<P> Protein catabolism is the breakdown of proteins into amino acids and simple derivative compounds, for transport into the cell through the plasma membrane and ultimately for the polymerization into new proteins via the use of ribonucleic acids (RNA) and ribosomes . Protein catabolism, which is the breakdown of macromolecules, is essentially a digestion process . Protein catabolism is most commonly carried out by non-specific endo - and exo - proteases . However, specific proteases are used for cleaving of proteins for regulatory and protein trafficking purposes . One example is the subclass of proteolytic enzymes called oligopeptidase . </P> <P> The amino acids produced by catabolism may be directly recycled to form new proteins, converted into different amino acids, or can undergo amino acid catabolism to be converted to other compounds via the Krebs cycle . </P> <P> The primary reason for protein catabolism is so organisms can convert proteins into a form of energy that they can use or store . To reuse their proteins, bacteria or soil microorganisms break down their proteins through protein catabolism into their individual amino acids and are used to form bacterial proteins or oxidized for energy . To convert to energy, once the proteins are broken down, they are typically deaminated (removal of an amino group) so that they can be processed into the Krebs / Citric Acid (TCA) Cycle . By proceeding into the Citric Acid Cycle, the original proteins will be converted into usable energy for the organism . </P>

Which of the following is a by product of the catabolism of proteins