Studies of the binding of adenine to adenine phosphoribosyltransferase

Abstract

The groups on adenine phosphoribosyltransferase and on adenine which are involved in the binding of this substrate were investigated by comparing the inhibition constant of adenine with those obtained for analogues of adenine, by determining the effect of pH on the Michaelis constant of adenine, and by protection by adenine against enzyme inactivation by N-bromosuccinimide. Those moieties of adenine which are probably involved in complexing with the enzyme include the 6-amino group, and the 3-nitrogen and the 7-nitrogen atoms. The first probably complexes as an electron acceptor and the last two as electron donors. There is probably a lack of bulk tolerance at the 6-amino, 2-carbon, and the 3-, 7-, and 9-nitrogen positions. The group on the enzyme which ionizes at pH 10.25 was tentatively identified as a phenolic hydroxyl group of tyrosine which complexes with either the 3- or 7-nitrogen atoms of adenine.