B vitamins and their coenzymes

Lesson 15 : Enzymes- Coenzymes and Co-Factors

B vitamins and their coenzymes

  1. Niacin (nicotinic acid) --> nicotinamide -->
  2. Get niacin in enriched cereals, meat, legumes.
  3. NAD+ and NADP+ are the coenzymes (cosubstrates).
  4. NAD + consists of 2 5’ribonucleotides (AMP and nicotinamide mononucleotide) joined by a phosphoanhydride linkage.
  5. NADP+, has a phosphoryl group on 2’-oxygen.
  6. Both coenzymes act as cosubstrates for dehydrogenases --> catalyze the oxidation of substrates by transfer of 2e- and 1H+ ---> NADH and NADPH.

Vitamin B2 (riboflavin)

  1. Coenzymes are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).
  2. Riboflavin found in milk, whole grains, liver.
  3. The coenzymes serve as prosthetic groups involved in 1e- or 2e- transfers.

    • FAD + 2e- + 2H+ ---> FADH2
    • FMN + 2e- + 2H+ --> FMNH2

  4. Enzymes that require FAD or FMN are called flavoenzymes or flavoproteins.
  5. Can actually donate 1 or 2 e- at a time --> form partially oxidized compound when only 1e- is donated --> relatively stable.

Vitamin B1 (thiamine)

  • Structure: pyrimidine ring and positively charged thiazolium ring.
  • Found in husks of rice and other cereals, liver, meat, particularly pork.
  • Deficiency in thiamine causes beriberi - extensive nervous system and circulatory system damage, muscle wasting, edema.
  • Coenzyme form is thiamine pyrophosphate (TPP) - synthesized by transfer of pyrophosphoryl group from ATP via thiamine pyrophosphate synthetase.
  • Used primarily in decarboxylases as a coenzyme.

Vitamin B6 family

  1. pyridoxine, pyridoxal, pyridoxamine are the vitamins.
  2. Act as prosthetic groups.
  3. Formed by the following reaction:

    • pyridoxine + ATP --> pyridoxine 5’phosphate --> pyrodoxal 5’ phosphate (PLP).

  4. Lack of B6 results in defects in protein metabolism.
  5. PLP found in enzymes that catalyze reactions involving amino acids, e.g. isomerizations, decarboxylations, R-group removal or replacements.
  6. Most frequent reaction is a transamination, where the a-amino group of a.a. is transferred to carbonyl group of a-keto acid --> new a.a. made or is excreted.
  7. PLP binds covalently with Lys residue in active site --> keeps PLP from running away.


  1. Synthesized by intestinal bacteria.
  2. Prosthetic coenzyme is called biocytin - covalently linked to Lys residue in active site.
  3. Involved in carboxyl group transfer reactions and ATP-dependent carboxylations.

    • E.g. pyruvate carboxylase
    • pyruvate + HCO3- ---> oxaloacetate

  4. Binds to HCO3- and acts as a CO2 carrier.

Folic Acid or Folate

  • Found in green leafy vegetables, liver, yeast.
  • Coenzyme form is tetrahydrofolate.
  • Used by enzymes that transfer 1-C units as methyl groups (CH3-).
  • Another folate coenzyme is tetrahydrobiopterin - used in hydroxylases.

Pantothenic Acid

  • Used in coenzyme A formation.
  • Reactive center is -SH group
  • Key in all acyl-group transfers
  • Coenzyme form is phosphopantethine - added to serine residue of protein --> acyl carrier protein (ACP) --> important in fatty acid synthesis.

Vitamin B12 or cobalamin

  1. Found in organ meat (kidney and liver).
  2. It is a prosthetic coenzyme.
  3. Ring structure similar to heme, with cobalt atom in center.
  4. Involved in molecular rearrangements.
  5. Deficiency in B12 results in pernicious anemia (decreased production of blood cells from bone marrow).

Vitamin C or ascorbic acid

  • Found in fresh fruit and vegetables.
  • Participates in hydroxylation reactions, e.g. collagen synthesis.
  • Deficiency causes scurvy.
Last modified: Friday, 20 January 2012, 6:26 AM