The inhibition of beta-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 (Escherichia 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 decrease 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 carbon 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 analogous to an oxonium-carbonium ion intermediate.