BIOCHEMISTRY AND HUMAN NUTRITION

Module 2. Enzymes

Lesson 11
ENZYME INHIBITION

11.1 Introduction

• Inhibitors are molecules that often resemble the substrate(s) or product(s) and bind to the active site thus they interfere with catalysis, slowing or halting enzymatic reactions.
• Many drugs are reversible enzyme inhibitors. They have their physiological effect by decreasing the activity of a specific enzyme. For example, aspirin (acetylsalicylate) inhibits the enzyme that catalyzes the first step in the synthesis of prostaglandins, compounds involved in many processes, including some that produce pain.
• The concentration of inhibitor needed to inhibit the enzyme depends on how tightly the inhibitor binds to the enzyme.
• The inhibition constant (Ki) is used to describe how tightly an inhibitor binds to an enzyme. The bigger the Ki, the weaker the binding.

11.2 Types of Inhibitors

There are two broad classes of enzyme inhibitors

• Irreversible
• Reversible

11.2.1 Irreversible

The irreversible inhibitors are those that bind covalently with or destroy a functional group on an enzyme that is essential for the enzyme’s activity, or those that form a particularly stable noncovalent association. Formation of a covalent link between an irreversible inhibitor and an enzyme is common. For example reaction of chymotrypsin with diisopropylfluorophosphate (DIFP) irreversibly inhibits the enzyme by binding with Ser195 in the active-site of chymotrypsin.

Fig. 11.1 Diisopropylfluorophosphate as irreversible inhibitors of chymotrypsin

11.2.2 Reversible

This type of inhibition involves equilibrium between enzyme and the inhibitor, the equilibrium constant (ki) being the measure of affinity of the inhibitor for the enzyme. This inhibition is further classified into three categories

• Competitive
• Uncompetitive
• Noncompetitive.

11.3 Competitive Inhibition

Competitive inhibitors bind only to the free enzyme and to the same site as the substrate. Competitive inhibitors are molecules that usually look like the substrate but can’t undergo the reaction. At an infinite concentration of the substrate the competitive inhibitor cannot bind to the enzyme since the substrate concentration is high enough that there is virtually no free enzyme present.

Since competitive inhibitors have no effect on the velocity at saturating (V_max) concentrations of the substrate, the intercepts of the doublereciprocal plots (1/V_max) at all the different inhibitor concentrations are the same. The lines at different inhibitor concentrations must all intersect on the y axis at the same 1/V_max. At low concentrations of substrate ([S] << Km), the enzyme is predominantly in the E form. The competitive inhibitor can combine with E, so the presence of the inhibitor decreases the velocity when the substrate concentration is low.

Fig. 11.2 Competitive Inhibition

Under competitive inhibition
Vmax remains unchanged ; Km increases

Example :

• Malonate is a competitive inhibitor of succinate dehydrogenase. The enzyme uses succinate as its substrate but inhibited by malonate which is structurally similar to succinate and differs in having one rather than two methylene groups.

11.4 Uncompetitive Inhibition

If the inhibitor combines only with ES (and not E), the inhibitor exerts its effect only at high concentrations of substrate at which there is lots of ES around. This means that the increasing substrate concentration (S) doesn’t prevent the binding of the inhibitor. Interestingly Km value is consistently smaller than Km value of the uninhibited reaction, which implies that S is more effectively bound to the enzyme in the presence of the inhibitor. The sequence of this type of reaction is

This type of inhibition is often observed for enzymes that catalyze the reaction between two substrates. Often an inhibitor that is competitive against one of the substrates is found to give uncompetitive inhibition when the other substrate is varied. The inhibitor does combine at the active site but does not prevent the binding of one of the substrates (and vice versa). In this type of inhibition Vmax as well as Km both are decreased

Fig. 11.3 Uncompetitive Inhibition

11.5 Noncompetitive Inhibition

Compounds that reversibly bind with either the enzyme or the enzyme substrate complex are designed as noncompetitive inhibitors and the following reaction describe these events.

Fig. 11.4 Noncompetitive Inhibition

Noncompetative inhibition therefore differs from competitive inhibition in that the inhibitor can combine with ES, and S can combine with EI to form in both instances EIS. This type of inhibition is not completely reversed by high substrate concentration since closed sequence will occur regardless of the substrate concentration. Since inhibitor binding site is not identical to nor does it modify the active site directly, the Km is not altered but Vmax is decreased.

For example, the amino acid alanine noncompetitively inhibits the enzyme pyruvate kinase. Alanine is one product of a series of enzyme-catalyzed reactions, the first step of which is catalyzed by pyruvate kinase. (Fig. 11.5 Enzyme inhibition)