Many enzymes are inactive in the absence of small amounts of other substances known as cofactors. In some cases, the cofactor is a trace metal (Chapter 2), such as magnesium, iron, zinc, or copper, and its binding to an enzyme alters the enzyme's conformation so that it can interact with the substrate (this is a form of allosteric modulation). Since only a few enzyme molecules need be present to catalyze the conversion of large amounts of substrate to product, very small quantities of these trace metals are sufficient to maintain enzymatic activity.

In other cases, the cofactor is an organic molecule that directly participates as one of the substrates in the reaction, in which case the cofactor is termed a coen-zyme. Enzymes that require coenzymes catalyze reactions in which a few atoms (for example, hydrogen, acetyl, or methyl groups) are either removed from or added to a substrate. For example:

R—2H + Coenzyme enzyme

What makes a coenzyme different from an ordinary substrate is the fate of the coenzyme. In our example, the two hydrogen atoms that are transferred to c

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions


Reaction 1


Reaction 3

Reaction 1


Reaction 3

Reaction 2 R2

R4 - 2H Reaction 4


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Essentials of Human Physiology

Essentials of Human Physiology

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