below pH 3.0 the dithioacyl enzymes denature and the RR spectra of the 13C = S substituted species confirm that the conformational population reverts to the mixture of conformers A and B found for the corresponding ethyl dithioesters in solution.     

Comparative resonance Raman spectroscopic and kinetic studies of acyl-enzymes involving papain, actinidin and papaya peptidase II.

Resonance Raman spectra are reported for a series of dithioacyl-enzymes involving actinidin (EC 3.4.22.14) and papaya peptidase II (the more basic monothiol cysteine proteinase of Carica papaya). The acyl groups are N-benzoylglycine and N-(beta-phenylpropionyl)glycine containing C = S or 13C = S at the ester function. Comparison of the data with those for the corresponding papain (EC 3.4.22.2) analogues [Storer, Lee & Carey (1983) Biochemistry 22, 4789-4796] allows us to define the conformation of the dithioacyl group in the catalytic site. In each case the dithioacyl group is bound in a single conformation known as conformer B, in which the glycinic nitrogen atom comes into close contact with the sulphur atom of the catalytic-site cysteine residue. For the N-(beta-phenylpropionyl)glycine dithioacyl-enzymes the torsional angles of the NH-CH2-C(= S) bonds assume values typical of an essentially relaxed non-strained state. However, for the N-benzoylglycine dithioacyl-enzymes there is evidence for a slightly perturbed conformer B, and the perturbation is most pronounced for N-benzoylglycine dithioacyl-actinidin. Values of k+2/Ks and k+3 for the reactions of papain, actinidin and papaya peptidase II with N-benzoylglycine and N-(beta-phenylpropionyl)glycine methyl thionoesters were obtained by a pre-steady-state kinetic study. Wide variation was found in k+2/Ks, but the values of k+3 are all similar. This general picture is supported by the results from a steady-state kinetic study of the reactions of the three enzymes with N-benzoyl-L-arginine-p-nitroanilide and with N-benzyloxycarbonyl-L-lysine p-nitrophenyl ester. The similarity of the values of k+3, together with the invariance of conformer B geometry at the P1 site, suggests that the chemistry of the deacylation process is highly conserved among these three cysteine proteinases.     

Comparison of the kinetics of the papain-catalyzed hydrolysis of glycine- and alanine-based esters and thiono esters

The kinetic constants for the papain-catalyzed hydrolysis of a series of substrates with glycine or alanine in the P1 position are discussed. The substrates have N-benzoyl, N-(p-nitrobenzoyl), N-(beta-phenylpropionyl), or N-(methyloxycarbonyl)phenylalanine attached to the P1 moiety, and kinetic constants are obtained for both esters and thiono esters. The results for the hydrolysis of esters can be readily interpreted in terms of the known specificity of papain. For any glycine ester the change in kcat/Km upon substituting C=S for C=O or upon substituting an alpha-CH3 group is minimal. However, upon making both these substitutions, i.e., going from a glycine ester to an alanine thiono ester substrate, larger changes are seen for this ratio. Data for N-benzoyl- and N-(beta-phenylpropionyl)glycine and -alanine methyl thiono esters show that k2 is the parameter most affected by the double C=S and alpha-CH3 substitution. A further conclusion is that the deacylation rate constants for any pair of glycine and alanine dithioacyl papains are similar; e.g., for the intermediates based on the "good" substrates PheAla and PheGly k3 differs by only 20%. This is a surprising finding in light of the very different conformations and interactions of the bound acyl groups revealed by resonance Raman spectroscopy and raises the possibility that specific stereochemical effects, such as the oxyanion hole and general base catalysis, are not operating in the hydrolysis of dithioacyl papains.     

