Kinetic parameters of metal-substituted leucine aminopeptidase from bovine lens

Leucine aminopeptidase (LAP) is a protease requiring two divalent metal cations per subunit for activity. Zn2+, Mg2+, and Co2+ metal-substituted forms of LAP have been prepared and investigated kinetically. Substitution of metal into the two binding sites independently resulted in the preparation of Zn2+Zn2+, Mg2+Zn2+, Co2+Co2+, Zn2+Co2+, Mg2+Co2+, and Co2+Zn2+ LAP derivatives that were characterized by atomic absorption spectrophotometry. Kinetic analysis of the metal-substituted enzymes indicated that site 1 (fast exchanging) metal substitution results in a Km decrease in the relative order Zn2+ greater than Mg2+ greater than Co2+. Similar comparisons for the site 2 metal (slow exchanging) involved only Zn2+ and Co2+, since only these metals have been shown to compete effectively for this site. Substitution of these two metals into site 2 revealed a Km decrease in the order Zn2+ greater than Co2+. It was suggested previously [e.g., Thompson, G. A., & Carpenter, F. H. (1976) J. Biol. Chem. 251, 1618-1624] that the fast-exchanging site 1 metal predominantly effects kcat while the slow-exchanging metal in site 2 exerts effects exclusively on Km. The present study, the first direct comparison of Km change resulting from metal substitution into both sites, clearly indicates that both metal sites exert significant effects on Km. In addition, the data suggest a more complex interaction between the two bound metals than previously suspected.