Chapter26

Chapter26 - BCH 4054 Chapter 26 Lecture Notes Slide 1...

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Chapter 26, page 1 BCH 4054 Chapter 26 Lecture Notes Slide 1 Chapter 26 Nitrogen and Amino Acid Metabolism Slide 2 Outline • No time to cover entire chapter, therefore concentrate on a few focal points • Assimilation of inorganic nitrogen • Transamination (aminotransferases) • Nitrogen removal from amino acids • Urea synthesis • A few metabolic defects in catabolism Slide 3 Assimilation of Inorganic Nitrogen • Primary sources are NO 3 - , NO 2 - , N 2 and NH 3 • Most reduction of first three species occurs in microorganisms and plants • N 2 to NH 3 is called nitrogen fixation • It occurs in bacteria, some in symbiotic relation with plants • Nitrifying bacteria convert NH 3 to NO 3 - • Denitrifying bacteria reduce NO 3 - to NH 3 • See Fig 26.1
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Chapter 26, page 2 Slide 4 Nitrogen Fixation • Enzyme is nitrogenase , catalyzing N 2 + 8 H + + 8 e - 2 NH 3 + H 2 • Two proteins in complex • Nitrogenase reductase (Fe-protein) • Nitrogenase (an MoFe-protein) • See Fig 26.5 for metal cluster structure • ATP required in the reaction • 16 ATP for each N 2 reduced • Electrons come from ferredoxin originally ATP requirement is explained as energy needed to overcome a high activation energy for breaking the N 2 triple bond. Other texts suggest the ATP lowers the reduction potential of the reductase complex. Rhizobia grow in symbiotic association with leguminous plants and fix nitrogen for them. Chemical fixation of nitrogen is by the Haber process , and is a major industrial chemical process used to produce fertilizer. Slide 5 Regulation of Nitrogen Fixation • ADP inhibits activity of nitrogenase • NH 4 + represses synthesis of many of the enzymes involved in nitrogen fixation • Known as the nif genes Slide 6 Nitrate Assimilation • Nitrate reductase • Two electron reduction of nitrate to nitrite • Involves a cytochrome and a molybdenum cofactor ( MoCo ) (Fig 26.2a) • Nitrite reductase • Six electron reduction of nitrite to ammonia Siroheme is prosthetic group (Fig 26.2b) • Ferredoxin produced in light reaction is electron donor • Plant enzyme is in chloroplasts Nitrate assimilation accounts for 99% of the inorganic nitrogen assimilation into organisms.
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Chapter 26, page 3 Slide 7 Ammonia Assimilation • Several reactions • Carbamoyl phosphate synthetase • For urea cycle • Glutamate dehydrogenase • Reversible, also a catabolic enzyme • Glutamine Synthetase • Primary assimilation mechanism • Glutamate Synthase • Equilibrium favors glutamate formation Slide 8 Carbamoyl Phosphate Synthetase • Two ATP’S required • One for activation of HCO 3 - • One for phosphorylation of carbamate NH 4 + + HCO 3 - + 2 ATP H 2 NCOPO 3 2- + 2 ADP + 2 P i O N-acetylglutamate activates the enzyme. This enzyme is found in mitochondria and begins the urea cycle.
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Chapter26 - BCH 4054 Chapter 26 Lecture Notes Slide 1...

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