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 (k cat , 5.41 s ؊1 ; K m , 32 M) and Met-Gly-ProArg*Gly-Phe-Pro-Gly-Ser (k cat , 351 s ؊1 ; K m , 214 M), 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 been defined.Collagen is an insoluble structural protein that accounts for approximately 30% of the total weight of animal proteins and is produced in large quantities as a by-product in livestock industries. It has recently been shown that the enzymatic degradation of collagen as well as that of gelatin, a denatured form of collagen, allows efficient utilization of these structural proteins. This degradation produces peptides, the collagen peptides, which have been shown to have several biological activities of industrial and medical interest (27), leading to the establishment of a wide variety of applications, e.g., an immunotherapeutic agent (7, 9), a moisturizer for cosmetics, a preservative (2), and seasoning and dietary materials (4).Generally, the enzymatic degradation of collagen is not affected by ordinary digestive proteinases but requires the collagen-specific, Zn 2ϩ -dependent metalloproteinases (collagenases). Although microbial collagenases have been found in a wide variety of mesophilic bacterial strains (3), the industrialscale application of known bacterial collagenases for collagen peptide production has been hampered because of their insufficient stability. In 2000...