Characterization of Highly Purified Photosystem I Complexes from the Chlorophyll d-dominated Cyanobacterium Acaryochloris marina MBIC 11017

Tatsuya Tomo,Yuki Kato,Takehiro Suzuki,Seiji Akimoto,Tatsunori Okubo,Takumi Noguchi,Koji Hasegawa,Tohru Tsuchiya,Kazunori Tanaka,Michitaka Fukuya,Naoshi Dohmae,Tadashi Watanabe,Mamoru Mimuro

doi:10.1074/jbc.m801805200

Abstract

Photochemically active photosystem (PS) I complexes were purified from the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017, and several of their properties were characterized. PS I complexes consist of 11 subunits, including PsaK1 and PsaK2; a new small subunit was identified and named Psa27. The new subunit might replace the function of PsaI that is absent in A. marina. The amounts of pigments per one molecule of Chl d' were 97.0 +/- 11.0 Chl d, 1.9 +/- 0.5 Chl a, 25.2 +/- 2.4 alpha-carotene, and two phylloquinone molecules. The light-induced Fourier transform infrared difference spectroscopy and light-induced difference absorption spectra reconfirmed that the primary electron donor of PS I (P740) was the Chl d dimer. In addition to P740, the difference spectrum contained an additional band at 728 nm. The redox potentials of P740 were estimated to be 439 mV by spectroelectrochemistry; this value was comparable with the potential of P700 in other cyanobacteria and higher plants. This suggests that the overall energetics of the PS I reaction were adjusted to the electron acceptor side to utilize the lower light energy gained by P740. The distribution of charge in P740 was estimated by a density functional theory calculation, and a partial localization of charge was predicted to P1 Chl (special pair Chl on PsaA). Based on differences in the protein matrix and optical properties of P740, construction of the PS I core in A. marina was discussed.

Highlights

In all oxygenic photosynthetic organisms, two photosystems (PSs)2 cooperatively function to drive photochemical reactions; both consist of many subunits and pigments, forming supramolecular complexes
Hu et al (20) reported that the redox potential of P740 was 335 mV. This is significantly lower than that of the T. elongatus PS I (ϩ423 mV) (22, 23) and of PS I from other species (24), and it is inconsistent with recent studies of P740 in A. marina (25, 26)
The new subunit was only poorly stained with Coomassie Brilliant Blue, but an equimolar amount of amino acids was detected by sequence analysis, indicating that this subunit is a real constit

Summary

Introduction

In all oxygenic photosynthetic organisms, two photosystems (PSs) cooperatively function to drive photochemical reactions; both consist of many subunits and pigments, forming supramolecular complexes (for reviews see Refs. 1, 2). Pigments function in two roles as follows: as light-harvesting components and as electron. Electron transfer reactions are organized as a sequence of absolute redox potentials. An absolute excitation energy of a pigment is not significant, but the energy levels relative to that of the reaction center are important For this reason, it is of particular interest to understand the effects of pigment changes on the electrochemical properties of the system and not to rely solely on changes in the light-harvesting properties. PS I complexes of A. marina have been isolated by many groups, and many aspects of their structure and function have been characterized, but inconsistencies in these data remain. This is significantly lower than that of the T. elongatus PS I (ϩ423 mV) (22, 23) and of PS I from other species (24), and it is inconsistent with recent studies of P740 in A. marina (25, 26)

Results

Discussion

Conclusion