Vitamins Introduction
Vitamins are organic compounds required in small amount in the diet of animals in order to ensure, healthy growth and production. Vitamins are essential nutrients because animal cannot synthesize such compounds inadequate amount for its daily needs. Historically the name vitamins arose from the fact that they are vital amines, hence were named vitamins. The alphabet ‘e’ from vitamins was removed when it was found that all such compounds were not amines, and named vitamins.
Classification of Vitamins
Vitamins may be classified into two groups according to their solubility
- The water soluble (B) vitamins.
- The fat soluble vitamins
Many of the water-soluble vitamins are components of larger coenzyme molecules, or in other words, the coenzymes are the derivatives of vitamins. Plants, however, have the ability to synthesize the vitamins (from CO2, NH3, and H2S) and in fact, serve as excellent source of these dietary essentials.
Thiamine
Thiamine, also in short B1, is a substituted pyrimidine joined by methylene bridge to a substituted thiazole.
It occurs in outer coat of seeds. The use of polished rice or refined wheat flour removes much of this vitamin. In animal tissue and yeast, it occurs primarily as the coenzyme thiamine pyrophosphate. Animals other than ruminants (whose bacteria can provide the vitamin) require thiamine in their diet.
Biochemical function:
It functions as coenzyme in many biochemical reactions as a component of enzymes. For example, Pyruvate dehydrogenase enzyme contains this TPP as coenzyme, catalyzes the reaction mentioned below. TPP is a nonprotein part of enzyme and helps in catalyzing the reaction.
Riboflavin
Riboflavin (vitamin B2) consists of the sugar alcohol (D-ribitol) attached to 7,8-dimethyl-isoallaxazine ring.
The vitamin occurs as a component of the two flavin coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
Riboflavin is synthesized by green plants, many bacteria, and fungi, but not by any animal. It is less soluble in water than thiamine. It is destroyed when exposed to bright light. Its main sources are milk, legumes, cereals, green leafy vegetables.
Biochemical function:
Riboflavin functions as coenzyme because of its ability to undergo oxidative-reduction reactions. On reduction, the yellow color disappears since the reduced flavin is colorless. Overall reaction is addition of two hydrogen atoms in a 1, 4 addition to form reduced, colorless flavin.
FMN and FAD functions as coenzymes for a group of proteins known as flavoproteins. The enzyme succinate dehydrogenase that catalyzes the oxidation of succinate to fumarate contains FAD as covalent linked prosthetic group
Nicotinic Acid
The vitamin known as a niacin or vitamin B3 is nicotinic acid. Another form of the vitamin is the amide, nicotinamide or niacinamide.
It is widely distributed in plants and animals tissues. The coenzyme form of the vitamin are the nicotinamide nucleotide, namely Nicotinamide adenine dinucleotide (NAD+) and Nicotinamide adenine dinucleotide phosphate (NADP+).
Biochemical Function: Oxidation of alcohol to acetaldehyde, occurs by transfer of a hydride ion (a hydrogen atom with additional electron) and the release of a proton. H+.
It is component of dehydrogenases and helps in oxidation reduction reactions. For example conversion of alcohol to acetaldehyde is carried enzymes alcohol dehydrogenase, that contains NAD+ as coenzyme.
Vitamin B6 Groups
They are pyridoxal, pyridoxine, and pyridoxamine. These three forms of vitamin B6 are widely distributed in animal and plant sources, cereal grains are expecially rich source of this vitamin. All these three forms are interconvertible i.e. if one is ingested it can be converted to other two forms.
Pyridoxal phosphate is a versatile coenzyme that participates in the catalysis of several important reactions of amino acid metabolism i.e. transamination, decarboxylation, and racemization.
Transamination:
The enzyme glutamic aspartic transaminase consists of pyridoxal phosphate as cofactor.
Lipoic Acid
Lipoic acid is covalently linked through its carboxylic group to the e-amino group of a specific lysine residue of some enzymes.
It can exist in either of the forms i.e. reduced or oxidized as shown above. It is involved in the reaction where pyruvate is converted to acetyl CoA with the help of enzyme pyruvate dehydrogenase. This enzyme contains the lipoic acid as one of the cofactor. Lipoic acid is attached to the protein though the lysine residue as shown below.
Biotin
The requirement of this vitamin is very low and its deficiency creation is very difficult. Its requirement is met through the intentinal bacteria as they can synthesize this vitamin. Its deficiency can be created by feeding large amount of raw avian egg white, which contains a protein avidin that has the ability to bind the biotin, and does not allow its absorption.
It is bound to protein through e-amino group of lysine residue of proteins as shown. Biocytin E-N-lysine is isolated when such proteins are hydrolysed. The bound form of this vitamin helps in carboxylation and transfer of carboxyl group in many biological reactions.
Biochemical Function
Pyruvate is converted to oxaloacetate by the help of enzyme pyruvate carboxylase. The enzyme contains biotin as a prosthetic group.
Folic Acid
Its requirement is very less in animal systems, hence its very difficult to produce its deficiency in them. Its main constitutents are 2, amino-hydroxy-6methyl pteridine, p-amino benzoic acid and gluatmic acid as shown below.
