A) amminase
B) nitrogenase
C) nitrate reductase
D) nitrite reductase
Correct Answer: B
Solution :
Rhizobium leguminosamm causes biological nitrogen fixation in root nodules of leguminous plants, A pigment leghaemoglobin is present in the root nodules which is oxygen scavanger, ie, protects the enzyme nitrogenase from \[{{O}_{2}}\]. Nitrogenase enzyme complex consists of two components, ie, the Fe-protein and Mo protein. Fe-protein: It is smaller component which have two identical subsunits of 30 to 70 KDa each, depending on the organisms. Each subunit contains an iron sulphur cluster (4Fe and 4S--) that participates in the redox reactions involved in the conversion of \[{{N}_{2}}\] to\[N{{H}_{3}}\]. The Fe protein is irreversibly inactivated by \[{{O}_{2}}\] with typical half decay times of 30-45 seconds. Mo-Fe protein: It has four sub units with a total molecular mass of 180-135 KDa, depending on the species. Each subunit has two Mo-Fe-S clusters. The Mo-Fe protein is also inactivated by oxygen with a half decay time in air of 10 minutes. In the overall nitrogen reduction reaction \[{{N}_{2}}+8{{e}^{-}}+8{{H}^{+}}+16ATP\xrightarrow{{}}2N{{H}_{3}}\] \[+{{H}_{2}}+16ADP+16Pi\] Ferredoxin serves as an electron donor to the Fe protein which in turn hydrolyzes ATP and reduces the MO-Fe protein. The Mo-Fe protein then can reduce numerous substrates some of which are as follows:| Reactions Catalyzed by Nitrogenase | |
| \[{{N}_{2}}\to N{{H}_{3}}\] | Molecular nitrogen fixation |
| \[{{N}_{2}}O\to {{N}_{2}}+{{H}_{2}}O\] | Nitrous oxide reduction |
| \[{{N}_{3}}\to {{N}_{2}}+N{{H}_{3}}\] | Azide reduction |
| \[{{C}_{2}}{{H}_{2}}\to {{C}_{2}}{{H}_{4}}\] | Acetylene reduction |
| \[2{{H}^{+}}\to {{H}_{2}}\] | Production |
| \[ATP\to ADP+Pi\] | ATP hydrolytic activity |
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