Metabolic reactions rely on catalysts, called enzymes. An enzyme is a substance, usually a protein, that speeds up a biological reaction without being consumed in the reaction. Metabolic enzymes, as the name suggests, aid in metabolism and also detoxification of substances in the body. Typically, enzymes are proteins made of long chains of amino acids. Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. Each enzyme is specific to the reaction it catalyzes; that is, each enzyme performs a single, specific task. For example, the enzyme lactate dehydrogenase converts lactate to pyruvic acid, while the enzyme glycogen synthase synthesizes glycogen from glucose for storage in the liver.
The enzyme's active site is the place where the chemical reaction occurs. The kind of molecule that reacts with an enzyme is known as a substrate. The active site consists of a specific sequence of amino acids that helps the reaction by forming and breaking bonds with the molecule with which the enzyme interacts. The physical shape of the enzyme's active site aids in this specificity. The amino acids that are found in the active site are entirely unique and will bind only to a specific target—the enzyme's substrate or substrates.
In the absence of oxygen, energy is produced via fermentation. Fermentation produces two ATP molecules without oxygen. Less energy is generated during fermentation than aerobic metabolic pathways. Aerobic means metabolic processes that produce energy with the use of oxygen. Enough energy is generated for the cell to continue working under anaerobic conditions. Anaerobic means metabolic processes that produce energy in the absence of oxygen. The common fermentation pathway in humans is called lactic acid fermentation.
The advantage of fermentation is that when oxygen is limited, the cell can still process pyruvate into ATP to use for energy when oxygen levels are low. Typically in cells such as muscle cells, fermentation is important when oxygen is used up by excessive exercise. In this case, the muscle cells can rely on fermentation to convert pyruvate into lactic acid.