Describe the relationship of various metabolic pathways with one another during the complete oxidation of glucose illustrating the major branching points.
To understand how glucagon stimulates the hepato- cyte to release glucose. we must first consider some of the biochemical that govern gluu.se metabo- lism in the liver, Biochemical pathways that link these reactions are illustrated in Fig. 3. It is important to recognize that not all atic reactions are freely reversible under conditions that prevail in living cells. Phosphorylation and dephosphorylation Of substrate usually require separate This sets up substrate cyc es. which would spin futilely in the absence of some regulatory influence on either or both opposing reactions. These reactions are often strategically situated at or near branch points in metabolic pathways and Can therefore direct of toward fate or another. Regulation is achieved both by modulating the acti v -ty Of enzymes already in cells and by increasing or decreasing rates of enzyme synthesis and therefore amounts or Enzyme activity be regulated by Changes in conformation produced by substrates or cofactors. or covalently by phosphorylation and dephosphorylation of regulatory sites in the enzymes themselves. Changing the activity Of an enzyme requires only seconds. Whereas.
FIGURE 3 Biochemical pathways of glucose metabolism in hepatocytes_ Reactions that are accelerated in the presence Of glucagon are shown in blue. many minutes or even hours are needed to change the amount of an enzyme.
Glycogenolysis Cyclic AMP formed in response to the interaction Of glucagon with its G-protein-coupled receptors 011 the surface of hepatocytes activates protein kinase A, which catalyzes phosphorylation, and hence activation, Of an enzyme called phosphorylase kinase (Fig. 4). This enzyme, in turn, catalyzes phosphorylation Of another enzyme, glycogen phosphorylase, which cleaves glycogen stepwise to release glucose-I -phosphate. Glucose-1- phosphate is the substrate for glycogen synthase, which catalyzes the incorporation Of glucose into glycogen. Glycogen synthase is also a substrate for protein kinase A and is inactivated when phosphorylated. Thus by increasing the formation of cyclic AMP, glucagon.
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