Thermolysin: kinetic study with oligopeptides

An interactive computer graphics study of thermolysin-catalyzed peptide cleavage and inhibition by N-carboxymethyl dipeptides

Interactive computer graphics was used as a tool in studying the cleavage mechanism of the model substrate Z-Phe-Phe-Leu-Trp by the zinc endopeptidase thermolysin. Two Michaelis complexes and three binding orientations of the tetrahedral intermediate to the crystal structure of thermolysin were investigated. Our results indicate that a Michaelis complex, which does not involve coordination of the scissile peptide to the zinc, is consistent with available experimental data and the most plausible of the two complexes. A tetrahedral intermediate complex wherein the two oxygens of the hydrated scissile peptide straddle the zinc in a bidentate fashion results in the most favorable interactions with the active site. The preferred tetrahedral intermediate and Michaelis complex provide a rationalization for the published substrate data. A trajectory for proceeding from the Michaelis complex to the tetrahedral intermediate is proposed. This trajectory involves a simultaneous activation of the zinc-bound water molecule concurrent with attack on the scissile peptide. A detailed ordered product release mechanism is also presented. These studies suggest some modifications and a number of extensions to the mechanism proposed earlier [Kester, W. R., & Matthews, B. W. (1977) Biochemistry 16, 2506; Holmes, M. A., & Matthews, B. W. (1981) Biochemistry 20, 6912]. The binding mode of the thermolysin inhibitor N-(1-carboxy-3-phenylpropyl)-L-leucyl-L-tryptophan [Monzingo, A. F., & Matthews, B. W. (1984) Biochemistry (preceding paper in this issue)] is compared with that of the preferred tetrahedral intermediate, providing insight into this inhibitor design.

Pressure dependence of thermolysin catalysis

A comparison of the pressure and temperature dependences of the catalytic reaction of thermolysin, a thermostable neutral protease from Bacillus thermoproteolyticus, with those of a non-thermostable neutral protease from Bacillus subtilis revealed a distinct difference in Km values of these enzymes for 3-(2-furyl)acrylyl-blocked dipeptide and tripeptide substrates, but not for Kcat. Namely, the volume changes for the binding process (delta V) for these substrates and several competitive inhibitors were -20- -30 ml/mol for thermolysin and nearly 0 ml/mol for the non-thermostable neutral protease. The enthalpy and entropy changes for the binding process were negative for thermolysin, but positive for the latter enzyme. The activation volumes (delta V not equal to) for the kcat process were 25 -35 ml/mol for both proteases, and activation enthalpy and entropy showed no significant difference between the two enzymes. The characteristic difference in the pressure and the temperature dependences seen for the binding process is discussed in relation to the thermostability of the proteases.

Inhibition of metalloendopeptidases by 2-mercaptoacetyl-dipeptides

A series of 2-mercaptoacetyl-dipeptides, a potential group of metalloendopeptidase inhibitors, has been synthesized by coupling the N-hydroxysuccinimide ester of S-acetyl-2-mercaptoacetic acid with hydrophobic dipeptide methyl ester hydrochlorides, followed by hydrolysis with NaOH in aqueous methanol and acidification with HCl. Thus, the 2-mercaptoacetyl derivatives of L-phenylalanyl-L-leucine, L-leucyl-L-phenylalanine and L-leucyl-D-phenylalanine were prepared. The first two compounds inhibit effectively thermolysin from Bacillus thermoproteolyticus and a metalloendopeptidase isolated from Streptomyces griseus, with Ki values in the micromolar range or below. The third compound inhibits the two enzymes only poorly, showing the stereospecificity of the inhibition process. These inhibitors should provide a useful tool for the study of bacterial and mammalian metalloendopeptidases (or dipeptidyl carboxypeptidases) and for the assessment of their physiological role.

Thermolysin-catalyzed peptide bond synthesis

The rates of the thermolysin-catalyzed synthesis of peptides have been determined by means of HPLC. In the condensation of various N-substituted amino acids and peptides with L-leucinanilide, the enzyme exhibits preference for a hydrophobic L-amino acid as the donor of the carbonyl group of the newly formed bond. The presence of another hydrophobic amino acid residue adjacent to the carbonyl-group donor markedly enhances the rate of synthesis. In general, the effect of structural changes in both the carboxyl and amine components of the condensation reaction is in accord with the available data on the primary and secondary specificities of the thermolysin-catalyzed hydrolysis of oligopeptide substrates. A kinetic study of the condensation of benzyloxycarbonyl-L-phenylalanine with various amine components has given data on the apparent kcat and Km values for the entry of the acidic component into the condensation reaction. The results are consistent with the behavior of rapid-equilibrium random bi-reactant systems leading to ternary enzyme-substrate complexes, with a synergistic effect in the binding of the two reactants at the active site. Because the changes in the apparent kcat for the entry of the same acidic component into reaction with different amine components are greater than those in the apparent Km, it is suggested that this synergism is largely expressed at the level of the transition-state complex.

pH and temperature dependences of thermolysin catalysis. Catalytic role of zinc-coordinated water

