Protein catabolism - Krebs cycle

Digested proteins are broken down into amino acids which are not stored, but are either oxidized to produce ATP or used to synthesize new proteins.
Many proteins function as enzymes, some are involved in transportation, serving as antibodies, clotting blood, being hormones, or being part of muscle fibers.

Protein catabolism (breaking down) yields amino acids which are converted to other amino acids, fatty acids, ketone bodies, or glucose.
Cells oxidize amino acids to generate ATP via the Krebs cycle.
Protein anabolism (synthesis) creates new proteins by bonding together amino acids on ribosomes.