Vitamins and Coenzymes 1


Vitamins and Coenzymes 1

Vitamins and Coenzymes 1

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  • Definition: General term for any of several organic substances essential for normal metabolic process and which, when absent in the diet, produce deficiency states
  • Required for growth, reproduction, normal body function
  • Lack of vitamins may result in a deficiency disease
  • The term 'vitamin' was first used in 1912 to describe a 'vital amine' derived from rice husks that cured beriberi
  • Further discoveries were also called vitamins although not all are amines
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Classes of Vitamins

  • Water soluble
    • Readily excreted via kidney in urine
    • Required daily (in small amounts)
    • E.g. vitamins B and C
  • Lipid soluble
    • Stored in the body (usually in liver)
    • Excessive intake can cause toxicity
    • E.g. vitamins A, D, E and K
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  • Cofactors are often required to activate enzymes
  • Cofactors can be essential ions, e.g. Na+, K+, or coenzymes
  • Coenzymes can be loosely bound co-substrates (e.g. ATP, NADH) or tightly bound prosthetic groups (e.g. thiamine pyrophosphate, pyridoxal phosphate)
  • In aminals, many coenzymes are syntheised from dietary precursors
  • These dietary precurosors are often vitamins

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  • Definition: a non-protein organic compound, produced in living cells, which plays an itnimatre and frequently essential role in the activation of enzymes
  • First enzyme regenerates the coenzyme so that it can play a role in the activation of the second enzyme
  • Co-substrates are altered during the course of the reaction and then dissociate from the enzyme. They are then regenerated by another enzymatic reaction
  • Prosthetic groups remain bound to the enzyme but still need to be regenerated
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B Group Vitamins

  • Water soluble
  • Originally thought to be one factor
  • Necessary for growth
  • Classed together as found in same foods although they differ in structure and function
  • Most can be synthesised by intestinal flora
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Vitamin B1 - Thiamine

  • Source: found in cells so present in all whole natural foods, e.g. pork, liver, kidneys, whole cereal grains, peas, beans, milk, eggs and yeast

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Vitamin B1 - Thiamine

  • Biological function: acts as a coenzyme as its pyrophosphate ester: thiamine pyrophosphate (TPP)

  • Synthesised in the intestinal mucosa from dietary thiamine and ATP
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Thiamine Pyrophosphate

  • Used as a coenzyme by a variety of enzymes
  • Helps break down food and release energy. Forms prosthetic group to enzymes at several stages in the metabolism of carbohydrates
  • E.g. pyruvate decarboxylase, pyruvate dehydrogenase (conversion of pyruvic acid to coenzyme A), transketalose (pentose phosphate pathway)

  • Also involved in conduction of action potentials 
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Vitamin B1 - Thiamine Deficiency

  • Prevalent in eastern Asia (rice contains little thiamine)
  • Also sometimes seen in chronic alcoholics (severely malnourished). Athletes and pregnant/lactating women may require supplements (increased carbohydrate consumption)
  • Wet beriberi
    • cardiac failure, decreased permeability of capillary walls, oedema
  • Dry beriberi
    • peripheral neuritis, wastage, paralysis
  • Wernicke-Korsakoff syndrome affects the central nervous system and peripheral nerves
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Vitamin B2 - Riboflavin

  • Source: milk, liver, kidneys, heart, eggs, whole cereal grains and green vegetables - only vitamin present in beer
  • Stable to cooking but not bright light

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Vitamin B2 - Riboflavin

  • Biological function: Important component of two coenzymes
    • flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
  • Many oxidoreductase enzymes require FAD or FMN (known as flavoproteins) as the prosthetic group
  • Metabolism of fats, carbohydrates and proteins
  • Heterocyclic system acts as hydrogen acceptor of donor, e.g. FAD to FADH2
  • E.g. glycerol phosphate shuttle in carbohydrate metabolism
  • Also involved in respiration, growth and reproduction
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Vitamin B2 - Riboflavin Deficency

  • Caused by malnutrition
  • Symptoms include inflammation of the mouth and tongue, lesions to intestinal mucosa, dermatitis, defective vision and injury to bone marrow
  • Rare in developed countries
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Vitamin B6 - Pyridoxine

  • Source: found in most foods of vetabole (pyridoxine) or animal (pyridoxal, pyridoxamine) origin
  • Interconvertible via their phosphates in vivo

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Vitamin B6 - Pyridoxine, Deficiency and Toxicity

  • Biological function: as the coenzyme pyridoxal-5'-phosphate (used in over 40 known enzymatic reactions)
  • Coenzyme for several enzymes concerned with tryptophan metabolism
  • Deficiency
    • Rare as most diets contain adequate amounts and some is synthesised by intestinal flora
    • Results in disorders of CNS, skin and mucous membranes
    • Those following high protein diets and patients taking isoniazid may benefit from increased intake
  • Toxicity: Peripheral neuropathy
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Vitamin B3 - Niacin (Nicotinic Acid) and Nicotinam

  • Source: nicotinic acid present in yeast, liver, lean meats, peanuts, peas, beans, whole wheat and rice grains
  • Stable to cooking

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Vitamin B3 - Niacin (Nicotinic Acid) and Nicotinam

  • Biological function: in the coenzymes NAD and NADP
  • Required by many oxidoreductase enzymes, e.g. reduction of folate to THF
  • NAD+ used as a co-substrate in oxidative processes (major electron acceptor in the oxidation of fuel molecules)
  • NADPH involved in reductions of intermediate metabolites
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Vitamin B3 - Niacin Deficiency and Toxicity

  • Body not entirely dependent on dietary intake as nicotinic acid is formed from tryptophan in vivo
  • Pellagra - characterised by dermatitis, diarrhoea and dementia. Can be common in areas in which maize is principal foodstuff (low in nicotinic acid and tryptophan)
  • Other risk factors: alcoholics, malabsorption, chronic diarrhoea, diabetis mellitus, vitamin B6 deficiency
  • Nicotinamide may be given as alternative supplements
  • Toxicity: Skin flushes, liver damage
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Vitamin B5 and Vitamin B7

  • Pantothenic acid (vitamin B5):
    • found in liver, yeast, egg yolk and fresh vegetables
    • constituent of coenzyme A
  • Biotin (vitamin B7):
    • found in egg yolk, liver, wheatgerm, milk and yeast
    • cofactor for carboxyl group transferases in fat metabolism
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