H-Pro-Thr-Glu-Phe-p-nitro-Phe-Arg-Leu-OH

Variation in the kinetic parameters, kcat and Km, with pH has been used to obtain evidence for significant acid-dissociation processes in the hydrolysis of octapeptide substrates by three aspartic proteinases. These substrates are all cleaved at the peptide bond between a Phe (P1) and a p-nitroPhe (P1') residue resulting in a shift in absorbance at 300 nm that facilitates kinetic measurements. The substrates differ in the amino-acid residues present in the P3 and the P2 positions. Porcine pepsin, calf chymosin, and the aspartic proteinase from Endothia parasitica all show pH dependencies that imply that favorable or unfavorable interactions can occur with the S3 or S2 areas of the enzyme-active site. Examination of the crystallographically determined structure of the E. parasitica proteinase and consideration of the amino-acid sequence differences between the three enzymes suggests that the origin of the pH effects arises from favorable interactions between Glu-13 (COO-) of pig pepsin and Thr (OH) or His (ImH+) in P3 of a substrate. Similarly, Lys-220 (NH3+) of chymosin and a Glu (COO-) in P2 of a substrate may produce a favorable interaction and Asp-77 (COO-) of E. parasitica proteinase and a Glu (COO-) in P2 of a substrate may produce an unfavorable interaction. These results lead to possible explanations for subtle specificity differences within a family of homologous enzymes, and suggest loci for study by site-directed mutagenesis.

Human plasma fibronectin contains a latent proteinase that after activation cleaves gelatin and fibronectin. The autoactivation propensity of the two purified cathepsin D-produced fragments of fibronectin (190 and 120 kDa) was compared. Both polypeptides were spontaneously activated in the presence of Ca2+. This activation was inhibited by EDTA. The active gelatinase was isolated from the autodigest of the 190-kDa fragment. Among various protein substrates, including laminin and native type I and IV collagens, the purified enzyme degraded only gelatin and fibronectin. We have named this proteinase FN-gelatinase. FN-gelatinase is inhibited by phenylmethanesulfonyl fluoride and also by pepstatin A like retroviral aspartic proteinases. The amino-acid composition of the purified enzyme (35 kDa) was compared with the entire fibronectin sequence using the computer programme FIT. The optimal fit indicated that the 35-kDa fragment corresponds to the stretch # 1043-1404. This sequence contains a 93-residue segment (# 1140-1233) analogous to retroviral aspartic proteinases, comprising the sequence DTG of their putative active site.

Fibronectin contains two latent gelatinolytic enzymes, FN-gelatinase and FN-laminase that can be activated in the presence of Ca2+ from the purified cathepsin D-produced 190-kDa fibronectin fragment. The results of this work show that Achromobacter collagenase cleaves fibronectin and generates an active FN-gelatinase. In contrast to the cathepsin D digest, the collagenase digest directly exhibits gelatinolytic activity without additional activation. The gelatinolytic activity of the total collagenase digest can be inhibited by phenylmethanesulfonyl fluoride, a serine proteinase inhibitor and by pepstatin A, an aspartic-acid proteinase inhibitor. FN-laminase activity, when assayed with its synthetic substrate GPAGPR and also with laminin was revealed after separation of the collagenase digest of fibronectin on heparin Ultrogel. FN-gelatinase and FN-laminase activities were found in heparin unretained and heparin strongly retained fractions. These results have demonstrated that in contrast to cathepsin D, Achromobacter collagenase activates two matrix-degrading proteinases from fibronectin, FN-Gelatinase und FN-Laminase.