Question

The structure of the active site of chymotrypsin is shown below. The catalytic triad shown (Ser 195, His 57, and aspl02) are shown. A peptide with a phenylalanine is shown bound in the active site. 1. Chymotrypsin is a serine protease. The Km for chymotrypsin with N-acetylvaline ethyl ester is 8.8 X 102 M. The Km for the reaction with N-acetyltyrosine ethyl ester is 6.6 X 10 (a) Which substrate has the higher affinity for this enzyme? (b) Give a logical explanation for your answer given in part (a) catalytic triad Ser 195 His 57 Gly 193 N-H N-H R NC H Ср -O CAsp 102 R' O U N-H

Answer 1

Solution a) N-acetyltyrosine ethyl ester (Km = $6.6*10^{-4}$)

Solution b). Affinity is the tendency of enzymes to bind their substrate. The value of Km describes about the concentration of the  substrate at the point when half of the enzyme site is occupied by the substrate.

The substrate having high value of km means that large amount of substrate is required to saturate the enzyme and the substrate having low value km means a little quantity of substrate is required to reach saturation point.

Hence, the one with low value of Km have highest affinity for the enzyme.