Control Of Enzyme Activity

Any metabolic pathways can be regulated by one or more enzymes. Speed and requirement of such pathways can be controlled by controlling the activity of the enzymes involved in that pathways. Some pathways can be completely inhibited, however some may be speed up or slow down by controlling the activity of enzymes involved in the metabolic pathways.

One of the example we can collect from “synthesis of cell wall” in bacteria. Penicillin act by blocking the pathway to synthesize of cell wall pathways.

In metabolic pathways there is one or more regulatory enzyme found. Controlling the activity of regulatory enzyme can change the pathway process.

Regulatory enzyme is so called, as they regulate the steps in the metabolic pathways. Adjustment of the activity of regulatory enzymes and other proteins controls the functioning of many metabolic pathways and cellular processes. 

This type of regulation is an example of posttranslational regulation because it occurs after the protein synthesized.

There are a number of post translation regulation mechanisms. Some are irreversible – for instance, cleavage of a protein can either activate or inhibit its activity. Other type of post translation regulation are reversible.

Allosteric Regulation

Most regulatory enzymes are allosteric enzymes. The activity of an allosteric enzyme is altered by a small molecule known as an effector or modulator. The effector binds reversible by non covalent forces to a regulatory site separate from the catalytic site and causes a change in the shape or conformation of the enzyme. As a result, the activity of the catalytic site is altered. A positive effector increases the enzyme activity, whereas a negative effector decreases activity or inhibits the enzyme. These changes in activity often result from alterations in the apparent affinity of the enzyme for its substrate, but changes in maximum velocity also can occur.

Covalent Modification Of Enzymes

Regulatory enzymes also can be switched on and off by reversible covalent modification.

Usually this can be achieved by addition or removal of a particular group, typically a phosphoryl, methyl or adrenal group. The enzyme with an attached group can be either activated or inhibited.

One of the most intensively studied regulatory enzymes is E. coli glutamine synthetase, an enzyme involved in nitrogen assimilation.

There are some advantages to using covalent modification for the regulation of enzyme activity. These inter convertible enzymes often are also allosteric.

Feedback Inhibition

Every pathway has at least one pacemaker enzyme that catalyzes the slowest or rate limiting reaction in the pathway.  Because other reactions proceed more rapidly that’s the pacemaker reaction, change in the activity of this enzyme directly alter the speed with which a pathway operates.

Usually first step in the pathway is a pacemaker reaction catalyzed by a regulator enzyme. The end products of pathway often inhibits this regulatory enzyme, a process known as feedback inhibition or end product inhibition. 

Feedback inhibition ensures balanced production of a pathway end products. If the end products becomes too concentrated, it inhibits the regulatory enzyme and slow its own synthesis. As the end product concentration decreases, pathway activity again increases and more product is formed.

In this way, feedback inhibition automatically matches end product supply with the demand.