THE BIOCHEMICAL ROLE OF VITAMINS AND THE METABOLIC PATHWAYS THEY PARTICIPATE IN
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Vitamins are a diverse group of organic molecules required in very small quantitiesin the diet for health, growth, and survival (Latin vita, life). The absence of a vitamin from the diet or an inadequate intake results in characteristic deficiency signs and, ultimately, death. The amount of each vitamin required in the diet is small (in the microgram or milligram range), compared with essential amino acid requirements (in the gram range). The vitamins are often divided into two classes, water-soluble vitamins and fat-soluble vitamins.
This classification has little relationship to their function but is related to the absorption and transport of fat-soluble vitamins with lipids.
Most vitamins are used for the synthesis of coenzymes, complex organic molecules that assist enzymes in catalyzing biochemical reactions, and the deficiency symptoms reflect an inability of cells to carry out certain reactions. However, some vitamins also act as hormones. Although the RDA or AI for each vitamin varies with age and sex, the difference is usually not very large once adolescence is reached.
For example, the RDA for riboflavin is 0.9 mg/day for males between 9 and 13 years of age, 1.3 mg/day for males 19 to 30 years of age, still 1.3 mg/day for males older than 70 years, and 1.1 mg/day for females aged 19 to 30 years.
The largest requirements occur during lactation (1.6 mg/day).
Vitamins, by definition, cannot be synthesized in the body, or are synthesized from a very specific dietary precursor in insufficient amounts. For example, we can synthesize the vitamin niacin from the essential amino acid tryptophan, but not in sufficient quantities to meet our needs. Niacin is therefore still classified as a vitamin.
Excessive intake of many vitamins, both fat-soluble and water-soluble, may cause deleterious effects. For example, high doses of vitamin A, a fat-soluble vitamin, can cause desquamation of the skin and birth defects. High doses of vitamin C cause diarrhoea and gastrointestinal disturbances.
TYPES OF VITAMINS
Vitamins are of two types;
1. WATER SOLUBLE VITAMINS
FAT SOLUBLE VITAMINS
1.WATER SOLUBLE VITAMINS:
· Non- B- complex
- Ascorbic Acid
- Thiamine (Vitamin B1)
- Ribroflavin( Vitamin B2)
- Niacin(Vitamin B3)
- Panthotenic acid
- Follic acid
- Vitamin B12
- Pyridoxine( Vitamin B6)
Plays a key role in one-carbon metabolism, is essential for biosynthesis of several compounds. Follic acid deficiency is probably the most common deficiency especially among pregnant women and alchoholics.
VITAMIN B12 (Cobalamin)
Required in humans for two essential enzymatic reactions, synthesis of methionine and the isomerisation of methylmalonyl CoA, that is produced during the degradation of some amino acids and fatty acids with odd numbers of carbon atoms. When the vitamin is deficient, abnormal fatty acids accumulate and become incorporated into cell membranes. This may account for some neurologic manifestation of Vitamin B12 deficiency.
VITAMIN C (Ascorbic acid)
The active form of Vitamin C is ascorbate acid. The main function of ascorbate is the reducing agent in several different reactions. It has a well- documented role as a co enyme in the hydroxylation reaction.
For example, hydroxylation of prolyl and lysyl-residues of collagen. Vitamin C is therefore required for the maintenance of normal connective tissue as well as for wound healing. Vitamin C also facilitates the absorption of dietary IRON from the intestine.
Is a collective term for pyridoxine, pyridoxamine and pyridoxal, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring. Pyridoxine occurs primarily in plant whereas Pyridoxal and Pyridoxamine are found in food obtained from animals. All three compounds can serve as precursors for the biologically active enzyme PYRIDOXAL PHOSPHATE. Pyridoxal phosphate functions as coenzyme for a large number of enzymes, particularly those that catalyze reactions involving amino acids.
VITAMIN B1 (Thiamine)
Thiamine pyrophosphate is the biologically active form of the vitamin, formed by the transfer of pyrophosphate group from ATP to thiamine.
Thiamine pyrophosphate serves as a coenzyme in the formation or degradation of alpha ketols by transketolase and in the oxidative decarboxylation of alpha keto- acids.