Identity of acyl group conformations in the active sites of papain and cathepsin B by resonance Raman spectroscopy

Resonance Raman spectroscopic data provide conclusive evidence for the existence of an acyl-enzyme intermediate during the reaction of a thionoester substrate, N-methyloxycarbonylphenylalanylglycine methyl thionoester (CH3OC(=O)-Phe-NHCH2C(=S) OCH3), with cathepsin B from porcine spleen. The resonance Raman spectrum of CH3OC(=O)-Phe-NHCH2C(=S)S-cathepsin B, where the thiol S is from the active-site cysteine residue, is compared to that of the corresponding papain acyl-enzyme. Within the limits of experimental error (+/-2 cm-1 for peak positions), there are no detectable spectral differences. Since the resonance Raman spectrum is sensitive to the torsional angles in the glycinic bonds and the cysteine linkages, the conformations are identical in those parts of the acyl-enzymes where chemical transformation occurs. A conformational analysis of the model compound CH3OC(=O)-Phe-NHCH2C(=S)SC2H5 demonstrates that the dithioacyl group in both dithioacyl-enzymes is present as a single population of a form known as conformer B. Conformer B is characterized by a small torsional angle about the glycinic NHCH2-CS(thiol) bond such that the nitrogen and S (thiol) atoms are in close contact. This conformer is widespread among the dithioacyl intermediates of plant cysteine proteinases, and it is apparent that the same chemistry is retained in a mammalian cysteine proteinase. Steady-state kinetic parameters are also reported for CH3OC(=O)-Phe-NHCH2C(=S)OCH3 reacting with papain and cathepsin B. The similarity of the Kcat values, 0.53 and 1.15 s-1, for papain and cathepsin B, respectively, provides further evidence for a conserved deacylation process.     

Comparison of the kinetics and mechanism of the papain-catalyzed hydrolysis of esters and thiono esters

The kinetic constants for the papain-catalyzed hydrolysis of the methyl thiono esters of N-benzoylglycine and N-(beta-phenylpropionyl)glycine are compared with those for the corresponding methyl ester substrates. The k2/Ks values for the thiono esters are 2-3 times higher than those for the esters, and both show bell-shaped pH dependencies with similar pKa's (approximately 4 and 9). The k3 values for the thiono esters are 30-60 times less than those for the esters and do not exhibit a pH dependency. Solvent deuterium isotope effects on k2/Ks and k3 were measured for the ester and thiono ester substrates of both glycine derivatives. Each thiono ester substrate showed an isotope effect similar to that for the corresponding ester substrate. Moreover, use of the proton inventory technique indicated that, as for esters, one proton is transferred in the transition state for deacylation during reactions involving thiono esters and the degree of heavy atom reorganization in the transition state is very similar in both cases. The k3 values for the hydrolysis of a series of para-substituted N-benzoylglycine esters were found to correlate with the k3 values for the corresponding para-substituted thiono esters [Carey, P. R., Lee, H., Ozaki, Y., & Storer, A. C. (1984) J. Am. Chem. Soc. 106, 8258-8262], showing that the rate-determining step for the deacylation of both thiolacyl and dithioacyl enzymes probably involves the disruption of a contact between the substrate's glycinic nitrogen atom and the sulfur of cysteine-25. It is concluded that the hydrolysis of esters and thiono esters proceeds by essentially the same reaction pathway. Due to an oxygen-sulfur exchange process the product released in the case of the N-(beta-phenylpropionyl)glycine thiono ester substrate is the dioxygen acid; however, for the N-benzoylglycine thiono ester substrate, the thiol acid is the initial product. This thiol acid then acts as a substrate for papain and reacylates the enzyme to eventually give the dioxygen acid product. It is shown that thiol acids are excellent substrates for papain.     

Benzoylamidoacetonitrile is bound as a thioimidate in the active site of papain

13C NMR spectroscopy has been used to demonstrate that 13CN-labeled benzoylamidoacetonitrile forms a covalent adduct with the thiol group of cysteine 25 in the active site of papain. Spectral comparison with model compounds indicates that the adduct is a thioimidate. On the basis of a proposed mechanism for the formation of the thioimidate, it is concluded that the -CH2C(= NH)S--imino nitrogen does not sit in the active site in the same manner as the thiol ester carbonyl oxygen of the thiol acyl enzyme (or the oxyanion of the tetrahedral intermediate). Thus, in this sense the stabilization of the thioimidate does not reflect a similarity in structure between the bound thioimidate and the transition state.     

