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Classification, Sources, Functions, Deficiency - Water soluble vitamins | 11th Biochemistry : Chapter 8 : Vitamins

Chapter: 11th Biochemistry : Chapter 8 : Vitamins

Water soluble vitamins

The members of this group are B-complex vitamins and vitamin C .

Water soluble vitamins:


The members of this group are B-complex vitamins and vitamin C . They are readily soluble in water and can be transported freely in the blood and the watery fluids between the cells. However, vitamin B12 needs a binding protein for the transport. Excess of these vitamins are eliminated via kidneys. Unlike other members of this family the vitamin B12 is eliminated through bile. As these are readily soluble in water these vitamins are easily lost during cooking.


1. B-complex vitamins


The vitamin B has a group of vitamins (B1, B2, B3, B5, B6, B12, biotin and folate). All these vitamins needs to be converted into the corresponding coenzymes which is their active form. They are useful in the synthesis of many neurotransmitters such as acetylcholine.


i. Vitamin B1 (Thiamine)


Vitamin B1 which is also called as thiamine contains pyrimidine and thiazole ring. Thiamine pyrophosphate (TPP) is known as active thiamine. Thiamine pyrophosphotransferase (ATP dependent enzyme) present in brain and liver is responsible for conversion of thiamine to thiamine pyrophosphate




Liver, pork, meat, and rice are rich sources. Other good sources are beans and nuts (Figure 8.13).




·     Thiamine act as a coenzyme in the form of thiamine pyrophosphate (TPP) in many enzymatic reactions. These are involved mainly in the breakdown of glucose to yield energy.


·     Thiamine pyrophosphate (TPP) also acts as a coenzyme for transketolase reactions in the phosphogluconate oxidative pathway of carbohydrate metabolism. This reaction is essential for ribose formation, a constituent of DNA and RNA.


·     Vitamin B1 is also required in tryptophan metabolism for the activity of the enzyme tryptophan pyrolase.


·     The adequate level of thiamine provides healthy nerves, a good mental outlook, a normal appetite and digestion.


Absorption and storage:


Free thiamine is readily absorbed by the small intestine. Thiamine is not stored in the human body. The excess thiamine is excreted in urine and also degraded by the enzyme thiaminase.




The symptoms of thiamine deficiency occur because the tissue cells are unable to receive sufficient energy from glucose. Therefore, they cannot carry out their normal functions. Early symptoms of thiamine deficiency include fatigue, irritability, depression and numbness of the leg and poor tone of the gastrointestinal tract together with constipation.


Beriberi is the disease caused by thiamine deficiency. Beriberi is characterized by oedema in the legs. Usually, beriberi is caused by carbohydrate rich and low thiamine diets such as polished rice.


ii. Vitamin B2 (Riboflavin)


Vitamin B2 which is also known as riboflavin consists of a heterocyclic isoalloxazine ring attached to ribitol (a pentose alcohol). It is a yellow coloured compound.




It is widely distributed in plants. Soy beans, green vegetables are good sources of this vitamin. High concentration occurs in yeasts, milk and egg (Figure 8.16).




Riboflavin is a component of two important coenzymes, namely flavin mono nucleotide (FMN) and flavin adenine dinucleotide (FAD). They play key roles in various enzymatic reactions.


FMN and FAD are synthesized as follows.


Riboflavin + ATP → FMN +ADP




FMN and FAD combine with different apoenzymes to form a large number of redox enzymes.


Eg. FMN is associated with the enzyme cytochrome c reductase.


FAD is found in xanthine oxidase and acyl CoA dehydrogenase


Riboflavin is essential for a healthy skin and for good vision in bright light.


Absorption and storage


The vitamin is phosphorylated in the intestinal mucosa during absorption. It is absorbed by the small intestine through the portal vein and is distributed to all tissues. The major part is excreted in urine and a small part is metabolized in the body.




Riboflavin deficiency leads to cheilosis. It is characterised by the development of fissures developing in the lips and at the corners of the mouth.

Riboflavin is extremely sensitive to strong light. Due to sensitivity, riboflavin deficiency may occur in new born infants with hyper bilirubinemia who are treated by phototherapy.


iii. Vitamin B3 (Niacin)


Vitamin B3 which is also called as niacin or nicotinic acid is pyridine 3-carboxylic acid.


It occurs in tissues as nicotinamide (Figure 8.18).




This vitamin is widely distributed in cereals, dark green leafy vegetables. Liver and kidney are rich sources of this vitamin.




·     Niacin is found in the form of its amide which is an important constituent of the coenzymes NAD+ and NADP+ and they take part in redox reactions which are associated with many dehydrogenases. For example NAD+ is the coenzyme for lacate dehydrogenase. NADP+ is the coenzyme for glutathione reductase.


