Nuclear-magnetic-resonance studies of carboxypeptidase B. Binding of inhibitors to the manganese enzyme

Purification and partial characterization of a neutral protease from a virulent strain of Bacillus cereus

The factors involved in the pathogenesis of Bacillus cereus (B. cereus) in non-gastrointestinal diseases are poorly investigated. Some researchers suggest that B. cereus proteases may be involved in these illnesses. The aim of this work was to purify and characterize a protease isolated from a virulent strain of B. cereus to explain its assumptive damaging effect. The enzyme was purified in a four-step procedure involving ammonium sulfate fractionation, acetone precipitation, Bio-Gel filtration and column chromatography on DEAE-cellulose (DE-52 cellulose). The enzyme appeared homogenous using disc electrophoresis. The specific activity of the protease was 72 U/mg of protein. The enzyme was shown to have a relative molecular mass of 29 kDa. The protease was most active at pH 7.0 and 40 degrees C with haemoglobin as the substrate. The enzyme was made completely inactive by ethylenediaminetetraacetic acid (EDTA), beta-mercaptoethanol, dithiothreitol (DTT) and benzamidine (at a concentration of 1 mM) and by diisopropylfluorophosphate (DIPF), L-cysteine, L-histidine, 1,10-phenanthroline (at a concentration of 10 mM). Divalent cations, especially Ca2+ increased enzyme activity. The enzyme hydrolysed haemoglobin, albumin and casein as the substrates. With haemoglobin and albumin as the substrates Michaelis-Menten kinetics was observed. The obtained Km values were 86 +/- 40 microM (SD, n = 3) and 340 +/- 100 microM (SD, n = 3) for haemoglobin and albumin, respectively. The corresponding Vmax values were 1.26 +/- 0.1 (SD, n = 3) and 0.38 +/- 0.07 (SD, n = 3) mumol of tyrosine liberated per min, per ml, and per mg, while those for casein were not determined. It is concluded that this enzyme is a metal-chelator-sensitive, neutral protease damaging haemoglobin and albumin.

Ostrich (Struthio camelus) carboxypeptidase B: purification, kinetic properties and characterization of the pancreatic enzyme

Carboxypeptidase B has been isolated from numerous mammalian and invertebrate species. In contrast, very little is known about carboxypeptidases of avian origin. To provide information for a comparative study, we have undertaken an investigation of the kinetic and physical properties of ostrich carboxypeptidase B. Carboxypeptidase B from the pancreas of the ostrich was purified by water extraction of acetone powder and aminobenzylsuccinic acid affinity and hydroxylapatite chromatography. The effects of pH and temperature on CPB activity were examined. K(i)-values for numerous inhibitors (PCI, ABSA, hipp-D-lys, epsilon-aminocaproic acid, D-arg and 3-phenylproprionic acid) and kinetic parameters (K(m), k(cat) and k(cat)/K(m)) for several substrates (hipp-arg, hipp-lys, FAAA, FAAL and hipp-AA) were determined. N-terminal sequencing and amino acid analysis were also performed. Purified ostrich carboxypeptidase B was assessed to be homogeneous by SDS-PAGE with a M(r) value of approx. 35,000. For ostrich carboxypeptidase B the K(m) values for the different substrates were of the same order as those reported for other species, whereas the k(cat) values were 8- to 21-fold lower than the reported values. FAAA and hipp-AA were the preferred substrates. PCI was the most effective inhibitor, with a K(i) in the nM region, and no inhibition was shown with 3-phenylpropionic acid. The N-terminal sequence showed a high degree of homology when aligned with CPB from other species. Amino acid analysis showed significantly lower levels of Asx and Cyh and higher levels of Trp and Leu when compared with other species. Ostrich carboxypeptidase B would appear to show many physical, chemical and kinetic properties similar to those of other known carboxypeptidases.

Metal ion effects on target sites of modification in metallocarboxypeptidase B

Native carboxypeptidase B and its Co2+-substituted derivative were oxidized by the active-site-directed agent m-chloroperbenzoic acid. The following results were obtained a) In the cobalt enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Peptide or ester pseudo-substrates protected the cobalt enzyme but not the zinc enzyme against inactivation. b) Upon oxidation and formation of Co3+, cleavage of peptide bonds occurred in the cobalt enzyme but not in the zinc enzyme. Both enzymes retained their original metal content. c) Following oxidation of the enzymes, amino acid analysis revealed a modification of a methionyl residue in the zinc enzyme only; the cobalt enzyme, on the other hand, showed a modification of a histidyl residue. d) Peptide mapping of the enzymes after cleavage by cyanogen bromide indicated that two methionyl peptides were missing in the oxidized zinc enzyme. These peptides point to Met-64 as the site of modification. The peptide map of the oxidized cobalt enzyme was similar to that of the unmodified native (i.e., zinc) enzyme. These studies indicate that the specific metal ion present in the enzyme imposes certain structural and functional differences on the active site, leading to differing reactivities of specific amino acid residues and to a different alignment of the active-site-directed reagent in the two enzymes.

Mechanistic implications of cyanide binding to carboxypeptidase B

The putative metal coordinating ligand cyanide was used to study the effects of modifications of the metal coordination sphere on the spectral properties and catalytic activity of cobalt and zinc carboxypeptidases. The absorption spectra of Co2+-carboxypeptidase B in the presence of cyanide pointed to a direct interaction of the ligands with the metal. Gel-filtration experiments showed that the binding of one mole of ligand per mole of enzyme metal ion resulted in maximal spectral effects. Binding of cyanide to the metal ion as measured by absorption spectroscopy was inhibited by acetyl-L-arginine, a peptide pseudosubstrate, and by acetyl-D-arginine, a competitive peptide inhibitor. Addition of acetyl arginine to the enzyme-cyanide complex caused displacement of the ligand, as evidenced by the spectral parameters. Cyanide inhibited peptide hydrolysis in a partially noncompetitive manner, i.e. it did not prevent binding of the substrate to the enzyme but the enzyme-substrate-cyanide complex was hydrolyzed at a slower rate than the enzyme-substrate complex. The dissociation constant evaluated from kinetic studies for the binding of cyanide to Co2+-carboxypeptidase B was in good agreement with that obtained from spectral measurements. Hydrolysis of the ester analog of the basis peptide substrate was not affected by cyanide. Based on these data a model is proposed in which the peptide carboxyl group displaces the water molecule from the metal coordination sphere during catalysis without increasing the coordination number.