Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Enzymes: Regulation of Activity
Covalent Modification of Enzymes

General Principles

Reversible Changes in the catalytic activity of Enzymes can be achieved through the covalent attachment of a phosphate group (predominant in mammals) or a nucleotide (predominant in Bacteria). Enzymes subject to covalent modification that results in altered activity are called covalently modified enzymes (Fig. 10.9).

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Fig. 10.9. Introduction/15.html">Regulation of enzyme Activity by covalent modification. Left: phosphorylation; right: nucleotide attachment. In both processes, the nucleoside triphosphate (NTP) is typically ATP.

Covalently modified enzymes can exist in two states, one characterized by high and the other by low catalytic efficiency. Depending on the specific case, the more active catalyst may be either the phospho- or the dephospho-enzyme (Table 10.3).

Sites of Phosphorylation

Typically, a specific Serine residue is phosphorylated to form an O-phosphoserine residue; less frequently, a Tyrosine residue is phosphorylated to yield an O-phosphotyrosine residue. Although a covalently modified enzyme may contain numerous serine or tyrosine residues, phosphorylation is highly selective and affects only a small number (1–3) of residues. These sites presumably do not form part of the catalytic center; thus, we have yet another example of allosteric effects.

Modifying (Converting) Enzymes

Phosphorylation and dephosphorylation are catalyzed by protein Kinases and protein Phosphatases, respectively (Fig. 10.10). In some cases, the converting enzymes are themselves covalently modified enzymes (Table 10.3). For instance, there are kinases and phosphatases of protein kinases that catalyze the modification of these converting Proteins (protein kinases). Evidence regarding whether protein phosphatases are themselves covalently modified enzymes is less conclusive, although their activity is subject to regulation. The activity of protein kinases and protein phosphatases is under hormonal control and is also regulated by The Nervous system, although the precise mechanisms of this regulation remain unclear.

Fig. 10.10. Covalent modification regulating enzyme function via phosphorylation/dephosphorylation of a serine residue.

Table 10.3. Selected mammalian enzymes whose catalytic activity differs in the phosphorylated and dephosphorylated states (E — dephospho-enzyme, EP — phospho-enzyme)

Energetics

The reactions shown in Fig. 10.10 are analogous to The conversion of glucose to glucose-6-phosphate or fructose-6-phosphate to fructose-1,6-bisphosphate (see Ch. 18). The phosphorylation and subsequent dephosphorylation of 1 mole of substrate (enzyme or sugar) results in the Hydrolysis of 1 mole of ATP.

The activity of kinases (catalyzing reactions 1 and 3) and phosphatases (catalyzing reactions 2 and 4) must, in turn, be regulated; otherwise, their simultaneous action would result in the catalysis of uncontrolled ATP hydrolysis.

Analogy with Feedback Inhibition

The regulation of enzyme activity by phosphorylation and dephosphorylation is, to some extent, analogous to feedback regulation. Both types of regulation provide a rapid means of altering metabolic flux in response to a physiological signal without affecting Gene Expression. In both cases, regulation targets the enzymes catalyzing the initial steps of a multi-step metabolic pathway—most often belonging to a biosynthetic pathway—acting not on the catalytic sites, but on allosteric sites. However, feedback inhibition targets a single enzyme selectively and is independent of hormonal or neural control. Conversely, the regulation of mammalian enzymes via phosphorylation-dephosphorylation encompasses multiple proteins, proceeds with the participation of ATP or other nucleoside triphosphates, and is under direct neural and hormonal control.

References

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