Q. What Are The Roles Of Vitamins In Health? – BCH
Vitamins are essential nutrients that are required in the diet, usually in trace amounts, because they cannot be synthesized by humans themselves.
Vitamins required in the human diet are listed in table below.
|Thiamine (vitamin B1)||Thiamine pyrophosphate|
|Riboflavin (vitamin B2)||Flavin adenine dinucleotide (FAD)
Flavin mononucleotide (FMN)
|Niacin (nicotinic acid)||Nicotinamide adenine dinucleotide (NAD+)
Nicotinamide adenine dinucleotide phosphate (NADP+)
|Pantothenic acid||Coenzyme A|
|Pyridoxine, pyridoxal, pyridoxamine (vitamin B6)||Pyridoxal phosphate|
|Biotin||Biotin-lysine complexes (biocytin)|
|Lipoic acid||Lipoyl-lysine complexes (lipoamide)|
|Retinol (vitamin A)|
|Retinal (vitamin A)|
|Retinoic acid (vitamin A)|
|Ergocalciferol (vitamin D)|
|Cholecalciferol (vitamin D3)|
|α-Tocopherol (vitamin E)|
Classification Of Vitamins
These important substances are traditionally distinguished as being either water-soluble or fat-soluble.
Except for vitamin C (ascorbic acid), the water-soluble vitamins are all components or precursors of important biological substances known as coenzymes. Coenzymes are low-molecular-weight molecules that bring unique chemical functionality to certain enzyme reactions.
Vitamin B1 is the precursor of thiamine pyrophosphate (TPP), a coenzyme involved in reactions of carbohydrate metabolism in which bonds to carbonyl carbons (aldehydes and ketones) are made or broken. In particular, the decarboxylation of α-keto acids and the formation and cleavage of α-hydroxyketones depend on thiamine pyrophosphate. Thiamine is essential for the prevention of beriberi, a nervous system disease.
Riboflavin, or vitamin B2, is a constituent and precursor of both flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). The name riboflavin is a synthesis of the names for the molecule’s constituent parts, ribitol and flavin. Flavin coenzymes exist in three different states. Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD+ and NADP+; one- or two-electron carriers, such as quinones; and a variety of one-electron acceptor/donors, such as cytochrome proteins.
Nicotinic acid is structurally related to nicotine, a highly toxic tobacco alkaloid. To avoid confusion of nicotinic acid and nicotinamide with nicotine itself, niacin was adopted as a common name for nicotinic acid. Pellagra, a disease characterized by dermatitis, diarrhea, and dementia, indicates niacin deficiency.
Pantothenic acid, sometimes called vitamin B3, is a vitamin that makes up one part of a complex coenzyme called coenzyme A (CoA). Pantothenic acid is also a constituent of acyl carrier proteins. The two main functions of coenzyme A are:
– activation of acyl groups for transfer by nucleotide attack, and
– activation of the α-hydrogen of the acyl group for abstraction as a proton
Pantothenic acid is an essential factor for the metabolism of fat, protein, and carbohydrates in the tricarboxylic acid cycle and other pathways. However, in view of this universal importance in metabolism, it I surprising that Pantothenic acid deficiencies are not a more serious problems in humans, but this vitamin is abundant in almost all food, so deficiencies are rarely observed.
Vitamin B6: Pyridoxine and Pyridoxal Phosphate
The biologically active form of vitamin B6 is pyridoxal-5-phosphate (PLP), a coenzyme that exists under physiological conditions in two tautomeric forms. PLP participates in the catalysiss of a wide variety of reactions involving amino acids, including transaminations, α- and β-decarboxylations, β- and γ-eliminations, racemizations, and aldol reactions. Note that these reactions include cleavage of any of the bonds to the amino acid alpha carbon, as well as several bonds in the side chain. The remarkable versatile chemistry of PLP is due to its ability to:
1. form stable Schiff base (aldimine) adducts with α-amino groups of amino acids and
2. act as an effective electron sink to stabilize reaction intermediates
Vitamin B12 or cyanocobalamin is converted in the body into two coenzymes. The predominant coenzyme form is 5’-deoxyadenosylcobalamin, but smaller amounts of Methylcobalamin also exist in liver, for example. The B12 coenzymes participate in three types of reactions:
1. Intramolecular rearrangements
2. Reductions of ribonucleotides to deoxyribonucleotides (in certain bacteria)
3. Methyl group transfer
Vitamin B12 is best known as the vitamin that prevents pernicious anemia. Vitamin B12 is not synthesized by animals or by plants. Only a few species of bacteria synthesize this complex substance. Carnivorous animals acquire sufficient amounts of B12 from meat in their diet, but herbivorous creatures typically depend on intestinal bacteria to synthesize B12 for them. This is sometimes insufficient and certain animals, including rabbits, occasionally eat their feces in order to accumulate the necessary quantities of B12.
