5.5: Proteins and Amino Acids

Proteins are long chains of amino acids linked by peptide bonds. While they serve as structural components of muscle, skin, and hair, they also function as enzymes, hormones, antibodies, and transport molecules. When dietary protein exceeds these functional needs, the excess can be broken down and used for energy.

Of the 20 amino acids in animal proteins, approximately ten are classified as essential for monogastric animals—they cannot be synthesized at rates sufficient to meet requirements. The requirement varies by species (e.g., ten for pigs, more for poultry/cats).

The limiting amino acid is the essential amino acid present in the diet at the lowest level relative to the animal's requirement.

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Protein digestion starts in the stomach. Where stomach acid converts pepsinogen into pepsin, which begins breaking down protein chains. The process continues in the small intestine, where the pancreas releases enzymes (trypsin, chymotrypsin, and carboxypeptidases) that further break down proteins. Finally, enzymes on the intestinal lining complete the job, creating amino acids and small peptides that the body can absorb.

Absorbed amino acids enter the blood and reach the liver, where they may be used for protein synthesis, converted to glucose, oxidized for energy, or released into general circulation. Excess amino acids are deaminated—the amino group is removed, converted to urea via the urea cycle (or uric acid in birds), and excreted. The remaining carbon skeleton can be used for energy or converted to glucose or fat.

Ruminant protein nutrition differs significantly from that of monogastric animals. Dietary protein is divided into rumen-degradable protein (RDP) and rumen-undegradable protein (RUP, also known as bypass protein). Microbes in the rumen break down RDP into amino acids, then deaminate them to release ammonia. Using the ammonia and carbon skeletons from carbohydrate fermentation, the microbes synthesize microbial protein. Both microbial protein and RUP then move to the abomasum and small intestine for normal enzymatic digestion.

Because rumen microbes can synthesize amino acids from ammonia, ruminants can utilize non-protein nitrogen (NPN) sources, such as urea. However, if RDP exceeds available energy, excess ammonia builds up, is absorbed into the bloodstream, converted to urea in the liver, and excreted—wasting energy and increasing nitrogen loss to the environment.

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