The inhibition of β-galactosidase (Escherichia coli) by amino sugars and amino alcohols

Abstract

Kinetically determined competitive inhibitor constants, which were estimates of the binding capacity of inhibitors interacting with the free enzyme form of .beta.-galactosidase (EC 3.2.1.23) (E. coli) showed that amino sugars and alcohols inhibited the enzyme much more than did their respective parent sugars and alcohols. In most cases the inhibition was 10-30 times greater, but the inhibition by 1-aminogalactopyranose was about 300 times greater than that of galactopyranose. When the amino groups were acetylated the inhibitory advantage was completely eliminated and partial neutralization of the amino group of an inhibitor by the presence of a group with a negative charge on the same inhibitor decreased the inhibitory advantage. Studies of binding to the galactosyl form of the enzyme (rather than to the free form) indicated that the binding at the glucose site was increased only slightly by the presence of the amino group. Overall, the findings suggested that .beta.-galactosidase has a negative charge near the anomeric C binding position of the galactose site. Since negative charges are unlikely to be of any importance in binding the normally neutral .beta.-galactosidase substrates, this charge may be important in stabilizing a positively charged reaction intermediate. Amino sugars and alcohols are thus probably inhibitors of .beta.-galactosidase, because they are positively charged and analogous to an oxonium-carbonium ion intermediate.