Deuteration Effects on the in Vivo EPR Spectrum of the Reduced Secondary Photosystem I Electron Acceptor A sub(1) in Cyanobacteria

The photoreduction of the secondary PSI electron acceptor A sub(1) in vivo has recently been detected via X-band EPR spectroscopy in intact spinach chloroplasts and in marine cyanobacteria Synechococcus PCC 7002 [Klughammer, C., and Pace, R. J. (1997) Biochim. Biophys. Acta 1318, 133-144]. A further...

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Bibliographic Details
Published in:Biochemistry (Easton) 1999-03, Vol.38 (12), p.3726-3732
Main Authors: Klughammer, C, Klughammer, B, Pace, R
Format: Article
Language:English
Subjects:
Cyanophyta
deuterium
double prime A1 protein
Synechococcus
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Summary:The photoreduction of the secondary PSI electron acceptor A sub(1) in vivo has recently been detected via X-band EPR spectroscopy in intact spinach chloroplasts and in marine cyanobacteria Synechococcus PCC 7002 [Klughammer, C., and Pace, R. J. (1997) Biochim. Biophys. Acta 1318, 133-144]. A further study of the A sub(1) super(-) EPR spectrum of Synechococcus PCC 7002 at room temperature with higher-field resolution revealed partially resolved hyperfine structure which was dominated by 0.4 mT splittings of three equivalent protons. The hyperfine splitting was not significantly affected by incubation of the cyanobacteria in super(2)H sub(2)O medium for 20 h, but was absent in fully deuterated cyanobacteria that were grown in super(2)H sub(2)O medium. Anisotropic g-factors consistent with a phylloquinone radical were derived by spectra simulation. Biosynthetic protonation of quinones via the CH sub(3) donor L-methionine in deuterated cells maintained hyperfine structure in the A sub(1) super(-) spectrum, indicating the incorporation of CH sub(3) groups in 60% of the deuterated, photoactive A sub(1) molecules. Conversely, biosynthetic quinone deuteration via L-[methyl-d sub(3)]methionine in protonated cells led to the loss of the 0.4 mT splittings in 54% of the A sub(1) molecules. These observations confirm the conclusion of Heathcote et al. [(1996) Biochemistry 35, 6644-6650] of the identity of EPR-detected, photoreduced A sub(1) super(-) in vivo with a phylloquinone (vitamin K sub(1)) radical in PSI. The partially resolved hyperfine structure of the A sub(1) super(-) spectrum indicates an altered spin distribution in the bound vitamin K sub(1) super(-) radical in vivo compared to that of unbound vitamin K sub(1) super(-) in vitro.
ISSN:0006-2960
DOI:10.1021/bi982431o