The tetrahymena ribozyme cleaves a 5'-methylene phosphonate monoester approximately 10(2)-fold faster than a normal phosphate diester: implications for enzyme catalysis of phosphoryl transfer reactions

Single-atom substrate modifications have revealed an intricate network of transition state interactions in the Tetrahymena ribozyme reaction. So far, these studies have targeted virtually every oxygen atom near the reaction center, except one, the 5'-bridging oxygen atom of the scissile phosphate. To address whether interactions with this atom play any role in catalysis, we used a new type of DNA substrate in which the 5'-oxygen is replaced with a methylene (-CH2-) unit. Under (kcat/Km)S conditions, the methylene phosphonate monoester substrate dCCCUCUT(mp)TA4 (where mp indicates the position of the phosphonate linkage) unexpectedly reacts approximately 10(3)-fold faster than the analogous control substrates lacking the -CH2- modification. Experiments with DNA-RNA chimeric substrates reveal that the -CH2- modification enhances docking of the substrates into the catalytic core of the ribozyme by approximately 10-fold and stimulates the chemical cleavage by approximately 10(2)-fold. The docking effect apparently arises from the ability of the -CH2- unit to suppress inherently deleterious effects caused by the thymidine residue that immediately follows the cleavage site. To analyze the -O- to -CH2- modification in the absence of this thymidine residue, we prepared oligonucleotide substrates containing methyl phosphate or ethyl phosphonate at the reaction center, thereby eliminating the 3'-terminal TA4 nucleotidyl group. In this context, the -O- to -CH2-modification has no effect on docking but retains the approximately 10(2)-fold effect on the chemical step. To investigate further the stimulatory influence on the chemical step, we measured the "intrinsic" effect of the -O- to -CH2- modification in nonenzymatic reactions with nucleophiles. We found that in solution, the -CH2- modification stimulates chemical reactivity of the phosphorus center by <5-fold, substantially lower in magnitude than the stimulatory effect in the catalytic core of the ribozyme. The greater stimulatory effect of the -CH2- modification in the active site compared to in solution may arise from fortuitous changes in molecular geometry that allow the ribozyme to accommodate the phosphonate transition state better than the natural phosphodiester transition state. As the -CH2- unit lacks lone pair electrons, its effectiveness in the ribozyme reaction suggests that the 5'-oxygen of the scissile phosphate plays no role in catalysis via hydrogen bonding or metal ion coordination. Finally, we show by analysis of physical organic data that such interactions in general provide little catalytic advantage to RNA and protein phosphoryl transferases because the 5'-oxygen undergoes only a small buildup of negative charge during the reaction. In addition to its mechanistic significance for the Tetrahymena ribozyme reaction and phosphoryl transfer reactions in general, this work suggests that phosphonate monoesters may provide a novel molecular tool for determining whether the chemical step limits the rate of an enzymatic reaction. As methylene phosphonate monoesters react modestly faster than phosphate diesters in model reactions, a similarly modest stimulatory effect on an enzymatic reaction upon -CH2- substitution would suggest rate-limiting chemistry.     

Thioamide substrate probes of metal-substrate interactions in carboxypeptidase A catalysis

Three thioamide peptides in which the oxygen atom of the scissile peptide bond is replaced by sulfur (denoted by (= S)) were synthesized and found to be good, convenient substrates for carboxypeptidase A. The thioamide bond absorbs strongly in the ultraviolet region, and enzymatic hydrolysis is monitored easily using a continuously recording spectrophotometric assay. The reaction follows Michaelis-Menten kinetics with kcat values of 68, 9.0, and 3.7 sec-1 and Km values of 0.83, 0.81, and 0.53 mM for Z-Glu-Phe(= S)-Phe, Z-Gly-Ala(= S)-Phe, and Z-Phe(= S)-Phe, respectively. Activities of the thioamides and their oxygen amide analogs were determined with a series of metal-substituted carboxypeptidases. The Cd(II), Mn(II), Co(II), and Ni(II) enzymes exhibit 30%-35%, 60%-85%, 150%-190%, and 40%-55% of the Zn(II) enzyme activity with the amide substrates; this compares with 240%-970%, 0%-15%, 340%-840%, and 30%-140% of the Zn(II) activity, respectively, with the thioamides. The activity of the Cu(II) and Hg(II) enzymes is less than 3% toward all substrates. Cadmium, a thiophilic metal, yields an enzyme which is exceedingly active with the thioamides; the kcat/Km values are 2.4-9.7-fold higher than with Zn(II) carboxypeptidase. In contrast, Mn(II), which has a relatively low affinity for sulfur, yields an enzyme with correspondingly low activity toward the thioamides. The results are consistent with a mechanism for peptide bond hydrolysis in which the metal atom interacts with the substrate carbonyl atom during catalysis.     

