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In this chapter, students will be able to understand: What are the biochemical pathways that form ammonium from inorganic nitrogen compounds prevalent in the inanimate environment? How is ammonium incorporated into organic compounds? How are amino acids synthesized and degraded? | Chapter 26 Nitrogen Acquisition and Amino Acid Metabolism to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline 26.1 The Two Major Pathways of N Acquisition 26.2 The Fate of Ammonium 26.3 Glutamine Synthetase 26.4 Amino Acid Biosynthesis 26.5 Metabolic Degradation of Amino Acids Major Pathways for N Acquisition All biological compounds contain N in a reduced form The principal inorganic forms of N are in an oxidized state Thus, N acquisition must involve reduction of the oxidized forms (N2 and NO3-) to NH4+ Nearly all of this is in microorganisms and green plants. Animals gain N through diet. Overview of N Acquisition Nitrogen assimilation and nitrogen fixation Nitrate assimilation occurs in two steps: 2e- reduction of nitrate to nitrite and 6e- reduction of nitrite to ammonium (page 854) Nitrate assimilation accounts for 99% of N acquisition by the biosphere Nitrogen fixation involves reduction of N2 in prokaryotes by nitrogenase Nitrate Assimilation Electrons are transferred from NADH to nitrate Pathway involves -SH of enzyme, FAD, cytochrome b and MoCo - all protein-bound Nitrate reductases are big - 210-270 kD See Figure 26.2 for MoCo structure MoCo required both for reductase activity and for assembly of enzyme subunits to active dimer Nitrite Reductase Light drives reduction of ferredoxins and electrons flow to 4Fe-4S and siroheme and then to nitrite See Figure 26.2b for siroheme structure Nitrite is reduced to ammonium while still bound to siroheme In higher plants, nitrite reductase is in chloroplasts, but nitrate reductase is cytosolic Enzymology of N fixation Only occurs in certain prokaryotes Rhizobia fix nitrogen in symbiotic association with leguminous plants Rhizobia fix N for the plant and plant provides Rhizobia with . | Chapter 26 Nitrogen Acquisition and Amino Acid Metabolism to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline 26.1 The Two Major Pathways of N Acquisition 26.2 The Fate of Ammonium 26.3 Glutamine Synthetase 26.4 Amino Acid Biosynthesis 26.5 Metabolic Degradation of Amino Acids Major Pathways for N Acquisition All biological compounds contain N in a reduced form The principal inorganic forms of N are in an oxidized state Thus, N acquisition must involve reduction of the oxidized forms (N2 and NO3-) to NH4+ Nearly all of this is in microorganisms and green plants. Animals gain N through diet. Overview of N Acquisition Nitrogen assimilation and nitrogen fixation Nitrate assimilation occurs in two steps: 2e- reduction of nitrate to nitrite and 6e- reduction of
