Quantitative assessment of intrinsic carbonic anhydrase activity and the capacity for bicarbonate oxidation in photosystem II.
On the basis of equilibrium isotopic distribution experiments using (18)O-labeled water, it is generally accepted that water is the sole substrate for O(2) production by photosystem II (PSII). Nevertheless, recent studies indicating a direct interaction between bicarbonate and the donor side of PSII have been used to hypothesize that bicarbonate may have been a physiologically important substrate for O(2) production during the evolution of PSII [Dismukes, G. C., Klimov, V. V., Baranov, S. V., Kozlov, Y. N., DasGupta, J., and Tyryshikin, A. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 2170-2175]. To test out this hypothesis and to determine whether contemporary oxygenic organisms have the capacity to oxidize bicarbonate, we employed special rapid-mixing isotopic experiments using (18)O/(13)C-labeled bicarbonate to quantify the inherent carbonic anhydrase activity in PSII samples and the potential flux of oxygen from bicarbonate into the photosynthetically produced O(2). The measurements were made on PSII samples prepared from spinach, Thermosynechococcus elongatus, and Arthrospira maxima. For the latter organism, a strain was used that grows naturally in an alkaline, high (bi)carbonate soda lake in Africa. The results reveal that bicarbonate is not the substrate for O(2) production in these contemporary oxygenic photoautotrophs when assayed under single turnover conditions.
Hillier
Warwick
W
Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia. Warwick.Hillier@anu.edu.au
McConnell
Iain
I
Badger
Murray R
MR
Boussac
Alain
A
Klimov
Vyacheslav V
VV
Dismukes
G Charles
GC
Wydrzynski
Tom
T
D001639:Bicarbonates / Q000378:metabolism*; D002256:Carbonic Anhydrases / Q000378:metabolism*; D000458:Cyanobacteria / Q000378:metabolism; D010084:Oxidation-Reduction; D010103:Oxygen Isotopes; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D018724:Spinacia oleracea / Q000378:metabolism
D001639:Bicarbonates; D010103:Oxygen Isotopes; D045332:Photosystem II Protein Complex; D002256:Carbonic Anhydrases