Vitamin C: Ascorbic Acid
L-Ascoribc acid, better known as vitamin C, is a reasonably strong reducing agent. The biochemical and physiologic functions of ascorbic acid most likely derive from its reducing properties (it functions as an electron carrier). A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-ascorbic acid. Ascorbic acid and dehydroascorbic acid form an effective redox system.
Ascorbic acid is effective in the treatment and prevention of scurvy, a potential fatal disorder characterized by anemia; alteration of protein metabolism; and weakening of collagenous structures in bone, cartilage, teeth, and connective tissues.
Folic acid derivatives (folates) are acceptors and donors of one-carbon units for all oxidation levels of carbon except that of CO2 (where biotin is the relevant carrier. The active coenzyme form of folic acid is Tetrahydrofolate (THF). THF is formed via two successive reductions of folate by dihydrofolate reductase. The biosynthetic pathways for methionine and homocysteine, purines, and the pyrimidine thymine rely on the incorporation of one-carbon units from THF derivatives.
Vitamin A or retinol often occurs in the form of esters, called retinyl esters. The aldehyde form is called retinal or retinaldehyde. Retinol can be absorbed in the diet from animal sources or synthesized from β-carotene from plant sources. Rod and cone cells of the eye accumulate retinol from blood. In the rods, retinol is oxidized by a specific retinol dehydrogenase to become all-trans retinal and then converted to 11-cis-retinal by retinal isomerase. The aldehyde group of retinal forms a Schiff base with a lysine on opsin, to form light-sensitive rhodopsin. Retinoic acid is essential for proper cell division and differentiation, the immune response, and embryonic development. Night blindness is the major disorder ascribed to vitamin A deficiency.
The two most prominent members of the vitamin D family are Ergocalciferol (known as vitamin D2) and cholecalciferol (vitamin D3). Cholecalciferol is produced in the skin of animals by the action of ultraviolet light (sunlight, for example) on its precursor molecule, 7-dehydrocholesterol. Based on its mechanism of action in the body, cholecalciferol should be called a prohormone that is, a hormone precursor. The active form of vitamin D, 1,25-dihydroxycholecalciferol acts on target tissues, where it acts like a hormone to regulate calcium and phosphate metabolism.
1,25-dihydroxyvitamin D3 , together with two peptide hormones, calcitonin and parathyroid hormone (PTH), functions to regulate calcium homeostasis and plays a role in phosphorus homeostasis. Calcium is important in many processes including muscle contraction, nerve impulse transmission, blood clotting, and membrane structure. Phosphorus of course I of critical importance to DNA, RNA, lipids and many metabolic processes.
Phosphorylation of proteins is an important regulatory signal for many biological processes. Phosphorus and calcium are also critically important for the formation of bones. Any disturbance of normal serum phosphorus and calcium levels will result in alterations of bone structure, as in rickets
The mechanisms of calcium homeostasis involves precise coordination of calcium
1. absorption in the intestine,
2. deposition in the bones
3. excretion in the kidneys
If a decrease in serum calcium occurs, vitamin D is converted to its active form, which acts in the intestine to increase calcium absorption. PTH and vitamin D act on bones to release calcium into the blood, and PTH acts on the kidney to cause increased calcium absorption. If serum calcium levels get too high, calcitonin induces calcium excretion from the kidneys and inhibits calcium mobilization from bone inhibiting vitamin D metabolism and PTH secretion.
Vitamin E is an antioxidant. The most active form of vitamin E, α-tocopherol is a potent antioxidant, and its function in animals and humans is often ascribed to this property. One possible role for vitamin E may relate to the protection of unsaturated fatty acids in membranes because these fatty acids are particularly susceptible to oxidation. When human plasma levels of α-tocopherol are low, red blood cells are increasingly subject to oxidative hemolysis. Infants, especially premature infants are deficient in vitamin E. When low-birth-weight infants are exposed to high oxygen levels for the purpose of alleviating respiratory distress, the risk of oxygen-induced retina damage can be reduced with vitamin E administration.
Vitamin K is essential for carboxylation of protien glutamate residues. The function of vitamin L in the activation of blood clotting is well established. A post-translational modification of prothrombin is essential to it function. In this modification, ten glutamic acid residues on the amino terminal end of prothrombin are carboxylated to form γ-carboxylglutamyl residues. These residues are effective in the coordination of calcium, which is required for the coagulation process. The enzyme responsible for this modification, a liver microsomal glutamyl carboxylase, requires vitamin K for its activity. Not only prothrombin (called “factor II” in the clotting pathway) but also clotting factors VII, IX, and X and several plasma proteins (proteins C, M< S, and Z) contain γ-carboxylglutamyl residues in proteins are known.