Km CO2 of RUBISCO

larson eric elarson at ux1.cso.uiuc.edu
Wed Mar 8 18:39:36 EST 1995


aj at biolsci.dundee.ac.uk (ANDY JOHNSTON) writes:

>Would anyone like to enlighten me as to the current thinking as to the 
>affinity of RUBISCO for CO2 in the fully activated state, taking into 
>consideration the effect buffers and salts have on the equilibrium constants 
>of carbonic acid.
=============
 
I don't believe thinking has changed much over the last few years.
Discussing the Km for CO2 in terms of buffer, salts, etc., has an impact on
the calculation of the Km of course, but this isn't a major concern for
most labs (i.e., many labs just report their Km in terms of bicarbonate
concentration and allow the reader to do the conversion.)
 
However, your use of the word "affinity" with respect to CO2 is less
favored.  A formal binding site for CO2 (or O2) has not been identified.
If there is one, it is probably less than the (weak) site identified for
Carbonic Anhydrase.  Current thinking is that saturation with respect to
CO2 is an expression of some other limitation in the catalytic cycle.
 
The use of Km for CO2 is fine, trying to equate this Km to an affinity is
probably not.

-- 
Eric Larson                  | University of Illinois at Urbana-Champaign
USDA/Agronomy                | 190 PABL; 1201 W. Gregory; Urbana, IL 61801
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