Chapter 17: Amino Acid Metabolism

The process begins with biological nitrogen fixation, where specialized bacteria and cyanobacteria use the nitrogenase enzyme complex to convert atmospheric nitrogen gas into ammonia, an energetically expensive reaction requiring substantial ATP investment. Organisms then assimilate ammonia primarily through glutamate dehydrogenase and glutamine synthetase, which funnel nitrogen into central metabolic pathways via transamination reactions catalyzed by transaminases using pyridoxal phosphate as a coenzyme. The chapter details how carbon skeletons for all twenty amino acids derive from intermediates in glycolysis, the citric acid cycle, and the pentose phosphate pathway, with mammals capable of synthesizing only about half while requiring the remainder as dietary essentials. Beyond their role in protein synthesis, amino acids serve as precursors for critical biomolecules including porphyrins, heme, nitric oxide, and melanin pigments. The text addresses continuous protein turnover in cells, where ubiquitin tagging and proteasomal degradation selectively remove proteins in response to physiological demands. During catabolism of excess amino acids, the alpha-amino group is removed through transamination to form alpha-keto acids, while the remaining carbon skeletons are converted into one of seven central metabolites, classified as either glucogenic intermediates feeding into gluconeogenesis or ketogenic products forming ketone bodies and fatty acids. The chapter concludes with nitrogen excretion mechanisms, emphasizing the urea cycle as the primary pathway for detoxifying waste ammonia in terrestrial vertebrates, integrating mitochondrial and cytosolic reactions to convert ammonia into water-soluble urea for safe urinary excretion, while also addressing how kidneys regulate acid-base balance through glutamine catabolism.