CELLULAR BIOCHEMISTRY AND METABOLISM (CLS 331) Dr. Samah Kotb Nasr Eldeen Dr. Samah Kotb Nasr Eldeen 1 Dr Samah Kotb Lecturer of Biochemistry

THE ROLE OF VITAMINS IN METABOLISM 2 Dr Samah Kotb Lecturer of Biochemistry

What is a vitamin???

A vitamin is both: An organic compound (contains carbon). An essential nutrient, the body cannot produce enough of on its own, so it has to get it (tiny amounts) from food.

Classification of vitamins

Types of vitamins

1 ) Fat-soluble vitamins are stored in the fat tissues of our bodies, as well as the liver. Fat-soluble vitamins are easier to store than water-soluble ones, and can stay in the body as reserves for days, some of them for months. Fat-soluble vitamins are absorbed through the intestinal tract with the help of fats (lipids). Vitamins A, D, E and K are fat-soluble.

2) Water-soluble vitamins do not get stored in the body for long - they soon get expelled through urine. Water-soluble vitamins need to be replaced more often than fat-soluble ones. Vitamins C and all the B vitamins are water-soluble.

WATER SOLUBLE VITAMINS

Introduction Water-soluble vitamins are alike in that, with the exception of B12, they can be supplied by plants in the diet. These vitamins are not stored in the body for very long and therefore need to be consumed regularly. Primarily the water-soluble vitamins serve as coenzymes in metabolic reactions. The majority of the water-soluble vitamins are 'B' vitamins and these play a major role in energy metabolism.

1-Thiamin

Thiamin : vitamin form Pyrimidine ringThiazole ring Thiamin pyrophosphate: coenzyme form

Function of thiamin

Function Thiamin, or vitamin B1, plays a major role in carbohydrate metabolism. Thiamin acts as a coenzyme along with phosphorus in important cellular reactions such as decarboxylation and transketolation. Thiamin pyrophosphate (TPP), a coenzyme, allows pyruvate to enter the citric acid cycle (Krebs' cycle) to produce energy for cellular functions. TPP acts in fat synthesis by transketolation, providing glyceraldehyde for the conversion of glucose to fat. Thiamin is thought to be involved in neurotransmission and nerve conduction.

2-Riboflavin

Riboflavin :- vitamin form

Function of Riboflavin

Function Riboflavin, or B2, is a constituent of coenzymes called flavoproteins. Flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) are vital in the respiratory chain of cellular energy metabolism. FMN is used in deamination, which is the process of removing the amino group from amino acids. FAD is used in the deamination of glycine, an amino acid. FAD is also involved in the oxidation of some fatty acids.

3-Niacin

Function Niacin, also called nicotinic acid, is involved in oxidation-reduction reactions as coenzymes. Niacin is involved in the synthetic pathway of adenosine tri-phosphate (ATP) and in ADP-ribose transfer reactions. The two niacin coenzymes involved in these processes are nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). NAD is a substrate for the enzyme poly (ADP- ribose) polymerase (PARP) which is involved in DNA repair. Tryptophan is a precursor to niacin.

4-Pyridoxine

Vitamin B 6 is a collective term for pyridoxine, pyridoxal, and pyridox- amine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring

Function Pyridoxine, or vitamin B6, is a coenzyme in reactions of amino acid, carbohydrate, and fat metabolism. Vitamin B6 is actually a term used for a group of vitamins with similar functions: pyridoxine, pyridoxal, and pyridoxamine. All are precursors to pyridoxine coenzyme pyridoxalphosphate (PLP). This coenzyme is involved in reactions involving many systems within the body. PLP has a role in gluconeogenesis through transaminase reactions. Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals.

The conversion of tryptophan to niacin utilizes an enzyme that requires PLP. In red blood cells PLP is a coenzyme for transaminases. PLP is also involved in the synthesis of several neurotransmitters, such as serotonin, taurine, dopamine, and norepinephrine. Intake of vitamin B6 has been associated with immune function.

5- Biotin

Some animal carboxylase enzymes (enzymes that add CO2 to substrates) require the water-soluble vitamin biotin. Biotin is covalently attached to the enzyme by an amide link to a lysine side chain.

An ATP-dependent process covalently links CO2 (using HCO3– as the actual substrate) to one of the biotin nitrogens; the carboxybiotin then acts as a carboxylate donor for the substrate.

Function

Animals have four biotin dependent enzyme complexes: 1) Pyruvate carboxylase, the first step of the gluconeogenic pathway from pyruvate, and an important source of oxaloacetate for the TCA cycle. Function

2 ) Acetyl-CoA carboxylase, the control step for fatty acid synthesis (this enzyme converts acetyl-CoA to malonyl-CoA).

3 ) Propionyl-CoA carboxylase, which produces methylmalonyl-CoA, the first step in the conversion of propionyl CoA (generated from odd-chain fatty acid and some amino acid oxidation) to succinyl- CoA, which can enter the TCA cycle.

4) β-Methylcrotonyl-CoA carboxylase, an enzyme required for oxidation of leucine and some isoprene derivatives.

6-Folic acid

Function Folic acid, also called folate, is involved in 1) DNA and protein synthesis. 2) It has a role in the synthesis of the amino acid methionine which is involved in lipid metabolism.

3)It has a primary role in purine synthesis, pyrimidine nucleotide synthesis, and the conversion of three amino acids. A) interconversion of the non-essential amino acids serine and glycine. B) catabolism of histidine to glutamic acid. C) conversion of homocysteine to methionine.

glycine serine histidine Glutamic acid methionine homocysteine

7-Vitamin B12

Function Vitamin B12, or cobalamin, is a coenzyme for methylmalonyl-CoA mutase and methionine synthetase. Methylmalonyl-CoA mutase is involved in the conversion of propionic to succinic acid, a factor of fatty acid metabolism.

Methionine synthetase is involved in the synthesis of DNA and RNA via purine and pyrimidines and influences the entry of folate into cells.

8-Vitamin C

Function Vitamin C is often called ascorbic acid. It has main functions: 1) Provides reducing equivalents for biochemical reactions, it is a reductive cofactor in the hydroxylation of the amino acids proline and lysine during the formation of collagen.

2) Serves as a cofactor for reactions requiring reduced metal ions. 3) Serves as a protective antioxidant that it can decrease free radicals, and can easily return to its reduced state.

4) It protects against the peroxidation of plasma lipid and low-density lipoprotein (LDL), provides antioxidant protection in the eye, and protects DNA from oxidative damage. 5) It is involved in the neurotransmitter synthesis.

6) It is involved in the regulation of iron metabolism. 7) Vitamin C and iron play a role in the synthesis of carnitine. Vitamin C enhances vasodilatory and anticlotting effects.