Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

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6 Phosphorylase kinase integrates signals from the cyclic-AMP-dependent and dependent signaling pathways that control glycogen breakdown in liver and muscle cells (Figure Q15-4). Phosphorylase kinase is com- posed of four subunits. One is the protein kinase that cata- lyzes the addition of phosphate to glycogen phosphorylase to activate it for glycogen breakdown. The other three sub- units are regulatory proteins that control the activity of the catalytic subunit. Two contain sites for phosphorylation by PKA, which is activated by cyclic AMP. The remaining sub- unit is calmodulin, which binds when the cytosolic concentration rises. The regulatory subunits control the equilibrium between the active and inactive confor- mations of the catalytic subunit, with each phosphate and nudging the equilibrium toward the active confor- mation. How does this arrangement allow phosphorylase kinase to serve its role as an integrator protein for the mul- tiple pathways that stimulate glycogen breakdown?