UDPGlucose 4-epimerase from Saccharomyces fragilis. Allosteric kinetics with UDP-glucose as substrate

UDPglucose 4-epimerase from Saccharomyces fragilis catalyzes a freely reversible reaction between UDP-galactose and UDP-glucose. With UDP-galactose as the substrate the enzyme shows a classical hyperbolic kinetics but when UDP-glucose is used as the substrate a distinct allostericity is observed. As a consequence, at low concentrations of UDP-glucose, the enzyme fails to establish the equilibrium at a significant rate. Glucose 6-phosphate acts as a strong activator for the enzyme with low concentrations of UDP-glucose as the substrate. In view of these rather unusual kinetic data for an enzyme catalyzing a freely reversible reaction, UDPglucose 4-epimerase may play a regulatory role in controlling the flux of galactose metabolism.