NATURE OF ACTIVE SITE OF A SUBUNIT OF FIRST COMPONENT OF HUMAN COMPLEMENT

A highly purified subunit of human complement (Cl̅s) was found to hydrolyze the amino acid ester, N-carbobenzoxy-L-tyrosine p-nitrophenyl ester. The Km of N-carbobenzoxy-L-tyrosine p-nitrophenyl ester for the purified enzyme was 5.6 × 10-5 m, the Vmax was 2.19 × 10-6 mmole/min, and the kcat was 1.22 sec-1. The reaction was inhibited competitively by a variety of guanidine, amidine, and aromatic compounds of low molecular weight. The results indicated that the active center of human Cl̅s consists of an anionic binding site in conjunction with a hydrophobic binding site. © 1969, American Chemical Society. All rights reserved.