Tuesday, August 25, 2015

630 - Diastereomer-dependent substrate reduction properties of a dinitrogenase containing 1-fluorohomocitrate in the iron-molybdenum cofactor

The normal nitrogenase central cofactor contains homocitrate near the central metal atom; it is required for formation of the cofactor. This study looks at compounds similar to homocitrate (diastereomers of fluorohomocitrate) incorporated into the cofactor, and how they affect the enzyme's activity.

What They Saw
They purified enzyme from Klebsiella pneumoniae and Azotobacter vinelandii I think; it's hard to tell because this paper (and others they cite) are really badly written with regard to methods. (They seriously put in a footnote saying "The experimental details of the synthesis of 1-fluorohomocitrate may be obtained from [author initials]." How is that acceptable, especially in a PNAS paper?). Somehow they got FeMo-cofactor with different analogs of homocitrate, and tested their activity with different substrates.

With threo-fluorohomocitrate, there was hardly any nitrogen fixation, similar to with citrate, but with erythro-fluorohomocitrate, there was about 3.5x less than with plain homocitrate, so it was somewhat active. Both were about half as good at acetylene reduction (measured by ethylene), and just as good at cyanide reduction (measured by methane).

In terms of hydrogen production, if there was a difference between plain homocitrate and the fluoros, it wasn't very big; they produced almost as much as the normal one. The same was true of other analogs (homoisocitrate, isocitrate, etc), though some were impaired (such as citrate, producing only about half as much).

The addition of inhibitors, carbon monoxide (CO) or carbonyl sulfide (COS), affected these different enzymes differently. The amount of hydrogen from homocitrate increased (maybe significantly) but decreased some or none for analogs, depending on the analog, up to 63%. The inhibitors reduced acetylene reduction from 35-100% in all cases, and cyanide reduction a little or a lot too.

Cyanide itself affected hydrogen production. (Dang, this paper is so badly written, it's giving me a headache trying to figure it out.) With all tested analogs (and homocitrate), cyanide inhibited hydrogen production 85-95%. Adding CO with cyanide prevented this inhibition with homocitrate and partially with fluorohomocitrate, but not much the other analogs. CO also inhibits cyanide reduction, at least with some analogs (especially the more active ones).

What This Means
This study can tell a lot about the biochemistry of different substrates binding to the enzyme and the enzyme acting on them. I wonder if the results are skewed somewhat because they only measured some of the products, not all possible ones (i.e. might some analog-based cofactors produce ethane from acetylene in addition to ethylene?). In terms of application, even if some of these analogs have desirable properties, it seems like it would be difficult to get them incorporated in vivo.


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