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Lehninger Principles ofBiochemistry

Fourth Edition

Chapter 22:Biosynthesis of Amino Acids,

Nucleotides, and Related Molecules

Copyright © 2004 by W. H. Freeman & Company

Lecture 2a

Lecture 2a: Nitrogen-fixation and entry of N

Net reaction:

N2 + 10 H+ + 8 e- + 16 ATP --> 2 NH4+ + 16 ADP + 16 Pi + H2

Nitrogenase reductase: 60 k dalton

Dimer, α2 , contains single 4 Fe - 4 S center

Dinitrogenase : 240K dalton

Tetramer, α2 β2 , contains Fe-Mo and S. (some use V instead ofMo).

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Reduction of N2 to 2 NH3involves e- transport.

Some features

a) 2 ATP’s/e-

b) Obligatory H2 evolved

c) At least 8 e- transferred

d) Source of electrons:ferredoxins orflavodoxins

e) Nitrogenase complex:dinitrogenase reductase+ dinitrogenase

Nitrogenase complex: Nase docks with Nase reductase to obtainthe e- which will be give to N. This enz is very O2 sensitive.

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Glutamate: source of amino groups for most amino acids.

NH4+ is assimilated into organism with Glu and Gln as critical

entry points

Assimilation of NH4 into Glu requires 2 rxns:

(1) Gln synthetase:

a) Glu + ATP --> glutamyl phosphate + ADP

b) glutamyl phosphate + NH4+ --> Gln + Pi + H+

Net:?

(2) α-k-Glutarate + Gln + NADPH +H+ --> 2 Glu + NADP+

Net of (1) and (2):

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