Question

Explain two mechanisms of proteins regulation. Include one mechanism that uses covalent modification (include enzymes that modify in your explanation) and one mechanism of allosteric regulation. Give specific examples of each.

Answer 1

The activity of regulatory enzymes is modulated through various types of signal molecules, which are generally small metabolites or cofactors. There are two major classes of regulatory enzymes in metabolic pathways.

a. Allosteric enzymes function through reversible, noncovalent binding of a regulatory metabolite called a modulator. Allosteric enzymes are those having "other shapes" or conformations induced by the binding of modulators.

b. The second class includes enzymes regulated by reversible covalent modification. Both classes of regulatory enzymes tend to have multiple subunits, and in some cases the regulatory site(s) and the active site are on separate subunits.

Allosteric example:The bacterial enzyme system that catalyzes the conversion of L-threonine into r.-isoleucine. In this system, the first enzyme, threonine dehydratase, is inhibited by isoleucine, the product of the last reaction of the series. Isoleucine is quite specific as an inhibitor. No other intermediate in this sequence of reactions inhibits threonine dehydratase, nor is any other enzyme in the sequence inhibited by isoleucine. Isoleucine binds not to the active site, but to another specific site on the enzyme molecule, the regulatory site. This binding is noncovalent and thus readily reversible; if the isoleucine concentration decreases, the rate of threonine dehydratase activity increases. Thus threonine dehydratase activity responds rapidly and reversibly to fluctuations in the concentration of isoleucine in the cell.

Covalent example:Modifying groups include phosphate, adenosine monophosphate, uridine monophosphate, adenosine diphosphate ribose, and methyl groups. These are generally covalently linked to and removed from the regulatory enzyme by separate enzymes.Regulation by covalent modification is glycogen phosphorylase (M_r 94,500) of muscle and liver which catalyzes the reaction .

(Glucose)_n + P_i(glucose)_n-1 + glucose-1-phosphate

The glucose-1-phosphate so formed can then be broken down into lactate in muscle or converted to free glucose in the liver. Glycogen phosphorylase occurs in two forms: the active form phosphorylase a and the relatively inactive form phosphorylase b. Phosphorylase a has two subunits, each with a specific Ser residue that is phosphorylated at its hydroxyl group. These serine phosphate residues are required for maximal activity of the enzyme. The phosphate groups can be hydrolytically removed from phosphorylase a by a separate enzyme called phosphorylase phosphatase