[Formation and properties of sugar arsenate esters]

Kinetics and thermodynamics of the spontaneous formation of glucose-6-arsenate (G6As) and 6-arsenogluconate (6AsG) as well as the ability of these compounds for substituting their phosphorylated homologues in enzymic reactions have been studied. Formation of G6As and glucose-6-phosphate (G6P) shows similar thermodynamic constants. Both reactions are endothermic, endergonic, and occur with a decrease of entropy. However, the kinetic coefficients of the spontaneous formation of the arsenate esters are ca. 10(5) times greater than those of their homologous phosphate esters. The activation energy of the spontaneous formation of G6As (E = +12 kcal mol-1) is even smaller than that of the formation of G6P by alkaline phosphatase (E = +13 kcal mol-1). Similar to the case of monoalkylphosphates, the monoanion species of G6As is much more reactive than the dianion species. This is an important difference with respect to G6P. Arsenate esters are good analogs of the phosphate esters for a variety of enzymes. Glucose-6-phosphate dehydrogenase shows nearly similar values of Km and Vmax for either G6P or G6As, and hexokinase is similarly inhibited by both compounds. 6-phosphogluconate dehydrogenase has the same Vmax with respect to 6PG and 6AsG, although the enzyme shows a much lower affinity for the latter substrate. The calculated half-lives at 25 degrees C and pH 7 of G6As and 6AsG are only ca, 6 and 30 min respectively, they increase at lower temperature and alkaline pH. At 0 degrees C and pH 9 the half-life of G6As is ca. 20 h.