This lesson will define and explore the activity, structure and importance of the enzyme nitrogenase. You’ll also learn about the nitrifying bacteria found in some of the foods you eat every day.
Nitrogen fixation is a natural process where nitrogen gas is converted into ammonium either by microbes or by lightning. The process is vital to life on the planet and food production, as nitrogen is needed for biomass production. Nitrogen fixation would not occur without a special enzyme called nitrogenase, an enzyme found in some types of nitrifying microbes that allows for the reduction of nitrogen gas to ammonium.There are two main forms of nitrifying microbes: symbiotic nitrogen fixing bacteria and free-living nitrogen fixing bacteria.
1. Symbiotic nitrogen fixing bacteria live on plants and are found around the world.
They are most commonly associated with Rhizobia bacteria that live on a class of plants called legumes (which include the pinto beans on your burrito, chickpeas in your hummus, soybeans in your tofu, and green peas in your salad). These bacteria live in nodules on the roots of the plants, which look much like little pearls, for protection from the outside world. Nitrogenase is incapable of functioning when oxygen is present. The plant provides simple sugars to the microbe, and in return, the plant receives the excess nitrogen fixed by the microbe.Frankia bacteria can also fix nitrogen through symbiotic relationships with some trees and shrubs, like birches, roses, and Australian pines.
These bacteria also live in root nodules.Some cyanobacteria also have symbiotic relationships with some hornworts, liverworts, mosses, horsetails, ferns, cycads, lichens, and other plants. These bacteria have special cells called heterocysts, which do not carry out oxygen reactions in photosynthesis, allowing nitrogen fixation to occur.2. Free-living nitrogen fixing bacteria do not require associations with other organisms to survive. These include some cyanobacteria and green, purple, and non- sulphur bacteria. Cyanobacteria fix over half of the nitrogen found in marine ecosystems.
Structure and Activity
So now we know that nitrogenase is the enzyme responsible for fixing atmospheric nitrogen, and it’s found in various microbes that may live alone in the nature or in symbiotic relationships with plants. Now let’s discuss its structure. The nitrogenase enzyme looks similar to a sandwich. On both ends of the enzyme are two iron containing proteins that are bonded to a molybdenum/iron protein.At the heart of this nitrogenase enzyme is an energy-expensive protein that needs a lot of electrons to fix nitrogen and the molybdenum/iron protein. The job of the two other iron proteins is to pump electrons to the center protein.
A general rule of thumb is it takes two energy molecules to pump one electron to the center protein.The other reason this reaction is so energy intense is because breaking apart atmospheric nitrogen is hard work. Nitrogen from the atmosphere has a chemical formula of N2. This means that two nitrogen molecules are bonded together to form nitrogen gas. While this may seem simple, these two nitrogen molecules have three chemical bonds and to separate them requires a lot of energy.
However, this process is worth it, especially where nitrogen is in low supply and occurs naturally due to nitrogenase.
The next time you sit down and have a burrito, think about the beans you are eating. Beans have root nodules that house Rhizobia bacteria, a microbe that produces nitrogenase, the enzyme responsible for atmospheric nitrogen fixation to plant-usable nitrogen in the form of ammonium. Symbiotic nitrogen fixing bacteria are microbes that have interdependent relationships with plants. Free-living nitrogen fixing bacteria do not need other organisms to survive.The nitrogenase enzyme resembles a sandwich with two iron proteins on each end of a molybdenum/iron protein.
This enzyme requires a lot of energy, but it is vital to plant and food production on the planet.