Role of tetramer in equilibrium dimer equilibrium in the dephosphorylation and activity of phosphorylase a.

Skeletal muscle phosphorylase a exists as a tetramer or a dimer depending upon the temperature and protein concentration. The rate of dephosphorylation by phosphorylase phosphatase is very low at 18 degrees C where phosphorylase a exists as a tetramer. Caffeine markedly increases the rate of dephosphorylation of tetrameric phosphorylase a at 18 degrees C but has no effect on the dephosphorylation of the dimeric form. Caffeine also enhances the enzymic activity of phosphorylase a at 18 degrees C. The results presented here indicate that caffeine can shift the tetramer in equilibrium dimer equilibrium toward the dimeric form at 18 degrees C. This conclusion in supported by sedimentation analyses.