Chapter 23: Protein Turnover & Amino Acid Catabolism

A significant focus is placed on the removal of nitrogen, the first step in amino acid degradation, which primarily occurs in the liver. The text explains the critical role of aminotransferases (transaminases) and their prosthetic group, pyridoxal phosphate (PLP), in funneling alpha-amino groups to alpha-ketoglutarate to form glutamate. This glutamate subsequently undergoes oxidative deamination via glutamate dehydrogenase to release ammonium ions. The summary outlines the urea cycle in depth, describing how terrestrial vertebrates convert toxic ammonium into urea for excretion through a series of enzymatic steps involving carbamoyl phosphate synthetase, ornithine transcarbamoylase, and arginase. The connection between the urea cycle and gluconeogenesis is highlighted through the production of fumarate. Furthermore, the chapter categorizes amino acids as glucogenic or ketogenic based on the metabolic fate of their carbon skeletons, which enter the citric acid cycle or glycolysis at one of seven points: pyruvate, acetyl CoA, acetoacetyl CoA, alpha-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate. Specific degradation pathways are detailed, including the breakdown of branched-chain amino acids and the oxygenase-dependent cleavage of aromatic rings. Finally, the chapter connects biochemical pathways to clinical medicine by examining inborn errors of metabolism, such as phenylketonuria (PKU), maple syrup urine disease, and alcaptonuria, demonstrating the pathological consequences of enzymatic deficiencies in amino acid metabolism.