histidine(redirected from His or H)
Also found in: Dictionary, Thesaurus, Medical.
histidine(hĭs`tĭdēn), organic compound, one of the 22 α-amino acidsamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
..... Click the link for more information. commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein. Histidine is the direct precursor of histaminehistamine
, organic compound derived in the body from the amino acid histidine by the removal of a carboxyl group (COOH). Although found in many plant and animal tissues, histamine is specifically important in human physiology because it is one of the chemicals released from
..... Click the link for more information. ; it is also an important source of carbon atoms in the synthesis of purinespurine,
type of organic base found in the nucleotides and nucleic acids of plant and animal tissue. The German chemist Emil Fischer did much of the basic work on purines and introduced the term into the chemical literature in the early 20th cent.
..... Click the link for more information. . The imidazole group on the side chain of histidine can act as both an acid and a base, i.e., it can both donate and accept protons under some conditions. This turns out to be an important property when histidine is incorporated into proteinsprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
..... Click the link for more information. , particularly when it becomes a part of the primary structure of some enzymesenzyme,
biological catalyst. The term enzyme comes from zymosis, the Greek word for fermentation, a process accomplished by yeast cells and long known to the brewing industry, which occupied the attention of many 19th-century chemists.
..... Click the link for more information. . It is thought that the side chain of this amino acid acts as a general acid and base as it participates in the catalytic functions of chymotrypsinchymotrypsin
, proteolytic, or protein-digesting, enzyme active in the mammalian intestinal tract. It catalyzes the hydrolysis of proteins, degrading them into smaller molecules called peptides. Peptides are further split into free amino acids.
..... Click the link for more information. , as well as those of a number of enzymes dealing with the metabolism of carbohydrates, proteins, and nucleic acids. It has even been implicated in the workings of cocoonase, the enzyme that allows adult silk moths to escape from their cocoons. Histidine is considered to be an essential amino acid for infants (it must be supplied in the diet); experiments with adults indicate that they can go for at least short periods without dietary intake of this amino acid. It was isolated from protein in 1896; its structure was confirmed by chemical synthesis in 1911.
an amino acid with basic characteristics, essential in many animals; the human body is capable of limited synthesis of histidine. It is one of the ingredients of the active centers of many enzymes, in particular of ribonuclease and transketolase. The initial stage of the enzymatic destruction of histidine in the body is the splitting off of ammonia, with the formation of urocanic acid, which is discharged in the urine. The deamination of histidine is an irreversible reaction and is catalyzed by the enzyme histidine ammonia lyase (histidine-a-deaminase), which is found in the liver of animals and in bacteria. Histidine deficiency leads to many metabolic disturbances, including inhibition of hemoglobin synthesis. Histidine is a precursor of the specific dipeptides of the skeletal musculature—carnosine and anserine. Decarboxylation of histidine leads to the formation of the biologically active amine histamine; this process is catalyzed by histidine decarboxylase, an enzyme belonging to the class of lyases. This enzyme acts only on the L-isomer (natural form) of histidine. The reaction is reversibly inhibited by the respiratory inhibitors cyanide, hydroxylamine, and semicarbazide.
A. A. BOLDYREV and E. V. PETUSHKOVA