Niacin or nicotinic acid is a substituted pyridine derivative. The biologically active co enzyme forms are Nicotinamide adenine dinucleotide (NAD+) and its pyrophosphorylated derivative, nicotinamide adenine dinucleotide diphosphate (NADP+). Nicotinamide, a derivative of nicotinic acid that contains an amide group instead of a carboxyl group, also occurs in the diet. Nicotinamide is readily deaminated in the body and therefore, is nutritionally equivalent to nicotinic acid.NAD+ and NADP+ serve as coenzymes in oxidation-reduction reactions in which coenzyme undergoes reduction of the pyridine ring by accepting a hydride.
The reduced forms of NAD+ and NADP+ are NADH and NADPH, respectively.
VITAMIN B2 (Riboflavin)
Riboflavin fulfills its role in metabolism as the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). FMN is formed by ATP-dependent phosphorylation of riboflavin, whereas FAD is synthesized by further reaction of FMN with ATP in which its AMP moiety is transferred to the FMN. The main dietary sources of riboflavin are milk and dairy products. In addition, because of its intense yellow color, riboflavin is widely used as a food additive.
Flavin coenzymes are electron carriers in Oxidoreduction Reactions. These include the mitochondrial respiratory chain, key enzymes in fatty acid and amino acid oxidation, and the citric acid cycle.
Biotin is a coenzyme of carboxylase enzymes. Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. A holocarboxylase synthetase acts on a lysine residue of the apoenzymes of acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, or methylcrotonyl-CoA carboxylase to react with free biotin to form the biocytin residue of the holoenzyme.
Biotin also has a role in regulation of the cell cycle, acting to biotinylate key nuclear proteins.
As part of CoA and ACP, pantothenic acid acts as a carrier of acyl radicals. Pantothenic acid has a central role in acyl group metabolism when acting as the pantetheine functional moiety of coenzyme A or acyl carrier protein (ACP). The pantetheine moiety is formed after combination of pantothenate with cysteine, which provides the SH prosthetic group of CoA and ACP. CoA takes part in reactions of the citric acid cycle, fatty acid synthesis and oxidation, acetylations, and cholesterol synthesis. ACP participates in fatty acid synthesis. The vitamin is widely distributed in all foodstuffs, and deficiency has not been unequivocally reported in human beings except in specific depletion studies.
FAT SOLUBLE VITAMINS:
- Vitamin A (Retinol, Beta Carotene)
- Vitamin D ( Cholecalciferol)
- Vitamin K ( phylloquinone, menaquinones)
- Vitamin E ( tocopherol)
Retinoids comprise retinol, retinaldehyde, and retinoic acid (preformed vitamin A, found only in foods of animal origin); carotenoids, found in plants, Vitamin a has a function in vision In the retina, retinaldehyde functions as the prosthetic group of the light-sensitive opsin proteins, forming rhodopsin (in rods) and iodopsin (in cones). Any one cone cell contains only one type of opsin and is sensitive to only one color.
A most important function of vitamin A is in the control of cell differentiation and turnover.
Vitamin D is really a hormone and not strictly a vitamin since it can be synthesized in the skin, and under most conditions that is its major source. Only when sunlight is inadequate is a dietary source required. The main function of vitamin D is in the regulation of calcium absorption and homeostasis; most of its actions are mediated by way.
Vitamin K is required for synthesis of blood- clotting proteins. Vitamin K was discovered as a result of investigations into the cause of a bleeding disorder hemorrhagic (sweet clover) disease—of cattle, and of chickens fed on a fat-free diet. The missing factor in the diet of the chickens was vitamin K, while the cattle feed contained dicumarol, an antagonist of the vitamin. Antagonists of vitamin K are used to reduce blood coagulation in patients at risk of thrombosis—the most widely used agent is warfarin.
Three compounds have the biologic activity of vitamin K, phylloquinone, the normal dietary source, found in green vegetables; menaquinones, synthesized by intestinal bacteria, with differing lengths of side-chain; menadione, menadiol, and menadiol diacetate, synthetic compounds that can be metabolized to phylloquinone.
Menaquinones are absorbed to some extent but it is not clear to what extent they are biologically active as it is possible to induce signs of vitamin K deficiency simply by feeding a phylloquinone deficient diet, without inhibiting intestinal bacterial action.
Vitamin E does not have a precisely defined metabolic function. However, it does act as a lipid-soluble antioxidant in cell membranes, where many of its functions can be provided by synthetic antioxidants. VitaminE is the generic descriptor for two families of compounds, the tocopherols and the tocotrienols.
The main function of vitamin E is as a chain-breaking, free radical trapping antioxidant in cell membranes and plasma lipoproteins.
WATER SOLUBLE VITAMINS
FAT SOLUBLE VITAMINS
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