Nitrogen Fixation (Metabolism) reactions and Heme Metabolism

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Nitrogen Fixation (Metabolism):

• CO2 and N2 are the major providers of O and N in nature.

• CO2 and N2, however, are highly unreactive, thus, very difficult to fix.

• CO2 is metabolized/fixed mostly by green plants.

• N2 is converted to metabolically useful forms only be certain types of bacteria called diazatrophs.

• Diazatrophs of genus Rhizobium live in symbiotic relationship with root nodules cells of legumes where they convert N2 to NH3.

• NH3 is incorporated either into glutamate by glutamate dehydrogenase or into glutamine by glutamine synthetase.

• Thus, more than enough N is produced for the plant to use. The excess N is absorbed into the soil.

• Here is the reaction: N2 + 8 H+ + 8 e- + 16 ATP + 16 H2O---> 2 NH3 + H2 + 16 ADP + 16 Pi

Nitrogenase:

• This is the main enzyme responsible; it is a complex of two proteins.

• Fe-Protein is a homodimer that contains one [4Fe-4S] cluster and two ATP binding sites.

• MoFe-protein is a alpha2beta2 heterotetramer that contains Fe and Mo.

• Each MoFe-protein associates with two molecules of Fe-protein.

• Fe-protein resembles an iron butterfly.

• The MoFe-protein's two alpha and two beta subunits have a similar structure, and often form a pseudo-2-fold symmetric dimer. Each alpha-beta dimer has two bound redox centers: P-cluster and FeMo-cofactor.

• Nitrogenase is inactivated by oxygen.

• Cyanobacteria do their work inside heterocysts, specialized nonphotosynthetic cells.

• Another molecule called leghemoglobin is synthesized by root nodules. This molecule absorbs O2 with great affinity.

N2 reduction is energetically costly

• In addition to requiring nitrogenase, a source of N2, two additional entities are required: a source of electrons, and ATP.

• Electrons may be generated oxidatively, or photosynthetically.

• 12 ATPs are required to fix one N2 molecule.

• Total cost of N2 fixation is 8 electrons, 16 ATP (20-30 ATP physiologically).

• Nitrogen fixing bacteria in the roots of plants consumes 20% of ATP the plant produces.

Glutamine Synthetase

• Catalyzes the formation of glutamine.

• Glutamine is the amino group donor in the formation of many biosynthetic products, as well as being a storage form of ammonia.

• Mammalian Glutamine Synthetase is activated by alpha-ketoglutarate.

• Bacterial Glutamine Synthetase is much more complex.

• Bacterial Glutamine Synthetase is controlled by histidine, tryptophan, carbamoyl phosphate, glucosamine-6-phosphate, AMP, CTP, alanine, serine, and glycine.

Additional Readings:

Basic Biochemistry

1. Nucleic Acid Structure and Organization
2. DNA Replication and Repair
3. Transcription and RNA Processing
4. Genetic Code, Mutations, and Translation
5. Genetic Regulation
6. Recombinant DNA
7. Amino Acids, Proteins, Enzymes
8. Hormones
9. Vitamins
10. Energy Metabolism
11. Glycolysis and Pyruvate Dehydrogenase
12. Citric Acid Cycle and Oxidative Phosphorylation
13. Glycogen, Gluconeogenesis, and Hexose Monophosphate Shunt
14. Lipid Synthesis and Storage
15. Lipid Mobilization and Catabolism
16. Amino Acid Metabolism Disorders
17. Purine and Pyrimidine Metabolism
18. Electron Transport
19. Citric Acid Cycle and Glyoxylate Cycle
20. Glycolysis
21. Pyruvate Metabolism
22. Mitochondrial ATP formation
23. Gluconeogenesis
24. Glycogen Metabolism
25. Nitrogen Fixation (Metabolism) reactions, and Heme Metabolism
26. Amino Acid Metabolism
27. What is Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)?

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