Phosphorylase kinase integrates signals from thecyclic-AMP-dependent and Ca2+-dependent signalingpathways that control glycogen breakdown in liver andmuscle 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 phosphorylaseto activate it for glycogen breakdown. The other three sub-units are regulatory proteins that control the activity of thecatalytic subunit. Two contain sites for phosphorylation byPKA, which is activated by cyclic AMP. The remaining sub-unit is calmodulin, which binds Ca2+ when the cytosolicCa2+ concentration rises. The regulatory subunits controlthe equilibrium between the active and inactive confor-mations of the catalytic subunit, with each phosphate andCa2+ nudging the equilibrium toward the active confor-mation. How does this arrangement allow phosphorylasekinase to serve its role as an integrator protein for the mul-tiple pathways that stimulate glycogen breakdown?
Phosphorylase kinase integrates signals from the
cyclic-AMP-dependent and Ca2+-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 Ca2+ when the cytosolic
Ca2+ concentration rises. The regulatory subunits control
the equilibrium between the active and inactive confor-
mations of the catalytic subunit, with each phosphate and
Ca2+ 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?
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