Characterization of the papain active centre by using two-protonic-state electrophiles as reactivity probes. Evidence for nucleophilic reactivity in the un-interrupted cysteine-25-histidine-159 interactive system.

1.2,2'-Dipyridyl disulphide (2-Py-S-S-2-Py) and n-propyl 2-pyridyl disulphide (propyl-S-S-2-Py) were used as two-protonic-state reactivity probes to investigate the active centre of papain (EC 3.4.22.2).2. The existence of a striking rate optimum at pH approx. 4 in the reaction of papain not only with the symmetrical probe but also with the unsymmetrical probe is shown to constitute compelling evidence that the thiolate ion component of the cysteine-25-histidine-159 interactive system of papain possesses appreciable nucleophilic character. It is not a necessary requirement that the probe reagent should engage the imidazolium ion of histidine-159 in hydrogen-bonding for the sulphur atom of the interactive system to display nucleophilic character. The single proton-binding site of propyl-S-S-2-Py cannot simultaneously interrupt the active-centre ion pair and provide for rate enhancement as the pH is lowered towards 4. The possible implication of this for the mechanism of papain-catalysed hydrolysis is discussed. 3. The suspected difference in the active centres of papain and ficin (EC 3.4.22.3), which could be a lack in ficin of a carboxy group conformationally equivalent to that of aspartic acid-158 of papain is confirmed. The reactivity of the papain thiol group towards both probe reagents is controlled by two ionizations with pKa close to 4 that are positively co-operative. 4. In the reaction of papain with 2-Py-S-S-2-Py. the reactivity appears to be controlled also by an addition ionization with pKa approx. 5. Possible origins of this additional ionization are discussed. K. The spectral and ionization characteristics of propyl-S-S-2-Py are reported. 6. The reagent reacts rapidly with thiol groups at the sulphur atom distal from the pyridyl ring to provide, at pH values below 9, stoicheiometric release of 2-thiopyridone. This property, together with the ability of the reagent markedly to increase its electrophilicity consequent on protonation, suggests alkyl-2-pyridyl disulphides in general as valuable two-protonic-state reactivity probes with exceptional specificity for thiol groups.     

Purification and characterization of a novel thiol protease involved in processing the enkephalin precursor

Proteolytic processing enzymes are required to convert the enkephalin precursor to active opioid peptides. In this study, a novel 33-kDa thiol protease that cleaves complete precursor in the form of [35S]methionine preproenkephalin was purified from bovine adrenal medullary chromaffin granules. Chromatography on concanavalin A-Sepharose and Sephacryl S-200, chromatofocusing, and chromatography on thiopropyl-Sepharose resulted in an 88,000-fold purification with a recovery of 35% of enzyme activity. The thiol protease is a glycoprotein with a pI of 6.0. It cleaves [35S]methionine preproenkephalin with a pH optimum of 5.5, indicating that it is functional at the intragranular pH of 5.5-6.0. Interestingly, production of trichloroacetic acid-soluble products was optimal at pH 4.0, suggesting that processing of initial precursor and intermediates may require slightly different pH conditions. The protease requires dithiothreitol for activity and is inhibited by the thiol protease inhibitors iodoacetate, p-hydroxymercuribenzoate, mercuric chloride, and cystatin. These properties distinguish it from other thiol proteases (cathepsins B, H, L, N, and S), indicating that a unique thiol protease has been identified. The enzyme converted [35S]cysteine preproenkephalin (possessing [35S]cysteine residues specifically within the precursor's NH2-terminal segment) to 22.1-, 21.6-, 17.7-, 17.3-, and 15.0-kDa intermediates that contain the precursor's NH2-terminal segment; proenkephalin in vivo is converted to similar intermediates. The enzyme cleaves peptide F at Lys-Arg and Lys-Lys dibasic amino acid sites to generate methionine enkephalin and intermediates. The appropriate vesicular localization, pH optimum, proteolytic products, and cleavage site specificity suggest that this thiol protease may be involved in enkephalin precursor processing. Most interestingly, [35S]methionine beta-preprotachykinin, a precursor of substance P, is minimally cleaved, suggesting that the thiol protease may possess some selectivity for the enkephalin precursor.     

