Question

What three mechanisms have evolved in nitrogen-fixing microorganisms to limit the inhibitory effects of \(\mathrm{O}_{2}\) on the nitrogenase reaction?

Short answer

Three mechanisms have evolved in nitrogen-fixing microorganisms to limit the inhibitory effects of oxygen on the nitrogenase reaction: 1. Physical separation of nitrogenase from oxygen: In some organisms, such as heterocyst-forming cyanobacteria, nitrogenase activity is physically separated from oxygen-generating processes through specialized cells called heterocysts, which do not perform photosynthesis. 2. Oxygen consumption: Some nitrogen-fixing organisms rapidly consume oxygen through high rates of respiration or the presence of oxygen-scavenging enzymes, decreasing the concentration of oxygen in their environment and reducing its inhibitory effects on nitrogenase. 3. Protection by oxygen-binding proteins: Some nitrogen-fixing microorganisms produce proteins that bind to oxygen, preventing it from inhibiting the nitrogenase reaction. These proteins create a barrier around the enzyme, protecting it from the effects of oxygen.

Step-by-step solution

Mechanism 1: Physical separation of nitrogenase from oxygen

In some nitrogen-fixing organisms, such as heterocyst-forming cyanobacteria, nitrogenase activity is physically separated from oxygen-generating processes. This is achieved by the formation of specialized cells called heterocysts, which do not perform photosynthesis and, therefore, do not produce \(\mathrm{O}_{2}\). The nitrogenase reaction takes place within these heterocysts, resulting in limited exposure to oxygen and a reduced inhibitory effect.

Mechanism 2: Oxygen consumption

Another strategy to protect nitrogenase from the inhibitory effects of oxygen involves rapid consumption of \(\mathrm{O}_{2}\). This is achieved through the use of high rates of respiration or the presence of oxygen-scavenging enzymes, such as cytochrome oxidase, in nitrogen-fixing organisms. By rapidly consuming oxygen, the concentration of \(\mathrm{O}_{2}\) in the microorganism's environment is decreased, reducing its inhibitory effects on the nitrogenase reaction.

Mechanism 3: Protection by oxygen-binding proteins

The third mechanism involves the use of proteins that bind to oxygen, effectively sequestering it and preventing it from inhibiting the nitrogenase reaction. These proteins, known as oxygen-binding proteins or stress proteins, are produced by some nitrogen-fixing microorganisms in response to high levels of \(\mathrm{O}_{2}\). They can bind to oxygen and create a barrier around the nitrogenase enzyme, protecting it from the inhibitory effects of oxygen and allowing the reaction to proceed efficiently.