[Show abstract][Hide abstract] ABSTRACT: Kumamolisin-As is an acid collagenase with a subtilisin-like fold. Its active site contains a unique catalytic triad, Ser278-Glu78-Asp82, and a putative transition-state stabilizing residue, Asp164. In this study, the mutants D164N and E78H/D164N were engineered in order to replace parts of the catalytic machinery of kumamolisin-As with the residues found in the equivalent positions in subtilisin. Unlike the wild-type and D164N proenzymes, which undergo instantaneous processing to produce their 37-kDa mature forms, the expressed E78H/D164N proenzyme exists as an equilibrated mixture of the nicked and intact forms of the precursor. X-ray crystallographic structures of the mature forms of the two mutants showed that, in each of them, the catalytic Ser278 makes direct hydrogen bonds with the side chain of Asn164. In addition, His78 of the double mutant is distant from Ser278 and Asp82, and the catalytic triad no longer exists. Consistent with these structural alterations around the active site, these mutants showed only low catalytic activity (relative k(cat) at pH 4.0 1.3% for D164N and 0.0001% for E78H/D164N). pH-dependent kinetic studies showed that the single D164N substitution did not significantly alter the logk(cat) vs. pH and log(k(cat)/Km) vs. pH profiles of the enzyme. In contrast, the double mutation resulted in a dramatic switch of the logk(cat) vs. pH profile to one that was consistent with catalysis by means of the Ser278-His78 dyad and Asn164, which may also account for the observed ligation/cleavage equilibrium of the precursor of E78H/D164N. These results corroborate the mechanistic importance of the glutamate-mediated catalytic triad and oxyanion-stabilizing aspartic acid residue for low-pH peptidase activity of the enzyme.
[Show abstract][Hide abstract] ABSTRACT: Kumamolisin-As (previously called ScpA) is the first known example of a collagenase from the sedolisin family (MEROPS S53). This enzyme is active at low pH and in elevated temperatures. In this study that used x-ray crystallographic and biochemical methods, we investigated the structural basis of the preference of this enzyme for collagen and the importance of a glutamate residue in the unique catalytic triad (Ser(278)-Glu(78)-Asp(82)) for enzymatic activity. Crystal structures of the uninhibited enzyme and its complex with a covalently bound inhibitor, N-acetyl-isoleucyl-prolyl-phenylalaninal, showed the occurrence of a narrow S2 pocket and a groove that encompasses the active site and is rich in negative charges. Limited endoproteolysis studies of bovine type-I collagen as well as kinetic studies using peptide libraries randomized at P1 and P1', showed very strong preference for arginine at the P1 position, which correlated very well with the presence of a negatively charged residue in the S1 pocket of the enzyme. All of these features, together with those predicted through comparisons with fiddler crab collagenase, a serine peptidase, rationalize the enzyme's preference for collagen. A comparison of the Arrhenius plots of the activities of kumamolisin-As with either collagen or peptides as substrates suggests that collagen should be relaxed before proteolysis can occur. The E78H mutant, in which the catalytic triad was engineered to resemble that of subtilisin, showed only 0.01% activity of the wild-type enzyme, and its structure revealed that Ser(278), His(78), and Asp(82) do not interact with each other; thus, the canonical catalytic triad is disrupted.
[Show abstract][Hide abstract] ABSTRACT: Enzymatic degradation of collagen produces peptides, the collagen peptides, which show a variety of bioactivities of industrial
interest. Alicyclobacillus sendaiensis strain NTAP-1, a slightly thermophilic, acidophilic bacterium, extracellularly produces a novel thermostable collagenolytic
activity, which exhibits its optimum at the acidic region (pH 3.9) and is potentially applicable to the efficient production
of such peptides. Here, we describe the purification to homogeneity, characterization, gene cloning, and heterologous expression
of this enzyme, which we call ScpA. Purified ScpA is a monomeric, pepstatin-insensitive carboxyl proteinase with a molecular
mass of 37 kDa which exhibited the highest reactivity toward collagen (type I, from a bovine Achilles tendon) among the macromolecular
substrates examined. On the basis of the sequences of the peptides obtained by digestion of collagen with ScpA, the following
synthetic peptides were designed as substrates for ScpA and kinetically analyzed: Phe-Gly-Pro-Ala*Gly-Pro-Ile-Gly (kcat, 5.41 s−1; Km, 32 μM) and Met-Gly-Pro-Arg*Gly-Phe-Pro-Gly-Ser (kcat, 351 s−1; Km, 214 μM), where the asterisks denote the scissile bonds. The cloned scpA gene encoded a protein of 553 amino acids with a calculated molecular mass of 57,167 Da. Heterologous expression of the scpA gene in the Escherichia coli cells yielded a mature 37-kDa species after a two-step proteolytic cleavage of the precursor protein. Sequencing of the scpA gene revealed that ScpA was a collagenolytic member of the serine-carboxyl proteinase family (the S53 family according to
the MEROPS database), which is a recently identified proteinase family on the basis of crystallography results. Unexpectedly,
ScpA was highly similar to a member of this family, kumamolysin, whose specificity toward macromolecular substrates has not