Phosphoketolase in Rhodotorula graminis and Other Yeasts

Abstract

Phosphoketolase(s) [EC 4.1.2.9], catalyzing the cleavage of xyluose 5-phosphate and fructose 6-phosphate, were found in R. graminis, R. glutinis, Candida 107 and C. tropicalis. Activity towards the latter substrate only was found in C. humicola. Neither activity was found in Saccharomyces carlsbergensis. The enzyme(s) were isolated and purified 8-fold from R. graminis. Inorganic phosphate, thiamine pyrophosphate and Mg2+ were required for activity. The enzyme had a Km of 1.25 mM for xylulose 5-phosphate and was markedly sensitive to NADH, NADPH, ATP and acetyl-CoA and less sensitive to phosphoenolpyruvate, citrate and dodecanoyl-CoA. The activity of phosphoketolase was halved in the presence of an active transketolase which competes with it for the same substrate. The contribution of phosphoketolase to glucose catabolism may be only slight.