Regulation of Metabolic Pathways

The rate at which a metabolic pathway functions is often determined by a regulatory enzyme responsible for one of its steps. This regulatory enzyme is present in limited quantity. Consequently, it can become saturated when the substrate concentration exceeds a certain level. Once this happens, increasing the substrate concentration no longer affects the reaction rate. In this way, a single enzyme can control a 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 another location in the sequence were rate limiting.

Often the product of a metabolic pathway inhibits the rate-limiting regulatory enzyme. This type of control is called negative feedback. Accumulating product inhibits the pathway, and synthesis of the product falls. When the concentration of product decreases, the inhibition lifts and more product is synthesized. This stabilizes the rate of production (fig. 4.15).

H What is a rate-limiting enzyme?

How can negative feedback control a metabolic pathway?

Enzyme Off Position

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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