Vitamin

Vitamins are organic molecules that are needed in small amounts in the diet. They are frequently molecules that bind in the active site of an enzyme and thereby alter its structure in a way that permits it to react more readily. Vitamins serve nearly the same role in all forms of life and many are essential in the metabolism of all living organisms. They are synthesized by plants and micro-organisms and the absolute requirement for vitamins in the diet of higher animals is the result of the loss of this biosynthetic capability during evolution. The biosynthetic abilities and thus the dietary requirement of different species vary. For example, ascorbic acid (vitamin C) is a vitamin only for primates and a few other animals, such as the guinea pig, but most other animals can synthesize it, so for them it is not a vitamin. Certain vitamins can be synthesized from provitamins obtained from the diet. Some of the vitamin requirements of humans and higher animals are supplied by the intestinal flora, for example most of the vitamin K required by humans is provided in this way.

Several diseases resulting from vitamin deficiencies were prevalent until the last century and sailors on long sea voyages, where fresh vegetables were not readily available, were often victims. In the Orient, the disease beriberi was rampant and millions died of its associated polyneuritis. The condition could be relieved by feeding the patients rice polishings. The founder of the vitamin concept was Lumin (1853–1937). During subsequent decades, the importance of accessory food factors for normal growth and development was gradually recognized. Stanley Publishing. Reproduced by permission.
The Polish biochemist, Casimir Funk formulated the vitamin theory in 1912 and proposed that several common diseases such as beriberi, pellagra, rickets and scurvy resulted from lack in the diet of essential nutrients. It was Funk who suggested the name "vitamin" for these accessory factors, from the Latin vita + amine, the "amine" reflecting the fact that the first of these factors to be studied, vitamin B₁, contained nitrogen.

The metabolic role of vitamins is largely catalytic. Most vitamins serve as coenzymes and prosthetic groups of enzymes. For most of these, the nature of the biocatalytic function has been elucidated. Vitamin D, however, acts as a regulator of bone metabolism and is thus has an activity similar to hormones. As a component of the visual pigments, vitamin A acts as a prosthetic group, however, it is not known whether it is associated with catalytic proteins in its other functions. Nicotinamide and riboflavin are constituents of the hydrogen-transferring enzymes, such as those in the respiratory electron transport chain. Biotin, folic acid, pantothenic acid, pyridoxine, cobalamin and thiamine are coenzymes, or precursors of coenzymes, of group transfer reactions. The low daily requirements for vitamins reflect their catalytic and/or regulatory roles. Thus vitamins are nutritionally quite different from fat, carbohydrate, or protein, which are required in the diet in considerable quantities as substrates of tissue synthesis and energy metabolism.

Vitamins can be grouped according to whether they are soluble in water or polar solvents. The water-soluble vitamins are ascorbic acid, the vitamin B series (thiamain, B₁, riboflavin, B₂, pyridoxine, B₆, cobalamin, B₁₂,), folic acid, niacin and pantothenic acid. Ascorbate, the ionised form of ascorbic acid, is essential in the prevention of scurvy and acts as a reducing agent (an antioxidant). It serves, for example, in the hydroxylation of proline residues in collagen. The vitamin B series are components of coenzymes. For example, riboflavin (vitamin B₂) is a precurser of FAD, and pantothenate is a component of coenzmye A. Vitamin B₁ (thiamine) was found to cure beriberi.

Much is known about the molecular actions of the fat-soluble vitamins, which are designated by the letters A, D, E and K. Vitamin K, which is required for normal blood clotting, participates in the carboxylation of γ-carboxyglutamate, which makes it a much stronger chelator of Ca²⁺. Vitamin A (retinol) is the precurser of retinal, the light sensitive group in rhodopsin and other visual pigments. A deficiency of this vitamin leads to night blindness. Furthermore it is required for growth by young animals. Retinoic acid, which contains a terminal carboxylate in place of the alcohol terminus of retinal, activates the transcription of specific genes that mediate growth and development. The metabolism of calcium and phosphorus is regulated by a hormone derived from vitamin D. A deficiency of vitamin D impairs bone formation in growing animals and causes the disease rickets. Infertility in rats is a consequence of vitamin E (α-tocopherol) deficiency and this vitamin also protects unsaturated membrane lipids from oxidation.

Most vitamins were purified between 1920 and 1950. The last one was vitamin B₁₂, in 1948, whose chemical structure was elucidated by A. R. Todd in 1955. Chemical syntheses are known for all vitamins.