Sunday, February 04, 2007

Nitrogenase

Nitrogen is a fundamental element for life. There are no living organisms which do not depend on it, because it is found in DNA, RNA and protein. Therefore, obtaining it is absolutely essential.

The ultimate source of nitrogen is the atmosphere, which is 80% nitrogen gas, composed of two nitrogen atoms bound by three strong bonds. In this form, it is absolutely useless to life. It must be broken apart, and there are only a few ways of doing this. (Oxygen gas, which is composed of two oxygen atoms with two bonds, has been broken down every second of your life by enzymes.)

  1. Lightning.
  2. The Haber process.
  3. Enzymes.

If we relied on lightning we'd be dead, since it doesn't produce enough freed nitrogen. The Haber process was only invented in the early 20th century, and is the source of fertilizer. That leaves enzymes, which are the source of nearly all the nitrogen in your body.

The enzyme that does this is nitrogenase. It is found in bacteria that live in the roots of plants. If you ever pull up a plant by its roots and find the roots covered in small nodules, those bumps are probably filled with nitrogen-fixing bacteria. There, they break atmospheric nitrogen and form ammonia and hydrogen gas.

N2 + 8e + 16MgATP + 8H+ = 2NH3 + H2 + 16MgADP + 16Pi.


This equation says take nitrogen gas, electricity, energy, protons, and convert them into ammonia, hydrogen gas, and spent fuel. Ammonia is easier to convert into nitrate, which is now biologically useful.

The enzyme is one of the most difficult ones to study. What happens "inside" it as it breaks nitrogen has been nearly impossible to observe by traditional methods, because the delivery of electrons can't be controlled. Biochemists know that the bonds are broken one at a time. A new paper by Dmitriy Lukoyanov et al. (subscription required) has unravelled much of the first part of the reaction. They find that no bonds are broken, and that nitrogenase accumulates four of the eight electrons before breaking a single bond. Their discovery finally links electronic states of the enzyme to hypothesized intermediates, and gives a better idea of the order of events performed by nitrogenase.