Vitamins nonprotein

Many enzymes carry out their catalytic function relying solely on their protein structure. Many others require nonprotein components, called cofactors (Table 14.2). Cofactors may be metal ions or organic molecules referred to as coenzymes. Cofactors, because they are structurally less complex than proteins, tend to be stable to heat (incubation in a boiling water bath). Typically, proteins are denatured under such conditions. Many coenzymes are vitamins or contain vitamins as part of their structure. Usually coenzymes are actively involved in the catalytic reaction of the enzyme, often serving as intermediate carriers of functional groups in the conversion of substrates to products. In most cases, a coenzyme is firmly associated with its enzyme, perhaps even by covalent bonds, and it is difficult to... 

Lipids are naturally occurring organic molecules that have limited solubility in water and can be isolated from organisms by extraction with nonpolar organic solvents. Fats, oils, waxes, many vitamins and hormones, and most nonprotein cell-meznbrane components are examples. Note that this definition differs from the sort used for carbohydrates and proteins in that lipids are defined by a physical property (solubility) rather than by structure. Of the many kinds of lipids, we ll be concerned in this chapter only with a few triacvlglycerols, eicosanoids, terpenoids, and steroids. 

Coenzyme An organic nonprotein molecule, frequently a phosphorylated derivative of a water-soluble vitamin, that binds with the protein molecule (apoenzyme) to form the active enzyme (holoenzyme). [EU]... 

The cases of myoglobin and hemoglobin are not rare. Many enzymes are dependent for their function on the presence of a nonprotein group. For example, cytochrome c also contains a prosthetic group similar, but not identical, to heme, as do a number of other proteins. These are known generically as heme proteins. There is a family of enzymes that contain a flavin group, the flavoproteins. Another family contains pyridoxal phosphate, a derivative of vitamin Be. There are a number of other examples. 

Some enzymes associate with a nonprotein cofactor that is needed for enzymic activity. Commonly encountered cofactors include metal ions such as Zn2+ or Fe2+, and organic molecules, known as coenzymes, that are often derivatives of vitamins. For example, the coenzyme NAD+contains niacin, FAD contains riboflavin, and coenzyme A contains pantothenic acid. (See pp. 371-379 for the role of vitamins as precursors of coenzymes.) Holoenzyme refers to the enzyme with its cofactor. Apoenzyme refers to the protein portion of the holoenzyme. In the absence of the appropriate cofactor, the apoenzyme typically does not show biologic activity. A prosthetic group is a tightly bound coenzyme that does not dissociate from the enzyme (for example, the biotin bound to carboxylases, see p. 379). 

Homocysteine is a nonprotein-building amino acid formed as a metabolite in the methionine cycle. It was first associated with disease in 1962 (1,2). Individuals with a mutation in cystathionine-(3-synthase (CBS) develop classical homocystin-uria with extremely elevated plasma tHcy (> 100 xmol/L) (3). Homocystinuria is characterized by early atherosclerosis and thromboembolism as well as mental retardation and osteoporosis and is ameliorated by vitamin supplementation aimed at reducing the blood concentration of homocysteine (4). 

The terms cofactor, coenzymes, and prosthetic group are used to describe the nonprotein moieties of the enzyme active center. The distinction between these terms is not sharp. Some of the cofactors are derivatives of vitamins that form either covalent or noncovalent linkages at or near the active site of the enzyme, and some are metal ions. If a cofactor (coenzyme) is tightly bound to the protein moiety (the apoenzyme), it is often referred to as a prosthetic group. A coenzyme that is easily removed from the holoenzyme, leaving behind the apoenzyme, is often regarded as a second substrate. 

The chemical composition of honey (Table 2.3) is dominated by glucose and fructose. Honey also contains many other valuable components, like enzymes, organic acids, mineral elements, nonprotein nitrogenous compounds, vitamins, aroma substances, and pigments. 

Coenzymes are complex nonprotein organic molecules that participate in catalysis by providing functional groups, much like the amino acid side chains. In the human, they are usually (but not always) synthesized from vitamins. Each coenzyme is involved in catalyzing a specific type of reaction for a class of substrates with certain structural features. Coenzymes can be divided into two general classes activation-transfer coenzymes and oxidation-reduction coenzymes. 

Gofactors are nonprotein substances that take part in enzymatic reactions and are regenerated for further reaction. Metal ions frequently play such a role, and they make up one of two important classes of cofactors. The other important class (coenzymes) is a mixed bag of organic compounds many of them are vitamins or are metabolically related to vitamins. 

Tryptophan is an essential amino acid which is ingested by Americans in quantities that exceed the normal daily requirements for protein synthesis (Rl), and considerable amounts are converted to nonprotein substances such as nicotinic acid and serotonin (Fig. 1). The tryptophan-niacin pathway, which is also known as the kynurenine pathway (Fig. 1), is important for production of the vitamin, nicotinic acid, and provides also a means for degrading tryptophan to acetoacetyl-CoA, carbon dioxide, and ammonia (P7). The amount of tryptophan metabolized by the various pathways available depends greatly on the amount of... 

Vitamin Organic compound essential to health that must be supplied in small amounts by the diet Coenzyme A nonprotein molecule needed by an enzyme to make the enzyme function possible... 

Bussi et al. (1953) and Cox (1953) divided pernicious anemia serum into protein and nonprotein fractions by ultrafiltration. The inhibitor was usually in the ultrafiltrate. Cox (1953) found that the protein fraction, rid of this inhibitor by ultrafiltration, had maturing properties. Microbiological assays showed that this fraction contained not only bound vitamin B12 but bound forms of the folic acid group of substances including citrovorum factor (Cox, Hornsby et al., unpublished). 

In 1956 Ross and Pike studied experimental toxemia in vitamin Be-de-hcient rats. The same changes proved to he present in the serum protein pattern and in the nonprotein nitrogen levels as those seen in toxemic women. The effect was similar when the maternal stores were depleted before mating. 

A nonprotein substance which is a part of an enzyme and is needed for the full functioning of the enzyme. A cofactor may consist of an organic compound, e.g., a vitamin derivative or a metallic ion, e.g., potassium, manganese, magnesium, calcium, or zinc. In the latter case, the metals are usually activators of the enzymes to which they become attached. 

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