Figure 1

Enzyme cascades involved in the regulation of glycogen synthesis and degradation in (A) Skeletal Muscle (B) Liver. Nomenclature: Active enzyme form is indicated by an affix 'a' and the corresponding inactive form is indicated by an affix 'b'. R₂C₂, cyclic AMP dependent protein kinase (CAPK); C, catalytic subunit of CAPK; PP1, phosphatase-1; PrP2, phosphatases-2A; PK, Phosphorylase kinase; GP, glycogen phosphorylase; GS, Glycogen synthase; Glu6P, glucose-6-phosphate; PP1 Inhibitor-1, Inhibitor of PP1; Km₁ to Km₈ are Michaelis-Menten constants, k₁ to k₈ are rate constants, K₁₁, K₂₂, Kd are dissociation constants as shown in the figure. Positive and negative signs indicate the activation and inhibition of a reaction respectively. In the muscle (Fig. 1A), cAMP activated CAPK catalyzes the phosphorylation of GS, PK and inhibitor-1. Phosphorylated PK activates GP-b. Active phosphatase-2A is assumed to inactivate inhibitor-1, whereas PP1 catalyzes the dephosphorylation of GS, GP and PK. In liver (Fig. 1B), GP-a catalyzes the allosteric inactivation of GS phosphatase and inhibitor-1 does not appear to be involved in the regulation of PP1.