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 skeletons

•For mammals: Essential amino acids must be obtained from dietNonessential amino acids - can be synthesized

17.1 The Nitrogen Cycle and Nitrogen Fixation

• Nitrogen is needed for amino acids, nucleotides

• Atmospheric N2 is the ultimate source of biological nitrogen

• Nitrogen fixation: a few bacteria possess nitrogenase which can reduce N2 to ammonia

• Nitrogen is recycled in nature through the nitrogen cycle

Fig 17.1 The Nitrogen cycle

Nitrogenase

• An enzyme present in Rhizobium bacteria that live in root nodules of leguminous plants

• Some free-living soil and aquatic bacteria also possess nitrogenase

• Nitrogenase reaction:

N2 + 8 H+ + 8 e- + 16 ATP

2 NH3 + H2 + 16 ADP + 16 Pi

17.2 Assimilation of Ammonia

• Ammonia generated from N2 is assimilated into low molecular weight metabolites such as glutamate or glutamine

• At pH 7 ammonium ion predominates (NH4+)

• At enzyme reactive centers unprotonated NH3 is the nucleophilic reactive species

A. Ammonia Is Incorporated into Glutamate

• Reductive amination of -ketoglutarate by glutamate dehydrogenase occurs in plants, animals and microorganisms

• In mammals & plants, located in mitochondria.

B. Glutamine Is a Nitrogen Carrier in Many Biosynthetic

Reactions

• A second important route in assimilation of ammonia is via glutamine synthetase

Glutamate synthase transfers a nitrogen to -ketoglutarate

Prokaryotes & plants

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.

Box 17.1 How some enzymes transfer ammonia from glutamine

• CP synthetase has 3 active sites connected by a tunnel running through the interior

• Protects intermediates from being degraded by water

Carbamoyl phosphate synthase backbone structure

• Tunnel connecting active sites (blue wire)

C. Regulation of Glutamine Synthetase in E. coli

• Glutamine synthetase (GS) plays a critical role in nitrogen metabolism

• E. coli enzyme regulated by:(1) Cumulative feedback inhibition

(9 allosteric inhibitors with additive effects)(2) Covalent modification(3) Regulation of enzyme synthesis

Fig 17.4 Allosteric inhibition of GS in E. coli

Fig 17.5 Regulation of E. coli GS by covalent modification

Regulation of mammalian GS

• Regulation not as extensive as in microorganisms

• No covalent regulation

• Allosteric inhibitors: glycine, serine, alanine, and carbamoyl phosphate

• Allosteric activator: -ketoglutarate

17.3 Transamination Reactions

• Transfer of an amino group from an -amino acid to an -keto acid

• In amino acid biosynthesis, the amino group of glutamate is transferred to various -keto acids generating -amino acids

• In amino acid catabolism, transamination reactions generate glutamate or aspartate

Fig 17.6 Transamination reactions

Fig 17.7

• Ping-pong kinetics of aspartate transaminase

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Fig 17.7 (cont)

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