CELLULAR BIOCHEMISTRY AND METABOLISM (CLS 331) Dr. Samah Kotb
Nasr Eldeen Dr. Samah Kotb Nasr Eldeen 1 Dr Samah Kotb Lecturer of
Biochemistry
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THE ROLE OF VITAMINS IN METABOLISM 2 Dr Samah Kotb Lecturer of
Biochemistry
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What is a vitamin???
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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.
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Classification of vitamins
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Types of vitamins
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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.
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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.
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WATER SOLUBLE VITAMINS
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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.
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1-Thiamin
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Thiamin : vitamin form Pyrimidine ringThiazole ring Thiamin
pyrophosphate: coenzyme form
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Function of thiamin
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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.
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2-Riboflavin
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Riboflavin :- vitamin form
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Function of Riboflavin
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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.
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3-Niacin
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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.
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4-Pyridoxine
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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
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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.
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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.
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5- Biotin
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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.
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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.
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Function
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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
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2 ) Acetyl-CoA carboxylase, the control step for fatty acid
synthesis (this enzyme converts acetyl-CoA to malonyl-CoA).
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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.
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4) -Methylcrotonyl-CoA carboxylase, an enzyme required for
oxidation of leucine and some isoprene derivatives.
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6-Folic acid
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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.
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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.
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.
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Methionine synthetase is involved in the synthesis of DNA and
RNA via purine and pyrimidines and influences the entry of folate
into cells.
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8-Vitamin C
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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.
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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.
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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.
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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.