Warwick Silvester wbs at
Sun May 1 17:46:46 EST 2005

Effects of oxygen on nitrogenase are complex as they involve direct
effects on the enzyme and the physiological effects on symbiotiuc
systems. These are distinct in their effects and fall into three types.
1 Direct long term effects of O2 on nitrogenase are irreversible.  Ie If
you expose an organism to excess O@ (more O2 than can be scavanged by
its resiratory system the O2 ireversibly binds to the active site and
the N2ase is destroyed. Recovery times are coincident with N2ase
turnover and are of the oprder of 12 -24 h.
2. Short term transients of O2. ie a pulse of O2 of 1-2 minutes results
in instant switch off of the nezyme and rapid recovery. This is put down
to protectin of the enzyme by a Fe S protein called the Shethra protein
which binds to the active site and protects it but only seems to work
for a short time.
3. There are a variety of physiological protection mechanisms in
organisms such as the O2 diffusion barrier in legume nodules, the
diurnal switch off in some O2 producing cyanobacteria, heterocysts  etc
etc. all of these have the capacity to provide a low O2 environment and
in the case of the legume nodule to provide hemoglobin to transport O2
across this low O2 environment.  Perturbation of this  system can cause
changes in the O2 permeability of the diffusion barrier giving rise to
the variable gas diffusion barrier of the legume nodule.
Hope that helps

Warwick Silvester
Dept of Biological Sciences
University of Waikato
New Zealand
Ph  07 838 4613
Fax 07 838 4324
E-mail  w.silvester at

-----Original Message-----
From: owner-plant-ed at
[mailto:owner-plant-ed at] On Behalf Of Carl Pike
Sent: Thursday, 28 April 2005 12:34 a.m.
To: plant-ed at
Subject: nitrogenase

It is well-known that molecular oxygen inhibits nitrogenase.  I assume
that this may have something to do with causing oxidation of the metals
(Mo and Fe) that are in the enzyme.  A student asked if the inhibition
was reversible or irreversible.  No source that I have consulted answers
this question, although one used the word "denaturation" in reference to
the oxygen effect on the protein.

Any thoughts?




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