Cleavage of type I and type II procollagens by type I/II procollagen N-proteinase. Correlation of kinetic constants with the predicted conformations of procollagen substrates.
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The kinetic constants were examined for the cleavage of several types of procollagen by type I/II procollagen N-proteinase. The Km values were essentially the same (0.2 microM) for chick type I procollagen, human type I procollagen, and chick type II procollagen. However, the Vmax values differed over a 14-fold range. As reported previously, the enzyme did not cleave denatured type I or II procollagen. Also, it did not cleave human type III procollagen which contains the same scissle -Pro-Gln- bond as the pro-alpha 1(I) chain of type I procollagen. To explain the observations, Chou-Fasman rules were used to compare the secondary structures of the cleavage sites in the procollagens. The results supported a previous suggestion (Helseth, D. L., Jr., Lechner, J. L., and Veis, A. (1979) Biopolymers 18, 3005-3014) that the region carboxyl-terminal to cleavage site in the pro-alpha 1(I) chain of type I procollagen was in a hairpin conformation consisting of a beta-sheet, beta-turn, and beta-sheet. In both chick and human type I procollagen, the hairpin loop in the pro-alpha 1(I) chain consisted of about 18 amino acids. The cleavage site itself was in a short alpha-helical structure of four or five amino acids. The pro-alpha 2(I) chains had a similar hairpin loop of about 14 amino acids and alpha-helix of four or five amino acids containing the cleavage site. Chick type II procollagen, which had the highest Vmax value, had a longer hairpin structure of 22 amino acids, and the cleavage site was in a longer alpha-helical domain of 10 amino acids. In contrast, type III procollagen had a random-coil conformation in the same region. The results help to explain the unusual substrate requirements of type I/II N-proteinase. They also help explain why mutations that produce in-frame deletions of amino acids 84 or more residues carboxyl-terminal to the cleavage site make the protein resistant to the enzyme.
