Kinetic Study of a Phosphoryl Exchange Reaction between Fructose and Fructose 1-Phosphate Catalyzed by the Membrane-Bound Enzyme II of the Phosphoenolpyruvate-Fructose 1-Phosphotransferase System of Bacillus subtilis

Abstract

A phosphoryl exchange reaction between fructose 1-phosphate and fructose was catalyzed by a membrane preparation isolated from B. subtilis. The regulation of the biosynthesis of the activity in the wild type and in the regulation mutants fruB closely correlates with that of the membrane-bound enzyme II of the phosphoenolpyruvate fructose 1-phosphotransferase system which is known to mediate the transmembrane vectorial phosphorylation of fructose. The computed analysis of the kinetic data shows that the mechanism of the enzyme II is ping-pong, i.e., that a phosphoryl-enzyme intermediate occurs in the reaction. The apparent Kd of the enzyme II/fructose 1-phosphate complex and of the phosphoryl enzyme II/fructose complex are estimated. The value of the standard free energy of the hydrolysis of the bond between the phosphoryl moiety and the enzyme suggests a covalent bonding. This intermediate presumably occurs in the physiological functioning of the enzyme which utilizes the phosphocarrier protein HPr as phosphoryl donor. The exchange reaction is competitively inhibited by high fructose concentrations; the same site of the enzyme binds fructose and fructose 1-phosphate, this site being accessible to fructose on the external side of the membrane when the enzyme is phosphorylated.