·     It promotes the formation of fat from carbohydrates.


·     Niacin is essential for healthy skin, normal functions of the gastro intestinal tract and maintenance of the nervous system.


Absorption and storage


Nicotinic acid and nicotinamide are absorbed by the intestine through the portal vein into the general circulation. Excess nicotinic acid is not stored in the body.




·     Nicotinic acid deficiency causes a disease called pellagra (Figure 8.20).


·     Dermatitis-skin exposed to the sun, soreness of the mouth and swelling of the tongue.


·     Diarrhoea.


·     Dementia-mental changes including depression and confusion.


iv. Vitamin B6 (Pyridoxine)


Vitamin B6 or Pyridoxine is also called as adermin. Vitamin B6 consists of three closely related pyridine derivatives.


·     Pyridoxine       

·     Pyridoxal

·     Pyridoxal amine


The metabolically active form of vitamin B6 is pyridoxal phosphate. It is formed from pyridoxal.




Rich sources of this vitamin are yeast, whole grain, cereals and egg-yolk. Moderate amounts are present in organ meats like liver and kidney.




Pyridoxal phosphate acts as coenzyme for several enzymes of amino acid metabolism.



·     It is involved in the formation of heme in hemoglobin.


·     Pyridoxal phosphate acts as a coenzyme for the decarboxylase in decarboxylation reactions.Amino acids are decarboxylated to form their corresponding amines.

·     Vitamin B6 is involved in the synthesis of coenzyme A from pantothenic acid.


·     It is also involved in the production of antibodies.


·     Pyridoxal phosphate plays a key role in glycogenolysis.


Absorption and storage


Pyridoxine is readily absorbed by the small intestine. The excess amount if ingested is not stored in the body, and is excreted in urine.




Deficiency of vitamin B6 is extremely rare. In infants its deficiency causes irritabilities, insomnia, muscular weakness and convulsion. The cause of the convulsions are severe impairment of the activity of the enzyme glutanate ecarboxylase, which is dependant on pyridoxal phosphate. The product of Glutamate decarboxcylase in γ-aminobutyric acid (GABA), which is a reguletory neurotransmitter in the central nervous system.


v. Vitamin B12:


Vitamin B12 has complex ring structure, similar to porphyrin ring which has a cobalt ion (Co3+) at its center (Figure. 8.23). The active coenzyme forms of vitamin B12 are methylcobalamine and deoxyadenolsyl cobalamine. The six co-ordination valencies of cobalt ion (Co3+) are satisfied by the four nitrogens of reduced terapyrrole and the fifth by the nitrogen of 5, 6 dimethyl benzimidazole moiety and the sixth one either by CN (cyanocobalamine)

or          H2O (aquacobalamine)        or

OH (Hydroxycobalamine)               or

CH3 (methylcobalamine)




Vitamin B12 is found in animal but not in vegetable foods and is unique in that it is the only known vitamin that holds an ion of metal (cobalt) in its molecule. Eggs and meat supply ample amounts of this vitamin.




·     Many microorganisms require vitamin B12 for growth.


·     Vitamin B12 is required as coenzyme for the conversion of L-methylmalonyl CoA to Succinyl CoA by the enzyme methylmelonyl CoA mutase. This reaction is essential for in the metabolisom of some branched-chain amino acids and odd-chain fatty acids


·     Vitamin B12 is required for the maturation of red blood cells in the bone marrow and for the synthesis of proteins.


Absorption and Storage


Vitamin B12 is absorbed from ileum. For the absorption of vitamin B12 from the intestines, a factor called “Intrinsic Factor” (IF) secreted by the stomach is essential. Vitamin B12 is stored in fair amounts in the liver.




When absorption is prevented by lack of intrinsic factor, it leads to the condition called pernicious anemia. It is characterized by a drastic decrease in red blood cell count and leads to formation of macrocytic red blood cells.

Vitamin B12 deficiency causes an increased concentration of methyl malonic acid, which competes with malonyl CoA and impairs fatty acid synthesis.


vi. Biotin (Vitamin B7):


Biotin is a heterocyclic monocarboxylic acid. It is a Sulphur-containing water soluble vitamin.




Foods rich in biotin are liver, kidney milk and egg-yolk. Vegetables, grains are good sources.




·     It is essential for the synthesis of lipids.


·     Biotin functions as the coenzyme for the enzyme called carboxylases, which catalyzes carboxylation reactions. Eg. Acetyl CoA carboxylase converts acetyl CoA to malonyl - CoA, which is required for fatty acid synthesis.


