The hexose-ATP-kinase of wild-type Schwanniomyces occidentalis (CBS 8 19) is, like hexokinase PI1 of Saccharomyes cerevisiue, associated with carbon catabolite repression and phosphorylates D-glucose and D-fructose. The kinase of Schw. occidentulis repression-resistant (DogR) mutants phosphorylates D-glucose, but not D-fructose. Subjecting the wild-type enzyme to 45 "C for a few minutes appears to alter its activity, specificity and kinetic characteristics to that of the mutant enzyme. Fast protein liquid chromatography, with anion-exchange or gel-filtration columns, gel-electrophoresis, and DNA hybridization with the hexl' mutant of the HXK2 (hexokinase PII) gene of Succh. cerevisiue, all failed to resolve more than one hexose-ATP-kinase in Schw. occidentalis. Hence Schw. occidentulis appears to have a single hexose-ATP-kinase, M , -72 000, with two catalytic sites, analogous to the aspartate kinase/homoserine dehydrogenase of Escherichiu coli. One site is hexokinase-like and associated with catabolite repression and the other is glucokinase-like. Values for apparent K, were 7.2 mM-D-fructose and 0.55 mM-D-glucose ('hexokinase' site) and 0.078 mM-D-glucose ('glucokinase' site). t Present address :
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