MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 81

CHAPTER 3
MECHANISMS OF CHOSEN ENZYME REACTIONS
3.1. Nitrogenase
3.1.1. OVERVIEW
Microbiological nitrogen fixation is the global large-scale process of the atmospheric
nitrogen reduction to ammonia with the yield approximately 200,000 million tons per
annum. The process occurs in anaerobic and aerobic bacteria such as Azotobacter
vinelandii, Clostridium pasterianum, Klebsiella pneumonia, etc., and Rizobium from the
root nodules of legumes. Since the publication of the pioneer works of the Bulen (Bulen
and LeCompt, 1996) and Mortenson (Mortenson et al., 1997) groups, who reported the
isolation of the first partially purified dinitrogen fixing complex (nitrogenase), the efforts
of many biochemists have been concentrated on the preparation of individual
components of nitrogenase and on the study of their structure and action mechanism.
The central enzyme of biological nitrogen fixation catalyzes in the nitrogen-fixing
bacteria the reduction of molecular nitrogen to ammonia by biological (ferredoxin) and
non-biological reducing agents with the assistance of ATP hydrolysis hydrolysis
(Bulen and LeCompt, 1996; Mortenson et al., 1967, Newton, 1996, 1997, 2000).
1996, 1997, 2000).
The active form of nitrogenase is formed through the combined action of two
components: a protein containing cluster (FeP) and iron-molybdenum protein
(FeMoP) with two so called P-clusters and two iron-molybdenum cofactors
(FeMoCo). The FeP consists of a equivalent subunit with a total molecular
weight of 64 kDa. FeMoP is an tetramer of molecular weight 250 kDA containing
two molybdenum atoms and about 30 iron and acid labile sulfur atoms distributed into
(FeMoP) and (FeMoCo). The Fe protein passes electrons from FeP to MoFe protein in a
reaction, which requires hydrolysis of MgATP to MgADP.
Apart from and nitrogenase catalyzes reduction of many substrates
NO, HCN, cyclopropene, etc), which are also inhibitors of nitrogen
reductions. Besides well-characterized classical molybdenum nitrogenase, two
genetically distinct nitrogenases were isolated from Azotobacter vinelandii (Bishop et
al., 1980; Eady, 1996; Harvey et al., 1990). All three nitrogenase enzymes comprise two
separable components, Fe-protein and proteins containing iron P-clusters, and cofactors
iron-molybdenum, iron-vanadium or only iron clusters.
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