·     It helps in the conversion of pyruvic acid to oxaloacetic acid. The enzyme that catalyzes this reaction is pyruvate carboxylase.


·     It helps to maintain the skin and the nervous system in good condition.


·     It involves in deamination of certain amino acids like aspartate, serine and threonine.




Egg white contains a protein called avidin (egg white injury factor) which binds very tightly with biotin, preventing its absorption and thereby inducing biotin deficiency. The symptoms include depression, hair loss and muscle pain.


vii. Folic acid (Vitamin B9):


Folic acid otherwise known as vitamin B9, folavin or folate is essential for cell division. Its active coenzyme is tetrahydrofolate (THF). The naturally occuring folate is susceptible to high heat and UV light and also be subject to oxidation.




Folate naturally occurs in a wide variety of foods, including dark green leaf vegetables, fruits, nuts, soybeans, chickpeas, dairy products, poultry and meat, eggs, seafood, grains. Vegetables such as avocado, beetroot, spinach, liver, yeast, asparagus, kale, and brussels and sprouts are among the foods with the highest levels of folate.




·     Folic acid is essential for cell division and growth


·     It is important for preventing birth defects.


·     It is used in the synthesis of amino acid methionine.


·     Tetrahydrofolate (THF) is needed to transfer one carbon units in the biosynthetic reactions.


Absorption and storage


Folates are absorbed in the small intestine. The body can store a small amount in liver.


The excess is removed from liver via bile.




Folate deficiency signs include anemia (megaloblastic anemia) and dysfunction of the gastrointestinal tract. The anemia is caused by the abnormal blood cell division resulting in fewer and larger red blood cells.


Lack of folate in the diet can cause neural tube defects in an embryo of a pregnant woman. These can cause fatal birth defects.


viii. Pantothenic acid (Vitamin B5):


Pantothenic acid is found in every living cell including plant, animal and microbes. It is a part of coenzyme A which is an essential coenzyme involved in the catabolism of carbohydrates, proteins and fat.




Pantothenic acid is present in dried mushrooms, avocado, dried egg yolks and sunflower seeds in high amounts. Outer layer of whole grains contains the vitamin, but milling removes much of the pantothenic acid.




·     Pantothenic acid forms a part of coenzyme A which is an essential coenzyme for metabolising carbohydrates, fat and proteins.


·     It is necessary to synthesise vitamin D, steroid hormones and red blood cells


·     It also helps to boost the immunity


Absorption and storage


Free pantothenic acid is absorbed into intestinal cells via a saturable, sodium-dependent active transport system. However, in foods, most pantothenic acid is in the form of CoA or bound to acyl carrier protein (ACP). Since the intestinal cells can only absorb this vitamin in free pantathenic acid, it is converted into free panthothenic acid in intestine.




Pantothenic acid deficiency is very rare and seen only in cases of severe malnutrition. Pantothenic acid is found in many common foods and average diets are thought to have an adequate amount of it.

Vitamin C


Vitamin C is called as ascorbic acid. Ascorbic acid is an enediol-lactone of an acid with configuration similar to that of the sugar L-glucose.




Citrus fruits like orange, lemon are especially rich in vitamin C. Water melons, tomatoes, grape and leafy vegetables are also good sources.


Amla – 600-700mg/100g


Guava- 200-300mg/100g




·     Vitamin C is involved in cellular oxidation-reduction reactions inside the cell as hydrogen carrier.

·     Vitamin C is essential for building collagen, the connective tissue protein which cements the cells and tissues together.


·     It regulates carbohydrate metabolism.


·     Vitamin C is required as coenzyme for the enzyme dopamine hydroxylase which catalyzes the conversion of dopamine to nor-epinephrine.


·     It is involved in the maturation of red blood cells.


·     The absorption of iron is significantly enhanced by the presence of vitamin C.


·     It has a general antioxidant role , especially in the regeneration of oxidized vitamin E in membranes.


Absorption and storage


Ascorbic acid is rapidly absorbed from the intestines and passed on through the portal vein to the general circulation. Vitamin C is found in highest concentrations in the adrenals, the pituitary and retina than in circulation. Excessive intake of the vitamin do not increase beyond the optimal levels.




Scurvy is the classical syndrome of vitamin C deficiency. It is related to defective collagen synthesis which is indicated by fragile skin, muscle weakness, bleeding of the gums, loose teeth and delayed wound healing.

Table 8.1. Recommended Daily Allowances(RDA) of fat and water soluble vitamins for Adults


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11th Biochemistry : Chapter 8 : Vitamins : Water soluble vitamins | Classification, Sources, Functions, Deficiency

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