Dependence of the P2-S2 stereochemical selectivity of papain on the nature of the catalytic-site chemistry. Quantification of selectivity in the catalysed hydrolysis of the enantiomeric N-acetylphenylalanylglycine 4-nitroanilides.

1. N-Acetyl-L-phenylalanylglycine 4-nitroanilide and its D-enantiomer were synthesized and characterized and used as substrates with which to evaluate stereochemical selectivity in papain (EC 3.4.22.2)-catalysed hydrolysis. 2. Kinetic analysis at pH 6.0, I 0.1, 8.3% (v/v) NN-dimethylformamide and 25 degrees C by using initial-rate data with [S] much less than Km and weighted non-linear regression provided values of kcat./Km for the catalysed hydrolysis of both enantiomers as (kcat./Km)L = 2040 +/- 48 M-1.S-1 and (kcat./Km)D = 5.9 +/- 0.07 M-1.S-1. These data, taken together with individual values of kcat. and Km for the hydrolysis of the L-enantiomer (a) estimated in the present work as kcat. = 3.2 +/- 1.2 S-1 and Km = 1.5 +/- 0.6 mM and (b) reported by Lowe & Yuthavong [(1971) Biochem. J. 124, 107-115] for the reaction at pH 6.0 in 10% (v/v) NN-dimethylformamide and 35 degrees C, as kcat. = 1.3 +/- 0.2 S-1 and Km = 0.88 +/- 0.1 mM, suggest that (kcat./Km)L congruent to 2000 M-1.S-1 and thus that (kcat./Km)L/(kcat./Km)D congruent to 330.3. Model building indicates that both enantiomeric 4-nitroanilides can bind to papain such that the phenyl ring of the N-acetylphenylalanyl group makes hydrophobic contacts in the S2 subsite with preservation of mechanistically relevant hydrogen-bonding interactions and that the main difference is in the positioning of the beta-methylene group. 4. The dependence of P2-S2 stereochemical selectivity of papain on the nature of the catalytic-site chemistry for reactions involving derivatives of N-acetylphenylalanine is discussed. The variation in the index of stereochemical selectivity (ratio of the appropriate kinetic or thermodynamic parameter for a given pair of enantiomeric ligands), from 330 for the overall acylation process of the catalytic act, through 40 and 31 for the reaction at electrophilic sulphur in 2-pyridyl disulphides respectively without and with assistance by (His-159)-Im(+)-H, to 5 for the formation of thiohemiacetal adducts by reaction at aldehydic carbon, is interpreted in terms of the extent to which conformational variation of the bound ligand in the catalytic-site region permits the binding mode of the -CH2-Ph group of the D-enantiomer to approach that of the L-enantiomer.     

Phosphorylation modulates the activity of glycine N-methyltransferase, a folate binding protein. In vitro phosphorylation is inhibited by the natural folate ligand

