Anatomy and Physiology

In order for a chemical reaction to take place, the reacting molecules (or atoms) must first collide and then have sufficient energy (activation energy) to trigger the formation of new bonds. Although many reactions can occur spontaneously, the presence of a catalyst accelerates the rate of the reaction because it lowers the activation energy required for the reaction to take place. A catalyst is any substance that accelerates a reaction but does not undergo a chemical change itself. Since the catalyst is not changed by the reaction, it can be used over and over again.

Chemical reactions that occur in biological systems are referred to as metabolism. Metabolism includes the breakdown of substances (catabolism), the formation of new products (synthesis or anabolism), or the transferring of energy from one substance to another. Metabolic processes have the following characteristics in common:

• Enzymes act as catalysts for metabolic reactions. Enzymes are proteins that are specific for particular reactions. The standard suffix for enzymes is “ase,” so it is easy to identify enzymes that use this ending (although some do not). The substance on which the enzyme acts is called the substrate. For example, the enzyme amylase catalyzes the breakdown of the substrate amylose (starch) to produce the product glucose.The induced‐fit model describes how enzymes work. Within the protein (the enzyme), there is an active site with which the reactants readily interact because of the shape, polarity, or other characteristics of the active site. The interaction of the reactants (substrate) and the enzyme causes the enzyme to change shape. The new position places the substrate molecules in a position favorable to their reaction and accelerates the formation of the product.

• Adenosine triphosphate (ATP) is a common source of activation energy for metabolic reactions. In Figure 1, the wavy lines between the last two phosphate groups of the ATP molecule indicate high‐energy bonds. When ATP supplies energy to a reaction, it is usually the energy in the last bond that is delivered to the reaction. In the process of giving up this energy, the last phosphate bond is broken and the ATP molecule is converted to ADP (adenosine diphosphate) and a phosphate group (indicated by P _i). In contrast, new ATP molecules are assembled by phosphorylation when ADP combines with a phosphate group using energy obtained from some energy‐rich molecule (like glucose).
• Cofactors are nonprotein molecules that assist enzymes. A holoenzyme is the union of the cofactor and the enzyme (called an apoenzyme when part of a holoenzyme). If cofactors are organic, they are called coenzymes and usually function to donate or accept some component of a reaction, often electrons. Some vitamins are coenzymes or components of coenzymes. Inorganic cofactors are often metal ions, such as Fe ⁺⁺.

Figure 1. The high‐energy bonds of adenosine triphosphate (ATP).