Enzyme regulation by covalent modification

FIGURE 15.2 Enzymes regulated by covalent modification are called interconvertible enzymes. The enzymes protein kinase and protein phosphatase, in the example shown here) catalyzing the conversion of the interconvertible enzyme between its two forms are called converter enzymes. In this example, the free enzyme form is catalytically active, whereas the phosphoryl-enzyme form represents an inactive state. The —OH on the interconvertible enzyme represents an —OH group on a specific amino acid side chain in the protein (for example, a particular Ser residue) capable of accepting the phosphoryl group. 

Many enzymes are regulated by covalent modification, most frequently by the addition or removal of phosphate groups from specific serine, threonine, or tyrosine residues of the enzyme. In the fed state, most of the enzymes regulated by covalent modification are in Ihe dephosphorylated form and are active (see Figure 24.2). Three exceptions are glycogen phosphorylase (see p. 129), fructose bis-phosphate phosphatase-2 (see p. 98), and hormone-sensitive lipase of adipose tissue (see p. 187), which are inactive in their dephosphorylated state. 

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