Wednesday, July 22, 2015

516 - Role of magnesium adenosine 5'-triphosphate in the hydrogen evolution reaction catalyzed by nitrogenase from Azotobacter vinelandii

Nitrogenase evolves hydrogen in a reaction that depends on ATP. The amount of ATP varies from 2 ATP per electron to over 20 (supposedly). The 4Fe-4S cluster in the dinitrogenase reductase donates one electron to the dinitrogenase, binding 2 ATP to do so. It seems like the ATP-powered flow of electrons can determine how many go toward hydrogen and how many toward nitrogen fixation. This study wanted to see if powering this flux was ATP's only role.

What They Saw
They purified enzyme from Azotobacter vinelandii and separated the two components, then mixed them with MgATP in in vitro assays. They observed that there was a burst of rapid ATP hydrolysis at first, which leveled off and remained fairly constant for the rest of the time. It seemed like the rate was the same as the rate of electron transfer between components. They confirmed the rate of about 2 ATP per electron; this makes sense with the dinitrogenase accepting two electrons at a time, one for each of its molybdenum atoms. The rate depended on the concentration of ATP, which makes sense and indicates that ATP is essential. This is also true when measuring the amount of hydrogen produced.

What This Means
They appeared to succeed in their goal of showing that ATP's only role in this reaction was powering the electron transfer, though some ATP can be used up to no purpose in some conditions.

Hageman, R. V., Orme-Johnson, W. H. & Burris, R. H. Role of magnesium adenosine 5’-triphosphate in the hydrogen evolution reaction catalyzed by nitrogenase from Azotobacter vinelandii. Biochemistry 19, 2333–2342 (1980).

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