Vitamins and Coenzymes 1

Vitamins and Coenzymes 1

• 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

• 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

• 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

• 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

• 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

• 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

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

• Synthesised in the intestinal mucosa from dietary thiamine and ATP

• 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 

• 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

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

• 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

• 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

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

• 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

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

• 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

• 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

• 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