Glycogen breakdown must be rapidly turned off when necessary It is crucial that the high-gain system of glycogen breakdown be terminated quickly to prevent the wasteful depletion of glycogen after energy needs have been met

Glycogen breakdown must be rapidly turned off when necessary

It is crucial that the high-gain system of glycogen breakdown be terminated quickly to prevent the wasteful depletion of glycogen after energy needs have been met. When glucose needs have been satisfied, phosphorylase kinase and glycogen phosphorylase are dephosphorylated and inactivated. Simultaneously, glycogen synthesis is activated.

High Gain

Gain is the ratio of the output signal to the input signal. In the case of glycogen metabolism, high gain means that a few hormone signals can rapidly generate large amounts of glucose 6-phosphate.

The signal-transduction pathway leading to the activation of glycogen phosphorylase is shut down automatically when secretion of the initiating hormone ceases. The inherent GTPase activity of the G protein converts the bound GTP into inactive GDP, and phosphodiesterases always present in the cell convert cyclic AMP into AMP. Protein phosphatase 1 (PP1) removes the phosphoryl groups from phosphorylase kinase, thereby inactivating the enzyme. Finally, PP1 also removes the phosphoryl group from glycogen phosphorylase, converting the enzyme into the usually inactive b form.

The regulation of glycogen phosphorylase became more sophisticated as the enzyme evolved

 Analyses of the primary structures of glycogen phosphorylase from human beings, rats, Dictyostelium(slime mold), yeast, potatoes, and E. coli have enabled inferences to be made about the evolution of this important enzyme. The 16 residues that come into contact with glucose at the active site are identical in nearly all the enzymes. There is more variation but still substantial conservation of the 15 residues at the pyridoxal phosphate-binding site. Likewise, the glycogen-binding site is well conserved in all the enzymes. The high degree of similarity among these three sites shows that the catalytic mechanism has been maintained throughout evolution.

Differences arise, however, when we compare the regulatory sites. The simplest type of regulation would be feedback inhibition by glucose 6-phosphate. Indeed, the glucose 6-phosphate regulatory site is highly conserved among most of the phosphorylases. The crucial amino acid residues that participate in regulation by phosphorylation and nucleotide binding are well conserved only in the mammalian enzymes. Thus, this level of regulation was a later evolutionary acquisition.

THIS IS NOTE FROM A CHAPTER FOR GLYCOGEN METABOLISM I need help formulating a critical thinking question based off this chapter WITH MULTIPLE CHOICE ANSWERS.