VPB 121: Veterinary Physiology - II (2+1)

FORMATION OF VOLATILE FATTY ACIDS

Acetic acid formation

• Oxydative decarboxylation of pyruvic acid
  • Pyruvic acid is converted into Acetyl-CoA by the removal of CO₂ and H₂, in the presence of thiamine pyrophosphate (TPP) and lipomide. The Acetyl-CoA yields acetic acid.
• Phophoroclastic split
  • Two molecules of pyruvic acid yield one molecule of acetic acid and formic acid. The formic acid is converted to CO₂ and H₂. The methanogenic bacteria  utilises a portion of this  H₂ for CH₄ production, whereas  the other portion of  H₂ will be utilised for  the production of succinate, propionate, butyrate, lactate as well as hydrogenation of unsaturated fatty acids. 

 2CH₃-CO-COOH CH₃-COOH + HCOOH   CO₂ + H₂ +ATP + CH₄

Propionic acid formation

• By CO₂ fixation
  • CO₂ combines with pyruvic acid to form oxalo acetic acid (CH₂COOH CO COOH) which is then reduced by hydrogenation (+2H) to malic acid;
  • On removal of one water molecule malic acid is converted to fumaric acid (CHCOOH CHCOOH).
  • Addition of H₂ and one molecuel of ATP  to fumaric acid  results in the formation of succinic acid
  • Decarboxylation   (- CO₂) of  succinic acid yields  propionic acid (CH₃ CH₂ COOH).
• By acrylate pathway
  • Pyruvic acid on hydrogenation (+2H) forms lactic acid (CH₃CHOH COOH)
  • On removal of water  lactic acid is converted to acrylic acid (CH₂CH COOH) on hydrogenation (+2H) it yields propionate.
• Butyric acid
  • Two molecules of acetyl-CoA condense to form acetoacetyl-CoA  and 2H₂.  On reduction (+2H) acetyl-CoA is converted to beta hydroxy butyrl CoA, which  by the removal of one molecule of H₂O is  converted to crotonyl CoA. Reduction of crotonyl CoA leads to formation of Butyrl CoA   along with one molecule of ATP . The butyrl CoA yields butyrate (CH₃CH₂CH₂COOH).
  • During the formation of  acetate and butyrate NAD is generated.The production of acetate leads to generation of ATP and formation of excess NADH. During the formation of propionate NAD is regenerated with a release of free hydrogen and this H is subsequently used to reduce CO₂ to CH₄ and H₂O.
  • Thus there is a direct relationship between acetate and CH₄ production.when more acetate is produced from pyruvate, more CH₄ is also produced.
  • Likewise, there is a reciprocal relation between propionate production and CH₄ formation; as more pyruvate is converted to propionate production, CH₄ formation is reduced.