Nitrogenase, the enzyme complex responsible for biological nitrogen fixation (N2 to NH3), consists of which protein components?

Introduction:
Biological nitrogen fixation is mediated by the nitrogenase enzyme complex that reduces atmospheric nitrogen (N2) to ammonia (NH3). This item evaluates recognition of the two essential protein components that operate together under strict oxygen control and high ATP demand.

Given Data / Assumptions:

• Nitrogenase catalyzes N2 + 8 H+ + 8 e- → 2 NH3 + H2.
• The system requires ATP hydrolysis and electron transfer.
• Two separable proteins form the functional complex.

Concept / Approach:
Nitrogenase comprises dinitrogenase (often called the MoFe protein) and dinitrogenase reductase (the Fe protein). Dinitrogenase reductase delivers electrons from reduced ferredoxin or flavodoxin to dinitrogenase in an ATP-dependent manner. Dinitrogenase then reduces N2 to NH3 at its metal cofactor center, most commonly the MoFe cofactor in classical systems.

Step-by-Step Solution:
Recall that nitrogenase activity requires two proteins acting in series. Map each role: reductase (Fe protein) supplies electrons; dinitrogenase (MoFe) performs substrate reduction. Recognize that both components are indispensable for catalysis. Select the combined option listing both proteins.

Verification / Alternative check:
Biochemical purification and reconstitution experiments show loss of activity if either component is removed, confirming the two-part requirement.

Why Other Options Are Wrong:

• Dinitrogenase alone: Lacks electron delivery and ATP coupling.
• Dinitrogenase reductase alone: Cannot bind and reduce N2.
• None of these: Incorrect because both components are essential.

Common Pitfalls:
Confusing ferredoxin (an upstream electron carrier) with the reductase protein of the nitrogenase complex.

Final Answer:
both (a) and (b) — nitrogenase consists of dinitrogenase and dinitrogenase reductase.