Glycine N-methyltransferase (EC 2.1.1.20) was recently identified as a major folate binding protein of rat liver cytosol (Wagner, C., and Cook, R. J. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 3631-3634). Activity of the enzyme is inhibited when the natural folate ligand, 5-methyltetrahydropteroylpentaglutamate (5-CH3-H4PteGlu5), is bound. It has been suggested that glycine N-methyltransferase plays a role in regulating the availability of methyl groups in the liver. Purified transferase was phosphorylated in vitro by the catalytic subunit of cAMP-dependent protein kinase. If 5-CH3-H4PteGlu5 was first bound to the transferase, phosphorylation was inhibited. Phosphorylation of glycine N-methyltransferase in vitro increased its activity approximately 2-fold. 5-CH3-H4PteGlu5 inhibited the activity of newly phosphorylated enzyme as well as native enzyme. Freshly isolated rat hepatocytes incorporated 32P-labeled inorganic phosphate into this folate binding protein. Chemical analysis of purified enzyme showed about 0.55 mol of phosphate present per mol of glycine N-methyltransferase subunit. These results indicate that phosphorylation of glycine N-methyltransferase may provide a mechanism for modulating the activity of this enzyme and support its role in regulating the availability of methyl groups.     

Inhibition of glycine N-methyltransferase by 5-methyltetrahydrofolate pentaglutamate

Glycine N-methyltransferase (EC 2.1.1.20) catalyzes the methylation of glycine by S-adenosylmethionine to form sarcosine and S-adenosylhomocysteine. The enzyme was previously shown to be abundant in both the liver and pancreas of the rat, to consist of four identical monomers, and to contain tightly bound folate polyglutamates in vivo. We now report that the inhibition of glycine N-methyltransferase by (6S)-5-CH(3)-H(4)PteGlu(5) is noncompetitive with regard to both S-adenosylmethionine and glycine. The enzyme exhibits strong positive cooperativity with respect to S-adenosylmethionine. Cooperativity increases with increasing concentrations of 5-CH(3)-H(4)PteGlu(5) and is greater at physiological pH than at pH 9.0, the pH optimum. Under the same conditions, cooperativity is much greater for the pancreatic form of the enzyme. The V(max) for the liver form of the enzyme is approximately twice that of the pancreatic enzyme, while K(m) values for each substrate are similar in the liver and pancreatic enzymes. For the liver enzyme, at pH 7.0 half-maximal inhibition is seen at a concentration of about 0.2 microM (6S)-5-CH(3)-H(4)PteGlu(5), while at pH 9.0 this value is increased to about 1 microM. For the liver form of the enzyme, 50% inhibition with respect to S-adenosylmethionine at pH 7.4 occurs at about 0.27 microM. The dissociation constant, K(s), obtained from binding data at pH 7.4 is 0.095. About 1 mol of (6S)-5-CH(3)-H(4)PteGlu(5) was bound per tetramer at pH 7.0, and 1.6 mol were bound at pH 9.0. The degree of binding and inhibition were closely parallel at each pH. At equal concentrations of (6R,6S)- and (6S)-5-CH(3)-H(4)PteGlu(5), the natural (6S) form was about twice as inhibitory. These studies indicate that glycine N-methyltransferase is a highly allosteric enzyme, which is consistent with its role as a regulator of methyl group metabolism in both the liver and the pancreas.     

Chymopapain A. Purification and investigation by covalent chromatography and characterization by two-protonic-state reactivity-probe kinetics, steady-state kinetics and resonance Raman spectroscopy of some dithioacyl derivatives.

