Does carbonic anhydrase shift the HCO3-CO2 equilibrium,

Herman, Peter herman at cemo.nioo.nl
Thu Dec 21 12:40:37 EST 1995


Dear colleagues,
I am currently involved in modelling primary production by
microphytobenthos, related to the general biogeochemistry of the sediments
in which it occurs. Limitation by inorganic carbon is a serious possibility,
since the microphytobenthos produces only in the top half mm of the
sediment. Thus the production per unit volume is very high and diffusive
transport of inorganic carbon to the production zone may be limiting.
The presence of extracellular carbonic anhydrase, although to my knowledge
not well studied in microphytobenthos, may be quite important as it may
permit the cells to speed up transformation of HCO3- into CO2.
Alternatively, they may take up HCO3- directly, and transform it
intracellularly, but anyhow they would not be limited by the relatively slow
kinetics of HCO3-CO2 transformations. We think it is important to include
this aspect into our model.

My question is the following:

although the literature abounds with descriptions of relative activity of
carbonic anhydrase in different species and under different conditions, I
fail to find any reference to the basic chemistry of the enzymatic reaction.
Does the enzyme only speed up the two-way reaction

CO2+OH- <-> HCO3-    (this is the net reaction only!)

leading eventually to the same equilibrium between HCO3- and CO2 as in the
absence of the enzyme? Or does it rather speed up either of the two ways of
the reaction more than the other way, thus influencing the eventual
equilibrium concentrations ?
(I assume steady state here, i.e. no consumption of CO2 nor HCO3- by the
organism).
Does anyone know a reference giving the answer to this simple question?
Many thanks in advance

Peter M.J. Herman

Netherlands Institute of Ecology
Centre for Estuarine and Marine Ecology
Vierstraat 28
4401 EA Yerseke
The Netherlands
Tel. +31/113/571920
Fax. +31/113/573616
E-mail HERMAN at NIOO.NL





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