文档介绍：Chapter 22 Biosynthesis of amino acids, nucleotides and related molecules
1. Reduction (fixation) of N2 into ammonia (NH3 or NH4+)
2. Synthesis of the 20 amino acids.
3. Synthesis of other biomolecules from amino acids
4. The de novo pathways for purine and pyrimidine biosynthesis.
5. The salvage pathways for purine and pyrimidine reuse.
1. The plex in certain bacteria (diazotrophs,固氮生物) catalyzes the conversion of N2 to NH3
Cyanobacteria (photosynthetic) and rhizobia (symbiont) can fix N2 into NH3.
The reduction of N2 to NH3 is thermodynamically favorable :
N2 + 3H2 2NH3 G`o=-
But ically unfavorable: the bond energy for the triple bond in N2 is 942 kJ/mol.
The nitrogenase (固氮酶) complex mainly consists of two types of enzymes: the dinitrogenase and the dinitrogenase reductase.
The dinitrogenase (containing molybdenum, thus called the MoFe protein) is a tetramer of two different subunits, containing multiple 4Fe-4S centers and two Mo-Fe clusters.
The dinitrogenase reductase (also called the Fe protein) is a dimer of two identifcal subunits, containing a single Fe4-S4 redox center.
The plex is highly conserved among different diazotrophs.
Cyanobacteria and Rhizobia can fix N2 into ammonia
Rhizobia exist in
nodules of
leguminous plants
The plex
The dinitrogenase
(tetramer)
The dinitrogenase
reductase (dimer)
The dinitrogenase
reductase (dimer)
ADP
ADP
4Fe-4S
4Fe-4S
4Fe-4S
(P-cluster)
Fe-Mo cofactor
2. Electrons are transferred through a series of carriers to N2 for its reduction on the plex
Eight electrons are believed to be needed for each round of fixation reaction: with six for reducing one N2 and two for reducing 2 H+ (to form H2).
The electrons e from reduced ferredoxin (from photophosphorylation) or reduced flavodoxin (from oxidative phosphorylation) and are transferred to dinitrogenase via dinitrogenase reductase.
For each electron to be transferred from dinitrogenase reductase to dinitrogenase, two ATPs are hydrolyzed causing