Chymopapain A was isolated from the dried latex of papaya (Carica papaya) by ion-exchange chromatography followed by covalent chromatography by thiol-disulphide interchange. The latter procedure was used to produce fully active enzyme containing one essential thiol group per molecule of protein, to establish that the chymopapain A molecule contains, in addition, one non-essential thiol group per molecule and to recalculate the literature value of epsilon 280 for the enzyme as 36 000 M-1 X cm -1. The Michaelis parameters for the hydrolysis of L-benzoylarginine p-nitroanilide and of benzyloxy-carbonyl-lysine nitrophenyl ester at 25 degrees C, and I 0.1 at several pH values catalysed by chymopapain A, papaya proteinase omega, papain (EC 3.4.22.2) and actinidin (EC 3.4.22.14) were determined. Towards these substrates chymopapain A has kcat./km values similar to those of actinidin and of papaya proteinase omega and significantly lower than those of papain or ficin. The environment of the catalytic site of chymopapain A is markedly different from those of other cysteine proteinases studied to date, as evidenced by the pH-dependence of the second-order rate constant (k) for the reaction of the catalytic-site thiol group with 2,2'-dipyridyl disulphide. The striking bell-shaped component that is a characteristic feature of the reactions of S-/ImH+ (thiolate/imidazolium) ion-pair components of many cysteine-proteinase catalytic sites with the 2,2'-dipyridyl disulphide univalent cation is not present in the pH-k profile for the chymopapain A reaction. The result is consistent with the presence of an additional positive charge in, or near, the catalytic site that repels the cationic form of the probe reagent. Resonance Raman spectra were collected at pH values 2.5, 6.0 and 8.0 for each of the following dithioacyl derivatives of chymopapain A: N-benzoylglycine-, N-(Beta-phenylpropionl)glycine- and N-methoxycarbonylphenylalanylglycine-. The main conclusion of the spectral study is that in each case the acyl group binds as a single population known as conformer B in which the glycinic N atom is in close contact with the thiol S atom of the catalytic-site cysteine residue, as is the case also for papain and other cysteine proteinases studied. Thus the abnormal catalytic-site environment of chymopapain A detected by the reactivity-probe studies, which may have consequences for the acylation step of the catalytic act, does not perturb the conformation of the bound acyl group at the acyl-enzyme-intermediate stage of catalysis.     

The arsonomethyl group as an analogue of phosphate. An X-ray investigation.

The X-ray structure analysis of three compounds of interest as enzyme substrates is reported. They are the hydrated forms of (I) DL-2-amino-4-arsonobutanoic acid [HO-AsO2--CH2-CH2-CH(NH3+)-CO2H], (II) DL-2-amino-4-phosphonobutanoic acid [HO-PO2--CH2-CH2-CH(NH3+)-CO2H] and the hydrated barium salt of (III) D-3-phosphoglycerate [HO-PO2--O-CH2-CH(OH)-CO2-]. The structures were fully refined to R factors of 0.033, 0.053 and 0.046. For the compounds (I) and (II) the charge distribution was directly determined by locating all H atoms. The co-ordination around As and P is approximately tetrahedral, with the valency angle between the two charged O atoms enlarged to 112 degrees in compound (I), 166 degrees in compound (II) and 122 degrees in compound (III). The As-X bond distances are increased relative to P-X to accommodate the increased atomic radius. The analysis establishes that the compounds are structural analogues. Tables of co-ordinates for H atoms, anisotropic thermal parameters, bond lengths and bond angles for the three compounds have been deposited as Supplementary Publication SUP 50122 (5 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained directly [see Biochem J. (1983) 209, 5].     

CCK8 peptide derivatized with diphenylphosphine for rhenium labelling: synthesis and molecular mechanics calculations.

A novel CCK8 derivative bearing a chelating agent at its N- end and its oxo-rhenium(V) complex have been synthesized and characterized. The chelating agent N-[N-13-(diphenylphosphino)propionyl]glycyl]cysteine (PN2S) ligand, the coordination set of which is made by the phosphorus atom of phosphine, the nitrogen atoms of the two amido groups and the sulphur atom of cysteine, has been used due to its high affinity towards the oxo-rhenium(V) moiety. Molecular modelling studies indicate that the CCK8 peptide adopts the right conformation for cholecystokinin receptor binding, and that modifications on the N-terminal side of CCK8 obtained by introducing chelating agents and its metal complexes should not affect the interaction with CCK(A) receptor.     

Elimination of a reactive thiol group from the active site of chloramphenicol acetyltransferase.

