Enzyme Action

Like other chemical reactions, metabolic reactions require energy (activation energy) before they proceed. This is why heat is used to increase the rates of chemical reactions in laboratories. Heat energy increases the rate at which molecules move and the frequency of molecular collisions. These collisions increase the likelihood of interactions among the electrons of the molecules that can form new chemical bonds. The temperature conditions in cells are usually too mild to adequately promote the reactions of life. Enzymes make these reactions possible.

The antibiotic drug penicillin interferes with enzymes that enable certain bacteria to construct cell walls. As a result, the bacteria die. In this manner, penicillin protects against certain bacterial infections. The drug does not harm human cells because these do not have cell walls.

Enzymes are usually globular proteins that promote specific chemical reactions within cells by lowering the activation energy required to start these reactions. Enzymes can speed metabolic reactions by a factor of a million or more.

Enzymes are required in very small quantities, because as they work, they are not consumed and can, therefore, function repeatedly. Also, each enzyme has specificity, acting only on a particular kind of substance, which is called its substrate (sub'strat). For example, the substrate of an enzyme called catalase (found in the per-oxisomes of liver and kidney cells) is hydrogen peroxide, a toxic by-product of certain metabolic reactions. This enzyme's only function is to decompose hydrogen peroxide into water and oxygen, helping prevent accumulation of hydrogen peroxide that might damage cells.

Each enzyme must be able to "recognize" its specific substrate. This ability to identify a substrate depends upon the shape of an enzyme molecule. That is, each enzyme's polypeptide chain twists and coils into a unique three-dimensional form, or conformation, that fits the special shape of its substrate molecule.

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