Phosphorylase thermodynamics

Although, of course, in enzyme reactions the difference between nucleophile and leaving group is somewhat artificial enzymes catalyse reactions in both directions and fairly often, as with phosphorylases, the physiological direction of chemical flux is apparently in the thermodynamically disfavoured direction, small concentrations of the thermodynamically disfavoured component being mopped up by efficient enzymes further along the pathway. 

In view of the specificity of the phosphorylase that disrupts the a-l,4-glucosidic linkages at the nonreducing end of glycogen, one might readily conceive that reversal of the phosphorylase reaction is responsible for glycogen synthesis. The thermodynamics of the phosphorylase reaction are not in conflict with such a hypothesis. 

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