Friday, August 28, 2015

663 - Azotobacter vinelandii Vanadium Nitrogenase: Formaldehyde Is a Product of Catalyzed HCN Reduction, and Excess Ammonia Arises Directly from Catalyzed Azide Reduction

The V nitrogenase is similar to but distinct from the Mo nitrogenase in various ways. This study tests its activity and patterns with cyanide and azide as substrates; previously this had only been tested with the Mo nitrogenase.

What They Saw
As with the Mo version, cyanide inhibited hydrogen production and the overall electron flux through the V nitrogenase, though less than with the Mo nitrogenase; more cyanide had more effect, up to 75% inhibition of hydrogen and electrons at 50 mM cyanide. Methane formation from cyanide increased at first, and then decreased with more cyanide, while ammonia continued to increase. They didn't measure methylamine in most conditions though, so the electron flux numbers might be off. With low cyanide, there was significant methylamine relative to the methane, about 0.66 to 1, higher than the Mo nitrogenase; this ratio seems to increase to 1:1 as methane decreases.

Interestingly, the enzyme also produces formaldehyde from cyanide. It's not clear whether the Mo version does too and it's too hard to detect, or if it just doesn't. It can be tricky, since other components in the reaction react with it, and cyanide inhibits the Mo enzyme a lot more.

Azide inhibited hydrogen production from the V nitrogenase about 50%. Hydrazine was more of a product from azide than it is for the Mo nitrogenase, relative to dinitrogen and ammonia. Overall activity was less than the Mo version, which is typical, but also because azide seems to reduce the total electron flux in this version. The ammonia seems to come from azide directly, not from the dinitrogen produced, because adding hydrogen gas (which specifically inhibits nitrogen reduction) didn't change the values.

They tried seeing what adding carbon monoxide (CO) might do to affect these reactions. With azide, it rescued hydrogen production, though not the electron flux, and CO didn't entirely prevent some azide reduction. With cyanide, the effects were similar, except the electron flux inhibition was relieved too.

What This Means
It seems that these chemicals have similar effects on the V nitrogenase as they do on the Mo version, but not completely the same. I wonder, though, about the CO assays: CO can be a substrate for the V nitrogenase, reduced mostly to ethylene but also some propylene and methane; how did this affect their assays?


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