There was a question at this time of whether hydrogen in the air could inhibit nitrogen fixation in Azotobacter vinelandii, and in what conditions this might happen. There were various complicating factors though. This study attempted to do a more controlled investigation using purified enzyme.
What They Saw
They observed that the higher the concentration of nitrogen in the atmosphere, the more nitrogen was fixed in a given time (thus the higher the specific activity was). From this they could calculate the KN2, by plotting the inverse of the specific activity over the inverse of the concentration and taking the slope of the line. This is an indication of the enzyme's affinity for nitrogen, I think. This is higher than seen from intact cells (0.01).
Then they got to the hydrogen inhibition experiments. As they increased the amount of hydrogen, the nitrogen-fixing activity did seem to decrease, indicating competitive inhibition.
One issue that they didn't control for was the production of hydrogen by nitrogenase itself; this could've influenced the numbers. It could also influence the KN2 numbers, come to think of it. It's also unclear whether the extracts had any hydrogenase activity, which could influence things in the opposite direction.
Reference:
Strandberg, G. W. & Wilson, P. W. Molecular H2 and the pN2 function of Azotobacter. Proc Natl Acad Sci U S A 58, 1404–1409 (1967).
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