PARENT SESSION
Posters P8A Cytochrome b-c complexes. Abstracts (382-393)

The functional role of the monomeric chlorophyll a in the cytochrome b₆f complex of oxygenic photosynthesis. Naranbaatar Dashdorj¹, Huamin Zhang², Hanyoup Kim¹, Jiusheng Yan², William Cramer², Sergei Savikhin^*,1, ¹ Department of Physics, West Lafayette, IN, USA² Department of Biological Sciences, West Lafayette, IN, USA

ABSTRACT- The cytochrome b₆f (Cyt b₆f) complex couples electron transfer between the reaction centers of photosystems I and II to proton translocation across the membrane in the oxygenic photosynthesis through a Q-cycle mechanism. High-resolution X-ray crystallographic structures (Kurisu et al., Science, 302, 1009-1014, 2003; Stroebel et al., Nature, 426, 413-418, 2003) of the Cyt b₆f complex unambiguously confirmed earlier findings (Huang et al., Biochem., 33, 4401-4409, 1994) of a chlorophyll a (Chl a) molecule as an intrinsic component of the Cyt b₆f complex. That rises an intriguing question about the functional role of the Chl a in the complex (Kühlbrandt, W., Nature 426, 399-400, 2003) since the Cyt b₆f complex does not participate in light harvesting, which is the usual function of a chlorophyll in photosynthetic complexes. Moreover, the excited Chl a molecule is known to produce highly toxic singlet oxygen as the result of energy transfer from the excited triplet state of the Chl a to oxygen molecule. To prevent singlet oxygen formation, a carotenoid is typically positioned close to the Chl a molecule effectively quenching the triplet excited state of the Chl a. However, according to the recent structures of the Cyt b₆f complex, the -carotene is too far from the Chl a for effective quenching of the Chl a excited state. We studied the properties of the Chl a molecule and their role in photostability of the complex by means of transient spectroscopy. Based on these results we will discuss the mechanisms protecting protein from rapid photodegradation and propose possible functional role of the Chl a molecule in the Cyt b₆f complex.

KEY WORDS: chlorophyll, protection, cytochrome, ultrafast