Talk:Nitrogenase

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The article is extremely important, the reaction is perhaps the 2nd most fundamental bio-reaction after photosynthesis.

I don't want to grade this, but I do believe there is an error here. I am not [yet?] a registered wikian, so someone else may want to make this change. I will, however, freely identify myself:

John R. Hoaglund, III, Ph.D. Senior hydrogeologist Geotrans Inc. [formerly with Penn State University] Irvine, CA 92614

My edit [ this same edit may apply to http://en.wikipedia.org/wiki/Nitrogen_fixation ]:

In this sentence:

"Since this reaction does not occur very often, and in fact goes the other way more easily, with ammonia breaking down into nitrogen and hydrogen, without nitrogenase, there would be much less life than presently exists."

the clause:

"..., and in fact goes the other way more easily, ..." is in error. The thermodynamics of the reaction:

N2(g) + 3H2(g) → 2NH3(g)

has a Gibbs Free energy of -33.3 kJ/mol, and thus runs most favorably in the direction written.

Though the article lists the reaction as N2 + 6H + energy → 2NH3, the full reaction that occurs is what I list above. The reaction written in the article with "6H" should at least be written as "6H+" (perhaps the H+ is provided by an acid...otherwise hydrogen occurs diatomically as H2). It is possible that the reaction N2 + 6H+ + energy → 2NH3 is correctly asserted as energetically favorable in the other direction [I haven't checked the thermo on that]. It may also be possible that the reaction I've written would be energetically favorable in the other direction if the phases were aqueous (i.e. dissolved), written as (aq), rather than gases, written as (g). Again, I haven't checked the thermo on that either. However, the net reaction is what I have written. The reaction is important enough to include the phases and the correct associated thermodynamics.

The main issue / problem with the reaction is not the thermodynamics, but rather that the kinetics are extremely slow at room temperature. Thus, very large activation energies are required to get the reaction started. Thus the reaction only runs in the presence of certain enzymes (biological catalysts) biologically [namely nitrogenase], or in the presence of catalysts [usually Fe ] + high temperature + high pressure industrially (The Haber-Bosch process).

Please add this reference when editing:

Alberty*,Robert A., 2005. Thermodynamics of the mechanism of the nitrogenase reaction. Biophysical Chemistry, Volume 114, Issues 2-3, Pages 115-120.

Date of publication: April 22, 2005

• Dept. of Chemistry, MIT

Abstract

The fixation of molecular nitrogen by nitrogenase requires a lot of energy because 16 mol of ATP are hydrolyzed per mole of nitrogen converted to ammonia. Kim and Dees determined the crystallograpic structure of nitrogenase and this has led to a three-step mechanism that involves Feprotein and MoFeprotein in addition to ferredoxin. Each of these steps can be interpreted in terms of two half reactions that are connected through their transfer of electrons. Estimates can be made of the standard apparent reduction potentials of these three steps and their dependencies on pH and ionic strength. This mechanism is compared with the same type of analysis of an alternative three-step mechanism in which the hydrolysis of ATP is coupled with the reduction of molecular nitrogen, rather than the reduction of Feprotein. The problem with the first mechanism is that the second step produces 12 mol of hydrogen ions per mole of nitrogen fixed and the third step consumes 10 mol of hydrogen ions per mole of nitrogen fixed. The alternative mechanism does not have this problem.