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Removal of the PsaF Polypeptide Biases Electron Transfer in Favor of the PsaB Branch of Cofactors in Triton X‐100 Photosystem I Complexes from Synechococcus sp. PCC 7002 †
Author(s) -
Agalarov Rufat,
Byrdin Martin,
Rappaport Fabrice,
Shen Gaozhong,
Bryant Donald A.,
Van Der Est Art,
Golbeck John H.
Publication year - 2008
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2008.00457.x
Subject(s) - photosystem i , electron transfer , chemistry , cofactor , photosystem ii , photochemistry , photosynthesis , biochemistry , enzyme
Continuous wave (CW) and transient electron paramagnetic resonance studies have implied that when PsaF is removed genetically, the double reduction of A 1A is facile, the lifetime of A 1A − is shorter and the ratio of fast to slow kinetic phases increases in PS I complexes isolated with Triton X‐100 (Van der Est, A., A. I. Valieva, Y. E. Kandrashkin, G. Shen, D. A. Bryant and J. H. Golbeck [2004] Biochemistry 43 , 1264–1275). Changes in the lifetimes of A 1A − and A 1B − are characteristic of mutants involving the quinone binding sites, but changes in the relative amplitudes of A 1A − and A 1B − are characteristic of mutants involving the primary electron acceptors, A 0A and A 0B . Here, we measured the fast and slow phases of electron transfer from A 1B − and A 1A − to F X in psaF and psaE psaF null mutants using time‐resolved CW and pump‐probe optical absorption spectroscopy. The lifetime of the fast kinetic phase was found to be unaltered, but the lifetime of the slow kinetic phase was shorter in the psaF null mutant and even more so in the psaE psaF null mutant. Concomitantly, the amplitude of the fast kinetic phase increased by a factor of 1.8 and 2.0 in the psaF and psaE psaF null mutants, respectively, at the expense of the slow kinetic phase. The change in ratio of the fast to slow kinetic phases is explained as either a redirection of electron transfer through A 1B at the expense of A 1A , or a shortening of the lifetime of A 1A − to become identical to that of A 1B − . The constant lifetime and the characteristics of the near‐UV spectrum of the fast kinetic phase favor the former explanation. A unified hypothesis is presented of a displacement of the A‐jk(1) α‐helix and switchback loop, which would weaken the H‐bond from Leu 722 to A 1A , accounting for the acceleration of the slow kinetic phase, as well as weaken the H‐bond from Tyr 696 to A 0A , accounting for the bias of electron transfer in favor of the PsaB branch of cofactors.