Nitrogenase

Nitrogenase

Structure of nitrogenase enzyme complex.

Nitrogenase is an enzyme complex that catalyzes the reduction of nitrogen (N₂) to ammonia (NH₃), a process known as biological nitrogen fixation. This enzyme is essential for the conversion of atmospheric nitrogen into a form that can be utilized by living organisms. Nitrogenase is found in certain bacteria and archaea, often in symbiotic relationships with plants.

Structure

The FeMoco cluster, the active site of nitrogenase.

Nitrogenase is a complex enzyme composed of two main protein components: the dinitrogenase reductase (also known as the iron protein) and the dinitrogenase (also known as the molybdenum-iron protein). The active site of nitrogenase contains a unique iron-molybdenum cofactor (FeMoco), which is crucial for its catalytic activity.

The dinitrogenase reductase is a homodimer that contains a [4Fe-4S] cluster and binds ATP. The dinitrogenase is an α₂β₂ tetramer that contains the FeMoco cluster.

Mechanism

Mechanism of nitrogen fixation by nitrogenase.

The nitrogenase reaction involves the transfer of electrons from the dinitrogenase reductase to the dinitrogenase, which then reduces nitrogen to ammonia. This process requires the hydrolysis of ATP, which provides the necessary energy for electron transfer.

The overall reaction catalyzed by nitrogenase is:

N₂ + 8 H⁺ + 8 e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pi

Kinetics

Lowe-Thorneley kinetic model of nitrogenase.

The kinetics of nitrogenase activity are described by the Lowe-Thorneley model, which outlines the sequence of electron transfer and ATP hydrolysis events. This model helps in understanding the complex interactions and steps involved in the nitrogenase catalytic cycle.

Biological Significance

Nitrogenase plays a critical role in the nitrogen cycle, enabling the conversion of inert atmospheric nitrogen into a biologically accessible form. This process is vital for the synthesis of amino acids, nucleotides, and other nitrogen-containing biomolecules.

Symbiotic Relationships

Cartoon representation of nitrogenase with active sites highlighted.

Many nitrogen-fixing bacteria form symbiotic relationships with plants, particularly legumes. These bacteria reside in root nodules and provide the host plant with ammonia in exchange for carbohydrates and a protective environment.

Inhibition and Regulation

Nitrogenase activity is sensitive to oxygen, which can irreversibly damage the enzyme. Organisms that possess nitrogenase have developed various mechanisms to protect the enzyme from oxygen, such as the production of leghemoglobin in root nodules.

Related Pages

• Nitrogen cycle
• Biological nitrogen fixation
• Leghemoglobin
• Symbiosis