Biochemistry, Protein Catabolism

Excerpt

Protein catabolism is the breakdown of proteins into absorbable monomers for further degradation or reassembly. Protein catabolism in the intestinal lumen is important for several reasons, one of which is mobilizing essential amino acids for absorption. Essential amino acids can’t be synthesized in the human body but are needed for the biosynthesis of vital proteins, so their only source is polypeptide breakdown through digestive enzymes. This process begins in the stomach and continues in the small intestine. Large protein chains are disassembled to eventually leave free amino acids that can be taken up into the blood and transported to various cells around the body for further breakdown.

Endopeptidases in zymogen form are released by the stomach mucosa and the exocrine pancreas to cleave the polypeptide chain between particular amino acid residues. Once in a smaller form, exopeptidases remove the last amino acids from the C or N terminus of a di-peptide or tri-peptide one by one, aiding absorption at the microvilli. Cells can use those amino acids to construct vital proteins or as substrates for energy creation. Proteins created intracellularly can also be catabolized for the same reasons. Intracellular proteins that were either misfolded or are no longer functioning in the cell also undergo intracellular protein catabolism in the lysosome, with the help of ubiquitin and proteasome formation. If a cell is in a low energy state, the free amino acids in the cytosol are further degraded to produce citric acid cycle intermediates and are funneled there to produce ATP. While the carbon backbone enters energy-generating pathways, the nitrogen backbone is modified and excreted mostly through the kidneys.