1 Chapter 17 - Amino Acid Metabolism Metabolism of the 20 common amino acids is considered from the origins and fates of their: (1) Nitrogen atoms (2) Carbon skeletonsFor mammals: Essential amino acids must be obtained from diet Nonessential amino acids - can be synthesized
2 17.1 The Nitrogen Cycle and Nitrogen Fixation Nitrogen is needed for amino acids, nucleotidesAtmospheric N2 is the ultimate source of biological nitrogenNitrogen fixation: a few bacteria possess nitrogenase which can reduce N2 to ammoniaNitrogen is recycled in nature through the nitrogen cycle
4 NitrogenaseAn enzyme present in Rhizobium bacteria that live in root nodules of leguminous plantsSome free-living soil and aquatic bacteria also possess nitrogenaseNitrogenase reaction:N2 + 8 H+ + 8 e ATP2 NH3 + H ADP + 16 Pi
5 17.2 Assimilation of Ammonia Ammonia generated from N2 is assimilated into low molecular weight metabolites such as glutamate or glutamineAt pH 7 ammonium ion predominates (NH4+)At enzyme reactive centers unprotonated NH3 is the nucleophilic reactive species
6 A. Ammonia Is Incorporated into Glutamate Reductive amination of a-ketoglutarate by glutamate dehydrogenase occurs in plants, animals and microorganismsIn mammals & plants, located in mitochondria.
7 B. Glutamine Is a Nitrogen Carrier in Many Biosynthetic Reactions A second important route in assimilation of ammonia is via glutamine synthetase
8 Glutamate synthase transfers a nitrogen to a-ketoglutarate Prokaryotes & plants
9 Fig 17.3 Alternate amino acid production in prokaryotes Especially used if [NH3] is low. Km of Gln synthetase lower than Km of Glu dehydrogenase.
10 Box 17.1 How some enzymes transfer ammonia from glutamine CP synthetase has 3 active sites connected by a tunnel running through the interiorProtects intermediates from being degraded by water
12 C. Regulation of Glutamine Synthetase in E. coli Glutamine synthetase (GS) plays a critical role in nitrogen metabolismE. coli enzyme regulated by: (1) Cumulative feedback inhibition (9 allosteric inhibitors with additive effects) (2) Covalent modification (3) Regulation of enzyme synthesis
13 Fig 17.4 Allosteric inhibition of GS in E. coli
14 Fig 17.5 Regulation of E. coli GS by covalent modification
15 Regulation of mammalian GS Regulation not as extensive as in microorganismsNo covalent regulationAllosteric inhibitors: glycine, serine, alanine, and carbamoyl phosphateAllosteric activator: a-ketoglutarate
16 17.3 Transamination Reactions Transfer of an amino group from an a-amino acid to an a-keto acidIn amino acid biosynthesis, the amino group of glutamate is transferred to various a-keto acids generating a-amino acidsIn amino acid catabolism, transamination reactions generate glutamate or aspartate