Degradation of different carbohydrates

Degradation of different carbohydrates

Four steps are involved in the degradation of carbohydrates. these are Adherence, Disaggregation, Extra-cellular and Intra-cellular degradation.

1. Adherence

Adherence is the most important factor in fiber digestion. Coccoid show preference to get attached to plant cell wall. Adherence helps the bacteria to be retained for a longer time and facilitates sustained action.

This attachment helps the cell bound enzymes of adhered bacteria to come into intimate contact with substrate and ensure that resulting degradation products are preferentially available.

Factors affecting the adherence

pH– At a pH below 5.0 adherence decreases and is facilitated at the pH of 5.5 to 7.8

Temperature– Adherence is maximum at 40 ⁰C.

Lignin and lignified portions of the plant cell wall are inhibitory. Soluble cellulose derivatives like carboxy methyl cellulose and methyl cellulose are inhibitory.

Certain compounds like phenyl propionic acid increases attachment with plant cell wall.

2. Disaggregation

The fibrous feeds are soaked in the rumen fluid and are broken to small pieces. Starch granules are easily attacked when ground. Lignin inhibits degradation of structural carbohydrates.

3. Extra-cellular degradation

The rumen liquor is the best source of bacterial and protozoal enzymes, which are secreted by the microorganisms and released into the rumen liquor.

The food substances that are soaked in the rumen fluid are acted by these microbial enzymes and degraded into short chain oligosaccharides and sugars.

4. Intra-cellular degradation

The oligosaccharide and disaccharides are degraded to simple sugars prior to engulfment by bacteria.

The simple sugars enter into the bacterial cell for further metabolism by the microbes. The sugars are metabolized in the microbes by phosphoroclastic cleavage by intracellular enzymes and leads to the formation of pyruvate, phosphophenol pyruvate, short chain fatty acids,CO₂ and methane (CH₄). During this process, 2 NAD are reduced to NADH.

Starch is degraded by bacterial enzymes to maltose and some glucose. Maltose is further fermented to glucose. Glycolysis is the major pathway of fermentation of glucose and other monosaccharides. Conversion of hexose to two molecules of pyruvate yields two ATPs, the main energy source for growth and maintenance of bacteria.

Cellulose is converted to glucose by cellulases and then to pyruvate by a complex process. Cellulose exists in amorphous and crystalline forms; crystalline form is the most difficult to degrade in the rumen. Hemicellulose is also degraded by cellulases similar to cellulose. Pentose sugars are produced from hemicellulose.Pectins are degraded to galacturonic acid, methyl esters of galacturonic acid and finally converted to short chain fatty acids.

Pyruvate is the intermediate product of carbohydrate metabolism in ruminants which is converted to short chain fatty acids, CO₂ and CH₄. During this process, ATPs are formed which are utilised for microbial growth and multiplication.

The microbes do not require CO₂ and CH₄ for their growth, whereas the short chain fatty acids are the major energy source for the host animal (ruminants). Concentration of short chain fatty acids in the rumen is within a range of 60 to 120mmol/L of rumen liquor.

Formation of volatile fatty acids

Acetic acid formation

1. 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.
2. 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

1. 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).
2. 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.
3. 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.