1. The type III variant of chloramphenicol acetyltransferase (CATIII) is resistant to inactivation by ionizable modifying reagents such as 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and iodoacetate, whereas it is sensitive to inhibition by similar but uncharged reagents, including 4,4'-dithiodipyridine, methyl methanethiolsulphonate (MMTS) and iodoacetamide. The target for these thiol-modifying reagents has been postulated to be Cys-31. This residue is situated within a part of the chloramphenicol-binding site formed largely from the side chains of hydrophobic amino acid residues, which might be expected to discriminate against the access of ionized ligands to Cys-31. 2. The substitution of Cys-31 by alanine, serine, threonine or methionine yields an enzyme that is resistant to inactivation by thiol-specific reagents. Replacement of Cys-31 by alanine, serine or threonine results in increased Km values for chloramphenicol with only small changes in kcat.. In contrast, the Cys-31----Met substitution mainly affects kcat. values. Although the kcat. for chloramphenicol acetylation is decreased 13-fold compared with wild-type CAT, the kcat. for the acetyl-CoA hydrolysis reaction, which occurs in the absence of chloramphenicol, is increased 2.7-fold. 3. MMTS modification of cysteine residues results in an adduct (-CH2-S-S-CH3) that is structurally similar to the side chain of a methionine residue (-CH2-CH2-S-CH3). The kinetic properties of MMTS-modified CATIII closely resemble those of [Met31]CAT.     

Studies on conformational transitions of Ca2+,Mg2+-adenosine triphosphatase of sarcoplasmic reticulum. II. Conformational characteristics of stabilized reaction intermediates as revealed by fluorescent and paramagnetic probes

Various reaction intermediates of sarcoplasmic reticulum Ca2+,Mg2+-ATPase were stabilized and accumulated by modifying a specific SH group or by using nucleotide analogs. Conformational changes of the Ca2+,Mg2+-ATPase during the catalytic cycle were studied in the stabilized intermediates by the use of fluorescent and spin probes, which were introduced at specific SH groups of ATPase, namely one highly reactive but functionally nonessential (SHN) and one essential for the decomposition of the E-P intermediate (SHD) [Kawakita, M., et al. (1980) J. Biochem. 87, 609-617]. The fluorescence intensity of N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide attached to SHD decreased by 2.5% upon addition of 10 microM AMP-P(NH)P provided that Ca2+ was also present. The AMP-P(NH)P-induced fluorescence change could also be detected by using other fluorescent probes such as N-[p-(2-benzimidazolyl)phenyl]maleimide and N-(1-anilinonaphthyl-4)maleimide. Moreover, labeling at SHN gave similar results. When SHN was labeled with N-[p-(2-benzimidazolyl)phenyl]maleimide, the fluorescence intensity also decreased by 2.5% upon addition of ATP only in the presence of Ca2+, where E-P formation took place. A conformational difference between ECa1-P X ADP and ECa1-P was suggested from saturation transfer ESR measurement of spin-labeled ATPase by using ADP beta S as an ADP analog to cause accumulation of ECa1-P X ADP beta S complex. Possible structural similarities among some of the intermediates are discussed based on these findings.     

Molecular mechanism for the initial process of visual excitation. IV. Energy surfaces of visual pigments and photoisomerization mechanism.

Using the twisted conformations of the chromophores for visual pigments and intermediates which were theoretically determined in the previous paper, energy surfaces of the pigment at - 190 degrees C were obtained as functions of the torsional angles theta 9-10 and theta 11-12 or of the torsional angles theta 9-10 and theta 13-14. In these calculations, the existence of specific reaction paths between rhodopsin (R) and bathorhodopsin (B), between isorhodopsin I (I) and bathorhodopsin, and between isorhodopsin II (I') and bathorhodopsin were assumed. It was shown that the total energy surfaces of the excited states had minima C1 at theta 9-10 approximately -10 degrees and theta 11-12 approximately -80 degrees, C2 at theta 9-10 approximately -85 degrees and theta 11-12 approximately -5 degrees, and C3 at theta 9-10 approximately -0 degree and theta 13-14 approximately -90 degrees. These minima are considered to correspond to the thermally barrierless common states as denoted by Rosenfeld et al. Using the total energy surfaces in the ground and excited states, the molecular mechanism of the photoisomerization reaction was suggested. Quantum yields for the photoconversions among R, I, I' and B were related to the rates of vibrational relaxations, radiationless transitions and thermal excitations. Some discussion was made of the temperature effect on the quantum yield. Similar calculations of the energy surfaces were also made at other temperatures where lumirhodopsin or metarhodopsin I is stable. Relative energy levels of the pigments and the intermediates were discussed.