Abstract
Nonlinear Dynamics of the Peroxidase-Oxidase
Reaction:
II. Compatibility of an Extended Model with Previously
Reported Model-Data Correspondences.
The Journal of Physical Chemistry B. 105: 5331-5340
(2001).
In the course of formulating detailed models of complex chemical reactions, it is sometimes the case
that modifications intended to account for one set of experimental observations wind up destroying a
model's ability to account for other results. Here, we consider a recently proposed model (BFSO-14)
[1]
of the peroxidase-oxidase reaction which derives from an earlier
scheme
[2]
via the addition of NADH oxidation by superoxide anion or its protonated form, hydroperoxyl radical,
i.e.,
NADH + O2- --> NAD. +
H2O2
This modification was introduced to account for the observation of
bistability
and
bursting
[3,4]
at enzyme concentrations less than 0.5 mM. Left unanswered in our previous
paper was the matter of whether or not the proposed "fix" invalidates previously published examples of
model-data agreement
[2,5,6]
at higher enzyme concentrations. In the present paper, we show that under these latter circumstances,
the new mechanism is as good as, and in some instances superior to, its predecessor. More generally,
we argue that the
consequences
of NADH oxidation by O2-or HO2. should be manifest principally at low
enzyme concentrations, thereby offering a "global" explanation of our findings.
Neither our original model, nor the derivative scheme treated here, provides for
reactions involving NAD dimers (NAD2), a species in which there has recently been renewed
interest
[7].
Most importantly, it has been proposed to replace the reduction of coIII (an enzyme intermediate) by
NAD radicals, a reaction for which there is no direct evidence, with the corresponding reaction
involving NAD2. While a detailed assessment of the consequences of dimer chemistry to
theoretical peroxidase-oxidase dynamics is beyond the scope of the present investigation, it is
easily documented that this substitution, by itself,
abolishes
oscillatory behavior for all model parameterizations previously considered. Moreover, this result
appears to obtain for arbitrarily small values of the associated rate constant. Whether or not the
inclusion of additional dimer reactions can restore the model's ability to account for experimental
observations of complex dynamics remains to be determined.
[1] Bronnikova, T. V.; Schaffer, W. M.;
Olsen, L. F. 2001. J. Phys. Chem. B 2001, 105, 310-321.
[2] Bronnikova, T. V.; Fed'kina, V. R.; Schaffer, W. M.; Olsen, L. F. J. Phys. Chem.
1995, 99, 9309-9312.
[3] Olsen, L.F.; Degn, H.
Nature, 1977, 267, 177-178.
[4] Aguda, B. D.; Frisch, L. L. H.;
Olsen, L. F. J. Am. Chem. Soc. 1990, 112, 6652-6656.
[5] Hauser, M. J. B.; Olsen, L. F.;
Bronnikova, T. V.; Schaffer, W. M. J. Phys. Chem B. 1997, 101, 5075-5083.
[6] Bronnikova, T. V.; Schaffer, W. M.;
Hauser, M. J. B.; Olsen, L. F. J. Phys. Chem. B. 1998, 102, 632-640.
References.
