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BC368 Biochemistry of the Cell II

BC368 Biochemistry of the Cell II. Nitrogen Anabolism Ch 22 (22.1 and 22.2) May 5, 2015. Overview of amino acid anabolism. Biologically useful nitrogen compounds are generally scarce in nature.

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BC368 Biochemistry of the Cell II

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  1. BC368 Biochemistry of the Cell II Nitrogen Anabolism Ch 22 (22.1 and 22.2) May 5, 2015

  2. Overview of amino acid anabolism • Biologically useful nitrogen compounds are generally scarce in nature. • Most organisms maintain strict economy in their use of ammonia, amino acids, and nucleotides, often salvaging and reusing them.

  3. Overview of amino acid anabolism • Biologically useful nitrogen compounds are generally scarce in nature. • Most organisms maintain strict economy in their use of ammonia, amino acids, and nucleotides, often salvaging and reusing them. • The nitrogen cycle maintains a pool of biologically available nitrogen in nature. ~Fig 22-1

  4. The nitrogen cycle

  5. Nitrogen Fixation ~1.0 x 1010 kg/yr~8.0 x 1010 kg/yr~1.8 x 1011 kg/yr ~4% of total~30% of total~66% of total N2 + O2 2 NO  2 NO2 N2 + 3 H2 2 NH3 N2 + 10 H+  2 NH4+ + H2 2NO2 + H2O  HNO3 + HNO2

  6. Haber-Bosch Cycle N2 + 3 H2 --> 2 NH3 500oC, 300 ATM • Ammonia was first made on an industrial scale in 1913. • Critical for the German munitions effort. • Later, principally used to make fertilizer, allowing more efficient food production. • Nearly 80% of the nitrogen found in human tissues originated from the Haber-Bosch process. Fritz Haber article

  7. Nitrogen-fixing bacteria/archaea N2 + 8 e- + 10 H+ + 16 ATP 2 NH4+ + H2 + 16 ADP + 16 Pi 15-37oC, 0.8 ATM Azotobacter/Rhizobium/Clostridium

  8. Overview of Nitrogen Fixation Reduction of N2 is carried out by the nitrogenase complex, which uses ATP to transfer electrons from a donor (harvested from a redox protein such as ferredoxin). Nitrogenase Complex

  9. Nitrogenase Complex 2 enzymes in the nitrogenase complex: • Dinitrogenase reductase (Fe protein; dimer of two identical subunits; 2 dimers per complex) • Dinitrogenase (MoFe protein; a2b2 tetramer)

  10. Nitrogenase complex Green= Dinitrogenase Reductase Blue & Purple= Dinitrogenase • Note that the reductase requires 2 ATP to pass 1 electron

  11. Dimer Nitrogenase complex Tetramer Green= Dinitrogenase Reductase Blue & Purple= Dinitrogenase Dimer

  12. Nitrogenase Complex 2NH4+ + H2 Dinitrogenase Reductase Dinitrogenase 8 N2 + 10H+ Electrons are passed through redox centers to N2 and H+ (8 electrons total).

  13. Assimilation of NH3 into amino acids • Once ammonia has been formed via nitrogen fixation, the nitrogen can be incorporated into either glutamate or glutamine for further use: • Glu is the source of amino groups for synthesis of most amino acids • Gln is the source of amino groups for synthesis of most other nitrogen-containing molecules (e.g., nucleotides)

  14. Assimilation of NH3 into amino acids • Formation of Glu: reductive amination of α-KG via glutamate dehydrogenase • Formation of Gln: glutamine synthetase reaction

  15. Allosteric control of glutamine synthetase Fig 18-1 • Complex control with many allosteric regulators (end products of Gln metabolism)

  16. Fig 18-1

  17. In general, humans can’t synthesize amino acids that require more than 5 steps. • This one is arginine.

  18. Overview of amino acid biosynthesis Fig 18-1 • Carbon skeletons are made from intermediates of glycolysis, TCA, or pentose phosphate shunt • Amine groups are from Glu (or Gln)

  19. Amino acid biosynthesis • Six biosynthetic families (bold = essential):

  20. Essential Nonessential Amino acid biosynthesis His Pentose Tetrose + triose (PEP) Tyr/Trp/Phe (aromatics) (pentose phosphate shunt) Glucose (glycolysis) Ser 3PG Cys/Gly 1. Ala/Val/Leu/Ile Pyruvate 2. α-KG Glu Gln/Pro (urea cycle) Ornithine Arg 3. OA Asp Lys/Asn/Met/Thr

  21. Regulation • Typically, first enzyme in a pathway is allosterically inhibited by the end product (allosteric feedback inhibition).

  22. Regulation • Interlocking regulatory mechanisms ensure that amino acids are synthesized in the correct proportions for protein synthesis. E. coli regulation of various amino acids derived from Asp.

  23. Anabolic Window • The rate of skeletal muscle protein synthesis is regulated by food intake and exercise. • Ingestion of 15-20 g protein elevates muscle protein synthesis rates for up to 5 hours • Exercise stimulates skeletal muscle protein synthesis for up to 48 hours Dreyer et al. (2008) Am J Physiol Endocrinol Metab. 294(2): E392–E400.

  24. Anabolic Window • Ingestion of protein within an hour of exercise greatly increases the rate of skeletal muscle protein synthesis. Beverage containing essential amino acids and carbohydrate

  25. 3:1 ratio of carbs to protein also enhances recovery of glycogen stores. • CHO-Pro: carbohydrate-protein (80 g CHO, 28 g Pro, 6 g fat) • LCHO: carbohydrate only (equal grams: 80 g CHO, 6 g fat) • HCHO: carbohydrate only (equal calories: 108 g CHO, 6 g fat)

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