On the Mechanism of Substrate Binding to the Purine‐Transport System of Saccharomyces cerevisiae

Publisher Website
Google Scholar
Abstract
The yeast S. cerevisiae takes up adenine, guanine, hypoxanthine and cytosine via a common energy-dependent transport system. The apparent affinity to 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 1 proton being associated to the transport system together with 1 substrate molecule. The pKa values of 2 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 assumption that a carboxyl group of the transport system, a proton and a substrate molecule arrange to an uncharged ternary complex was supported.