Primary structure of human pancreatic elastase 2 determined by sequence analysis of the cloned mRNA

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Abstract
A cDNA encoding elastase 2 has been cloned from a human pancreatic cDNA library. The cDNA contains a translation initiation site and a poly(A) recognition site and encodes a protein of 269 amino acids, including a proposed 16-residue signal peptide. The amino acid sequence of the deduced mature protein contains a 12-residue activation peptide containing a cysteine at residue 1 similar to that of chymotrypsin. The proposed active enzyme contains all of the characteristic active-site amino acids, including His-57, Asp-102, and Ser-195. The S1 binding pocket is bounded by Gly-216 and Ser-226, making this pocket intermediate in size between chymotrypsins and elastase 1 or protease E, consistent with the substrate specificity of elastase 2 for long-chain aliphatic or aromatic amino acids. Computer modeling studies using the amino acid sequence of elastase 2 superimposed on the X-ray structure of porcine elastase 1 suggest that a change of Gln-192 in elastase 1 to Asn-192 in elastase 2 may account for the lower catalytic efficiency of the latter enzyme. In addition, modeling studies have been conducted to attempt to identify basic amino acids in elastases which are absent in chymotrypsins, and which could account for the specific property of elastolysis. Several basic residues appear to be near the ends of the extended binding pocket of elastases which might serve to anchor the enzyme to the elastin substrate. These studies indicate that elastases 2 and elastase 1 both contain an Arg-65A as well as a basic dipeptide at 223/224 which is not present in chymotrypsins. In addition, Arg-217A is present in human elastase 2 but absent in a rat pancreatic protein which has been proposed to be an elastase 2 on the basis of sequence homology [MacDonald, R. J., Swift, G. H., Quinto, C., Swain, W., Pictet, R. L., Nikovits, W., and Rutter, W. J. (1982) Biochemistry 21, 1453-1463], but which was not isolated during screening of rat pancreatic tissue extracts for elastolytic activity [Largman, C. (1983) Biochemistry 22, 3763-3770].