On the mechanism of substrate binding to the purine-transport system of Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae takes up adenine, guanine, hypoxanthine, and cytosine via a common energy-dependent transport system. The apparent affinity of the transport system to these and other purines and pyrimidines is correlated with their capability to be protonated to the positively charged form. Further organic molecules are competitive inhibitors when they are cationic, e.g. guanidine and octylguanidine in contrast to urea, or hexadecyltrimethylammonium in contrast to dodecylsulfate and Triton X-100. The influence of the pH on the kinetic constants of hypoxanthine transport points to a stoichiometry of one proton being associated to the transport system together with one substrate molecule. The pKa values of two ionizable groups that are involved in substrate binding are revealed; one of which (pKa = 1.8) may be attributed to the substrate, the other (pKa = 5.1) to an amino acid residue in the recognition site of the transport system. Studies with group-specific inhibitors indicate that this amino acid residue contains a carboxyl group. The results are in accordance with the assumption that a carboxyl group of the transport system, a proton and a substrate molecule arrange to an uncharged ternary complex.