Nutritional Biochemistry and Functional Foods

Enzymes are proteins that catalyze biochemical reactions by lowering the activation energy required for the reaction to proceed, accelerating processes that would otherwise occur far too slowly to sustain life, often by many orders of magnitude, without themselves being consumed in the reaction. Each enzyme typically has an active site with a shape specific to its substrate, a specificity often described by the induced-fit model, in which the enzyme's active site subtly changes shape to better accommodate the substrate as it binds. Many enzymes require additional non-protein components to function, and when these components are organic molecules, they are called coenzymes; a large proportion of coenzymes are derived directly from B vitamins, which is a central reason these vitamins are essential to metabolism—for example, niacin is a precursor to NAD+ and NADP+, riboflavin to FAD, and pantothenic acid to coenzyme A, all of which participate directly in the energy-yielding pathways of glycolysis, the Krebs cycle, and fatty acid metabolism. When these vitamin-derived coenzymes are insufficiently available, the enzymatic reactions that depend on them slow down, which is one of the key mechanisms by which vitamin deficiencies produce their characteristic metabolic and clinical effects.