Metabolic regulation is the process by which all cells — from bacteria to humans — control the chemical processes necessary for life. Metabolism is organized into complex, step-dependent reactions called metabolic pathways. Ubiquitous special proteins called enzymes are the main way that these pathways are regulated, although the concentration of nutrients, waste products, and hormones can control metabolic rates. Metabolic disorders are inherited diseases caused by the absence of key enzymes that disrupt the normal regulation of a given pathway.
Metabolism describes the chemical reactions by which organisms function, from cellular respiration to the events underlying digestion, growth, and reproduction. Systems called metabolic pathways coordinate these functions and are usually started or stopped by proteins called enzymes. Metabolic regulation is the basis of the biological control of metabolism, as it enables living cells to direct these pathways. In non-living systems, equilibrium with the outside environment occurs at the conclusion of chemical reactions, which would kill a living cell. Thus metabolic regulation helps keep the living system in a chemically balanced state, called homeostasis.
The most basic form of metabolic regulation occurs when genes instruct cells to produce enzymes and to regulate their quantity. Additionally, in a metabolic pathway, molecules undergo significant changes and are either used by the cell or processed to generate another step in the pathway. Some of these molecules, called substrates, are effective means of metabolic regulation through their concentration. The rate of a metabolic pathway will change, depending on the availability and concentration of a substrate, which must bind to an enzyme in order to work. In addition to substrates, enzymes are often dependent on other enzymes and on vitamins.
In addition to the more passive “feedback” kind of metabolic regulation by substrate concentration, there are direct controls in most multicellular organisms. Even plants use hormones to control their metabolism. In higher animals, external regulation of metabolism can come from chemical signals that control enzyme activity, either by directly acting on enzymes or by affecting the genes that regulate their production. Some forms of metabolic regulation change only the rate of time at which a biochemical process occurs; others activate a process or prevent it from starting. In animals, metabolic rate controls functions from respiration to body fat.
There are many disorders of the metabolism, including thousands of congenital deficiencies of genes that code for essential enzymes. Diseases of the thyroid can radically change the metabolic rate, causing obesity or near-starvation. Sometimes, human metabolism is excessively slow or fast due to disease states and may be treated medically. Some drugs or nutritional substances can be said to boost metabolic rates by changing the rate of pathways involved with carbohydrate or fat digestion. In patients with diabetes mellitus, for example, the hormone insulin's effects on sugar metabolism are compromised, and synthetic insulin must be administered to restore normal metabolic regulation.