Regulation of Metabolic Pathways

Regulation of Metabolic Pathways

The rate at which an enzyme controlled reaction occurs usually increases if either the number of substrate molecules or enzyme molecules is increased However, the rate at which a metabolic pathway functions is often determined by an enzyme responsible for one of its steps. 

This regulatory enzyme is present in limited quantity, Consequently, it can become saturated whenever the substrate increases above a certain amount. Once the enzyme is saturated, increasing the amount of substrate will no longer affect the rate of reaction. In this way, a single enzyme in a pathway can control the whole pathway As a rule, such a rate limiting enzyme is the first enzyme in a series This position is important because some intermediate substance of the pathway might accumulate if an enzyme occupying some other location in the sequence were involved.
 

Commonly a rate-limiting enzyme is controlled, in turn, by a negative feedback mechanism. In this case, the final product of a metabolic pathway acts to inhibit the regulatory enzyme. 

Thus, as the product increases, the pathway is inhibited and product production decreases; when the amount of product decreases, the pathway is not inhibited and more product is synthesized. Consequently, the rate at which the product is produced remains relative stable.