Properties of glycogen synthase and phosphorylase from Biomphalaria glabrata (mollusca)

Glycogen synthase and phosphorylase were characterized from the cephalopedal region of the snail, Biomphalaria glabrata. Glycogen synthase exhibited increases and decreases in its activity ratio (-G6P/+G6P) under conditions that are known to cause conversion of the two forms of the enzyme from mammalian systems, implying that the snail's synthase also possesses interconvertible forms. Each form had a distinct pH optimum, with the G6P-independent form (synthase alpha) exhibiting maximum activity at pH 7.4, whereas the G6P-dependent form (synthase beta) had optimal activity at pH 8.3. Both synthase alpha and beta displayed classical Michaelis-Menten kinetics for the substrates UDP-glucose and glycogen, and the beta form displayed sigmoidal kinetics for its modulator, G6P. Only UDP-glucose could function as a glucosyl donor in the synthase-catalyzed reaction. ADP, GDP, UDP, and ATP were all competitive inhibitors of synthase alpha, although at varying degrees of efficiency. Glycogen phosphorylase also demonstrated interconversion of its two forms (alpha and beta), as evidenced by changes in its activity ratio (-AMP/+AMP). AMP elicited hyperbolic kinetics from this enzyme. Concentrations of KF above 20 mM were found to inhibit glycogen synthase alpha while stimulating phosphorylase beta, thus causing erroneous activity ratios for both enzymes.