ENZYMES:

• •  Substances that can speed up a chemical reaction without being permanently altered themselves are called catalysts.

•  In living cells, enzymes serve as biological catalysts.

•  Enzymes are specific and act on specific substances called the enzyme substrate(s) and each catalyzes only one reaction.

Ex. Sucrose is the substrate of the enzyme sucrose, which catalyzes the hydrolysis of sucrose to glucose and fructose.

Enzyme specificity and efficiency:

Enzymes are large globular proteins that range in MW from about 10,000 to several million. Each enzyme has a characteristic three-dimensional shape with a specific surface configuration as a result of its primary, secondary and tertiary structures. This enables it to find the correct substrate from among the large number of diverse molecules in the cell. Enzymes are extremely efficient. Under optimum conditions can catalyze reactions at rates 10⁸ to 10¹⁰ times higher than those of comparable reactions without enzymes. Turnover number (maximum number of substrate molecules an enzyme molecule converts to product each second) is between 1 and 10,000 and can be high as 500,000.

Enzyme components:

• •  Some enzymes consist entirely of proteins.

•  Most consist of both a protein portion called an apoenzyme and a nonprotein component called a cofactor.

•  Ions of iron, zinc, magnesium or calcium are examples of cofactors. If the cofactor is an organic molecule, it is called a coenzyme.

•  Together, the apoenzyme and cofactor form a holoenzyme, or whole active enzyme. If the cofactor is removed, the apoenzyme will not function.

•  Coenzymes assist the enzyme by accepting atoms removed from the substrate or by donating atoms required by the substrate. Some act as electron carriers, removing electrons from the substrate and donating them to other molecules in subsequent reactions.

•  Many coenzymes are derived from vitamins. Two of the most important coenzymes in cellular metabolism are

Nicotinamide adenine dinucleotide (NAD⁺)

Nicotinamide adenine dicucleotide phosphate (NADP⁺)

These contain derivatives of B vitamin nicotinic acid (niacin)

NAD⁺ - involved in catabolic (energy-yielding reactions)

NADP⁺ - involved in anabolic (energy-requiring reactions)

•  Flavin coenzymes, such as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), contains derivatives of the B vitamin riboflavin and are also electron carriers.

•  Coenzyme A (CoA) contains derivatives of pantothenic acid, another B vitamin. This plays an important role in the synthesis and breakdown of fats and in a series of oxidizing reactions called the Krebs cycle.

•  Some cofactors are metal ions, including Fe, Cu, Mg, Mn, Zn, Ca and Co. They form a bridge between the enzyme and a substrate. Ex. Mg²⁺ is required by many phosphorylating enzymes (enzymes that transfer a phosphate group from ATP to another substrate).