The pH dependences of kcat/Km, kcat and Km of thermolysin-catalyzed hydrolysis of N-furylacryloylglycyl-L-leucinamide (Fua-Gly-LeuNH2) and N-furylacryloylglycyl-L-phenylalaninamide (Fua-Gly-PheNH2) were investigated. Taking the buffer dependences into account, the kcat/Km profile was explained by a simple bell-shaped curve with pKa1 = 5.0 and pKa2 = 8.25, at 25 degrees C. Both kcat and Km increased with pH at lower pH and took larger values for Fua-Gly-LeuNH2 than for Fua-Gly-PheNH2 at 25 degrees C. The pH dependence of inhibitory actions by amino acids and dipeptides, such as carbobenzyloxy-L-phenylalanine and L-phenylalanyl-L-leucinamide, showed characteristic features depending on their charge states: anionic or neutral ones inhibited the enzyme more strongly at lower pH while cationic ones did so at neutral pH. Temperature dependences of kcat/Km, Ki and the two pKa values in the kcat/Km profile were measured. The kcat/Km showed non-linear dependence while Ki increased linearly with temperature on a logarithmic scale. The calculated delta H values of deprotonation for pKa1 and pKa2 were 33.4 kJ/mol and 35.1 kJ/mol, respectively. The value for pKa1 is too large to be assigned to the carboxylic group of Glu-143, in contrast to the generally accepted view. A mechanism for thermolysin catalysis is presented with particular emphasis on the binding specificity and the catalytic role of zinc-coordinated water.

Hydrolysis of beta-casein and peptides by intracellular neutral protease of Streptococcus diacetylactis

The endopeptidase activity of mesophilic streptococci was characterized further by investigating the specificity of an intracellular endopeptidase from Streptococcus diacetylactis for beta-casein, derived peptides, and bradykinin. The inhibitory action of phosphoramidon as well as direct determinations of metal content showed this enzyme was a metalloprotein. Hydrolysis of native beta-casein was relatively low. Peptides obtained from the fraction soluble at pH 4.6 led to the demonstration that Pro186-Ile187 and Ala189-Phe190 were hydrolyzed by the enzyme. Two peptides derived from beta-casein by the action of chymosin were hydrolyzed efficiently: we observed hydrolysis of Lys176-Ala177, Lys169-Val170, and Pro206-Ile207. The Pro7-Phe8 bond of bradykinin was hydrolyzed rapidly, showing that this enzyme was efficient for the hydrolysis of prolyl peptide bonds. The protease was slightly less sensitive to phosphoramidon than was thermolysin. Metal analyses showed the enzyme contained 580 microgram of zinc and 4,760 microgram of calcium per gram protein. This protease is thus a true metalloenzyme (E.C.3.4.24.4), and its action may complete the hydrolysis initiated by chymosin remaining active in cheese curd by hydrolyzing peptides released by chymosin.

[Specificity of thermolysin action on dog myoglobin]

The proteolytic specificity of thermolysin has been studied by quantitative analysis of an enzymic digest of dog myoglobin. Results confirm main specificities of thermolysin towards Phenylalanine, Isoleucine, Leucine or Tyrosine bonds; the influence of neighbourhood was also determined and the conclusions are in a good agreement with the known structure of the active site of thermolysin.

Studies on the inhibition of thermolysin

Thei nhibition of the thermolysin catalyzed hydrolysis of FA-Gly-Leu-NH2 and FA-Gly-Phe-NH2 has been reported. The results suggest a model for substrate and inhibitor binding involving the hydrophobic specificity pocket, Arg-203 and Glu-143.

Superactivation of thermolysin by acylation with amino acid N-hydroxysuccinimide esters

Synthesis of a series of active N-hydroxysuccinimide esters of aliphatic and aromatic amino acids has yielded a new class of reagents for the covalent modification of proteolytic enzymes such as thermolysin. The activities of aliphatic acyl amino acid thermolysins are from 1.7 to 3.6 times greater than that of the native enzyme when hydrolyzing durylacryloyl-Gly-Leu-NH2, the substrate employed most widely. By comparison, the aromatic acylamino acid derivatives are "superactive," their activities being as much as 70-fold greater. Apparently, the aromatic character of the amino acid introduced is a critical variable in the determination of the functional response. The increased activity is completely restored to that of the native enzyme by deacylation with nucleophiles, such as hydroxylamine, and the rate of restoration of native activity is a function of the particular acyl group incorporated. Preliminary evidence regarding the chemical properties of the modified enzyme suggests that tyrosine, rather than lysine, histidine, or arginine, may be the residue modified. The functional consequences of successive modification with different reagents, moreover, indicate that each of them reacts with the same protein residue. The competitive inhibitors beta-phenyl-propionyl-Phe and Zn-2+ do not prevent modification with these active esters. Hence, the site(s) of their inhibitory action differ(s) from that at which modification occurs. The structure of the substrate is also a significant variable which determines the rate at which each acyl amino acid thermolysin hydrolyzes peptides. Depending on the particular substrate, the activity of aromatic derivatives can be as much as 400-fold greater than that of the native enzyme, and the resultant activity patterns can be ordered in a series characteristic for each enzyme derivative.