Enzyme Regulation

Enzyme Regulation

Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst. Enzymes are proteins, the functioning of the enzyme is determined by the shape of the protein. The arrangement of molecules on the enzyme produces an area known as the active site within which the specific substrate(s) will "fit". It recognizes, confines and orients the substrate in a particular direction.

Enzymes function under very specific conditions including temperature and pH and are highly regulated. Enzyme regulation, specifically allosteric regulation will be discussed in this essay.

The term allosteric derives from the Greek for "another shape (or state)," thereby indicating that all enzymes capable of allosteric regulation can exist in two different states ( Hammes ). In one of the two forms, the enzyme has a high affinity for its substrate, whereas in the other form, it has little or no affinity for its substrate. Enzymes with this property are called allosteric enzymes. The two different forms of an allosteric enzyme are readily interconvertible and are, in fact, in equilibrium with each other. Obviously, the reaction rate is high when the enzyme is in its high-affinity form and low or even zero when the enzyme is in its low-affinity form.

Whether the active or inactive form of an allosteric enzyme is favored depends on the cellular concentration of the appropriate regulatory substance, called an allosteric effector. In the case of isoleucine synthesis, the allosteric enzyme is threonine deaminase and the allosteric effector is isoleucine(Sacher). More generally, an allosteric effector is a small organic molecule that regulates the activity of an enzyme for which it is neither the substrate nor the immediate product.

An allosteric effector influences enzyme activity by binding to one of the two interconvertible forms of the enzyme, thereby stabilizing it in that state. In other words, an allosteric enzyme can exist in either a complexed or uncomplexed form, depending on whether it has an effector molecule bound to it or not. The effector binds to the enzyme because of the presence on the enzyme surface of an allosteric (or regulatory) site that is distinct from the active site at which the catalytic event occurs. Thus, a distinguishing feature of all allosteric enzymes (and other allosteric proteins, as well) is the presence on the enzyme surface of an active site to which the effector binds. In fact, some allosteric enzymes have multiple allosteric sites, each capable of recognizing a different effector.

An effector may be either an allosteric inhibitor or an allosteric activator, depending on the effect it has when bound to the allosteric site on the enzyme- that is, depending on whether the complexed form is the low affinity or high-affinity state of the enzyme. The binding of an allosteric inhibitor shifts the equilibrium between the two forms of the enzyme to favor the low-affinity state . In either case, binding of the effector to the allosteric site stabilizes the enzyme in one of its two interconvertible forms, thereby either decreasing or increasing the likelihood of substrate binding.

Most allosteric enzymes are large, multisubunit proteins with an active site or an allosteric site on each subunit. In fact, the active sites and allosteric sites are usually on different subunits of the protein, referred to as catalytic subunits and regulatory subunits, respectively. This means, in turn, that the binding of effector molecules to the allosteric sites affects not just the shape of the regulatory subunits but that of the catalytic subunits as well.

Many allosteric enzymes exhibit a property known as cooperativity. This means that, as the multiple catalytic sites on the enzyme bind substrate molecules, the enzyme undergoes conformational changes that affect the affinity of the remaining sites for substrate. Some enzymes show positive cooperativity, in which the binding of a substrate molecule to one catalytic subunit increases the affinity of the other catalytic subunits for substrate. Other enzymes show negative cooperativity, in which the substrate binding to one catalytic subunit reduces the affinity of the other catalytic sites for substrate (Strater et al).

The cooperativity effect enables cells to produce enzymes that are more sensitive or less sensitive to changes in substrate concentration than would otherwise be predicted by Michaelis-Menten Kinetics (Sacher). Positive cooperativity causes an enzyme's catalytic activity to increase faster than normal as the substrate concentration is increased, whereas negative cooperativity means that enzyme activity increases more slowly than expected.

In addition to allosteric regulation, many enzymes are also subject to control by covalent modification. In this form of regulation, the activity of an enzyme is affected by the addition or removal of specific chemical groups. Common modifications include the addition of phosphate groups, methyl groups, acetyl groups, or derivatives of nucleotides (Auzat et al ). Some of these modifications can be reversed, whereas other cannot. In each case, the effect of the modification is to activate or to inactivate the enzyme, or at least to adjust its activity upward or downward.

One of the most frequently encountered and best understood covalent modifications involve the reversible addition of phosphate groups. The addition of phosphate groups is called phosphorylation and occurs most commonly by transfer of the phosphate group from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue in the protein (Jiin-Yu et al). Enzymes that catalyze the phosphorylation of other enzymes (or of other proteins) are called protein kinases. The reversal of this process, dephosphorylation, involves the removal of a phosphate group from a phosphorylated protein, catalyzed by enzymes called protein phosphatases.

This mode of regulation is illustrated by glycogen phosphorylase, and enzyme found in skeletal muscle cells. This enzyme breaks down glycogen by successive removal of glucose units as glucose-1-phosphate. Regulation of this dimeric enzyme is achieved in part by the presence in muscle cells of two interconvertible forms of the enzyme, an active form called phosphorylase a and an inactive form called phosphorylase b (Schlessinger). When glycogen breakdown is required in the muscle cell, the inactive b form of the enzyme is converted into the active