Folic acid is the vitamin and its reduction products is the coenzymic form. Enzyme L-folate reductase, reduces folic acid to dihydrofolate (H2F). The double bond at position 7-8 is lost as two hydrogen atoms are added. Dihydrofolate is further reduced to (H4F) tetrahydrofolate. The double bond at position 5-6 is lost by further addition of two hydrogen atoms. The reduction is carried out with the help of NADH and H+.
The central role of tetrahydrofolate is to act as a carrier of one carbon unit, at the oxidation level of formate. The formate unit is used in the biosynthesis of pyrimidine, purines, serine and glycine.
Biochemical function:
The enzyme serine hydroxymethyl transferase contains THF as coenzyme to carry out one carbon transfer reaction, e.g. as shown below.
Vitamin B12
This enzyme is isolated from liver as a cyanocobalamine. It may also be isolated with anions other than cyanide, viz. hydroxyl, nitrate, chloride or sulphate. It is found only in animals and microorganisms and not in plants. It is involved in reactions like transmethylation and intramolecular migration. One of the reactions in which it is involved is given below.
Biochemical function:
The enzyme contains the cyanocoblamine as the coenzyme
Pantothenic Acid
It occurs as a component of coenzyme A and acyl carrier protein (ACP).
In 1954 Lipman found that a heat stable cofactor was required in many enzymes catalyzing acetylation. This cofactor was named coenzyme A, where A stands for acetylation. The terminal sulfhydryl group of CoA is the reactive site. The acyl group is attached to the CoA by thioester found.
Acyl carrier protein (ACP): Vagelos discovered that the intermediates in fatty acid synthesis in E.Coli are linked to an acyl carrier protein. Specifically they are linked to the sulfhydryl terminus of phosphantotheine group.
Vitamin C
This enzyme is water soluble. It is also named as ascorbic acid and can be isolated in pure crystalline form from lemon juice. Vitamin C also required in diet of only a few vertebrates- man, monkey, the guinea pigs and certain fishes, which are unable to synthesize them. Most other higher animals and plants can synthesize ascorbic acid from glucose or other simple precursors. It is requried for the normal formaiton of connective tissues (collagen). The vitamin especially is involved in the hydroxylation of proline and lysine residues of the collagen.
Its deficiency leads to scurvy.
Fat Soluble Vitamins
These vitamins are designated by the letters A, D, E and K. Unlike water soluble vitamins, they are stored in the organisms. Therefore, their supply in excess can be harmful. No specific coenzyme function has yet been found for any of the fat soluble vitamins.
Vitamin-A
Vitamin A1 or retinol and its aldehyde derivative, retinal, has the following structure:
These compounds are formed from their parent substance beta-carotene, which is called its provitamin.
An oxygenase located in the intestinal mucosa cleaves the beta-carotene as shown above, yielding two molecules of vitamin A1 aldehyde or retinal, which is then reduced to retinol by alcohol dehydrogenase. Beta-carotene is synthesized in plants by not in animals. The deficiency of vitamin A leads to night blindness. It is stored in liver. Excess intake leads to toxicity. Retinol and its aldehyde, retinal are the intermediates in chemical changes that occur during the visual process in the rods cell of an eye.
Rhodopsin consists of 11 cis-retinal bound to a protein (opsin). Light isomerizes 11-cis-retinal and it leads to the conversion of rhodopsin to opsin and all trans-retinal, which ultimaely leads to cascade of reactions that make us to see.
Vitamin D
Vitamin D also called calciferol, has two important forms i.e. D2 (ergocalciferol) found in plants and D3 (cholecalciferol) found in animals. They are the derviatives of cholesterol and are involved in the calcium and phosphorous metabolism.
The active form of vitamin D3 is 1.25 dihydrocholecalciferol (calcitriol), which is required for the normal formation of bones. The vitamin increases the calcium absorption into intestinal cells and brings about normal calcification of bone matrix. Its deficiency leads to rickets. Vitamin D2 is formed from ergosterols in plants.
Vitamin E
They are also named as tocopherols (group of related compounds). It contains substituted aromatic ring and long hydrocarbon chain as shown.
This vitamin prevents the oxidatin of unsaturated fatty acids. It occurs in plant oils, wheat germ, rice bran and soyabean. Its deficiency leads to destruction of erythrocytes and causes muscular dystrophy. Tocopherols are used commercially to retard spoilage of foods.
Vitamin K
Also called coagulation vitamin as its deficiency increases blood clotting time. At least two forms are known: Vit-K1 (Phylloquinone) in plants, Vit-K2 (menaquinone) in intestinal microflora
Vitamin K2 consists of 6-9 isoprene units that are indicated in the brackets. The vitamin helps in the formation of prothrombin, a blood plasma protein which is essential in blood clot formation.
Nitrogen Base
The nitrogenous bases are derivatives of two heterocyclic compounds, pyrimidine and purine
Nucleic acids have three principal derivatives of pyrimidine base i.e. cytosine, thymine, and uracil, whereas two principal purine bases i.e. guanine and adenine. Thymine is only present in DNA and uracil in RNA.