Protein digestion in ruminants
Rumen microbial population have proteolytic activity. The diet of ruminants contains proteins and non-protein nitrogenous (NPN) substances (ammonia, nitrates, urea). Of the total proteins entering the rumen, 20 to 100% will be degraded to ammonia which are referred as rumen degradable protein (RDP).
The fraction which is not degraded by the microbes in the rumen escape and by-pass the microbial digestion, called as rumen undegradable protein (UDP); this fraction reaches the small intestine for enzymatic digestion by the animal.
When the dietary proteins enter the rumen, the RDP fraction is hydrolysed by extracellular microbial proteases and produce short-chain peptides as end products.
The peptides are absorbed into the microbial cell bodies. Within the microbial cells, the peptides can either be used for the formation of microbial protein or enter in to VFA pathways for energy production.
The amino acids on deamination yield ammonia and a carbon skeleton; the carbon structures of many amino acids enter directly into various steps of the VFA pathways, leading to the formation of three major VFAs (Acetic, Butyric and Propionic acids).
Three branched-chain amino acids valine, leucine and isoleucine are involved in the biosynthesis of branched-chain VFAs, the iso acids (isobutyrate, isovalerate and 2–methyl butyrate) which are growth promoters of cellulolytic rumen microbes.
Nitrogen metabolism in rumen
Some proteins are converted to amino acids extra cellularly. The amino acids are absorbed by the microbial cells and are utilised by the bacteria for incorporating into the microbial protein or microbial cell wall or in the nucleic acids.
The amino acid requirement of the ruminant is met by microbial proteins synthesized by bacteria utilizing dietary protein,NPN substances,urea from saliva,urea through rumen epithelium and un degraded proteins that escapes degradation by rumen microbes (UDP).
Protein breakdown in the rumen is proportional to solubility, the degree of secondary and tertiary structures, cleavage of disulphide bonds, cross linking between amino acids and also the concentration of ammonia in rumen fluid.
The merit of conversion of dietary protein to microbial protein,depends upon the composition of food protein.
Proteolytic activity is well pronounced in Bacteroides sp, Selenomonas sp, Butyrivibrio sp, megasphaera elsdeni and Succinivibrio dextrisolvens.
Proteolysis results in oligopeptides which undergo degradation to smaller peptides and amino acids. Peptides are more effectively incorporated into bacterial protein whereas amino acids undergo rapid deamination providing NH3 for bacterial growth. The amino acids on deamination give rise to NH3, CO2 and VFA.
Ammonia nitrogen in the rumen fluid
Ammonia is an important source of N for the growth of rumen microorganisms. About 92% of rumen bacteria utilize ammonia as a nitrogen source.
Ammonia nitrogen in the rumen fluid varies with diet( 2mg to 100 mg/ dl) ( normal range is 5-25mg/dl)
High concentration of starch diet tends to reduce rumen ammonia. Urea, frequently used to replace true protein in ruminant ration as a non-protein source is rapidly cleaved in rumen by ureolytic bacteria to ammonia.
Ammonia toxicity is most frequent sequelae of urea feeding where rumen pH may rise to 7.3. But urea toxicity is tolerable if pH is maintained within 7.0.
The route by which NH3 utilized depends upon the concentration of NH3 in the rumen fluid. When the NH3 concentration is low, the energy dependent GS/GOGAT systems is utilized whereas when the concentration is high, GDH system is utilized. This is the pathway by which NH3 is converted to microbial protein. When NH3 present in the form of ammonium ion and it requires active transport.
Glutamine occupies a central role in the nitrogen metabolism of rumen bacteria. As there is no functional TCA cycle (Ketoglutarate is not formed) it is synthesized by reverse TCA cycle.
Amino acid biosynthesis
- The NH3 is taken up by the bacteria and fixed to glutamate. Based on the source of carbon atom amino acids are divided into groups like glutamate family (glutamine, proline, arginine).
- Serine family (Serine, glycine, cysteine).
- Aspartate family (Aspartate, lysine, methionine, threonine, isoleucine).
- Pyruvate family (Alanine, isoleucine, leucine, valine).
- Aromatic family (Phenylalanine, tyrosine, tryptophan and histidine).