Photosynthesis Research 13:225 260 (1987) © Martinus Nijhoff Publishers, Dordrecht - Printed in the Netherlands
225
Minireview
Primary photochemistry of reaction centers from the photosynthetic purple bacteria CHRISTINE KIRMAIER & DEWEY HOLTEN Department of Chemistry, Washington University, St. Louis, MO 63130, USA Received 5 January 1987; accepted 20 April 1987
Key words: electron transfer, bacterial photosynthesis, reaction center Abstract. Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions. Abbreviations: B C h l - bacteriochlorophyll, P - a dimer of BChl molecules, BPh bacteriopheophytin, QA and QB - quinone molecules, L, M and H - light, medium and heavy polypeptides of the reaction center
I. Introduction
Scope of this review Since the time the first reaction center was isolated from the purple bacterium Rhodobacter sphaeroides [122] (formerly Rhodopseudomonas sphaeroides [65]), an enormous literature has grown on the properties of bacterial reaction centers (see general reviews [56, 106, 112, 114]). This review will focus on the primary charge separation process in reaction centers isolated from the purple photosynthetic bacteria. Thus, we will be concerned here with the events which begin with excitation of the primary electron donor P, a dimer of bacteriochlorophyll (BChl) molecules, and which culminate, less than a nanosecond later, in the formation of the stable (relatively long-lived) state P+Q~. Subsequent electron transfer from the reduced
226 primary quinone (Q£) to the secondary quinone (QB), leading ultimately to the formation and utilization of QBH2, has been recently reviewed [31], and will not be described here. Nor will we describe in detail the interesting and often novel photochemistry observed in reaction centers in which electron transfer to QA is prevented by its removal or chemical reduction (see [15, 56, 101, 112] and references therein), except insofar as such experiments reveal information about the nature and energetics of states which take part in the normal sequence of events leading to P+ QA formation. This review will especially focus on progress made during the last three to five years, with the intent of highlighting current efforts to understand better the relationship between the structure of the reaction center and how, at a molecular level, it functions so efficiently. The reaction center structure and absorption spectroscopy
All of the purple bacterial reaction centers that have been isolated and analyzed contain six pigment molecules (four BChls and two bacteriopheophytins (BPh)), two quinones (QA and QB), and a divalent nonheme iron atom [26, 36, 143, 150] (in one case manganese [126]). These p
BChI~/\ BChl BChI'M~ .~/BChlL
BPhM~
BPhL QB F 0e
225
Minireview
Primary photochemistry of reaction centers from the photosynthetic purple bacteria CHRISTINE KIRMAIER & DEWEY HOLTEN Department of Chemistry, Washington University, St. Louis, MO 63130, USA Received 5 January 1987; accepted 20 April 1987
Key words: electron transfer, bacterial photosynthesis, reaction center Abstract. Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions. Abbreviations: B C h l - bacteriochlorophyll, P - a dimer of BChl molecules, BPh bacteriopheophytin, QA and QB - quinone molecules, L, M and H - light, medium and heavy polypeptides of the reaction center
I. Introduction
Scope of this review Since the time the first reaction center was isolated from the purple bacterium Rhodobacter sphaeroides [122] (formerly Rhodopseudomonas sphaeroides [65]), an enormous literature has grown on the properties of bacterial reaction centers (see general reviews [56, 106, 112, 114]). This review will focus on the primary charge separation process in reaction centers isolated from the purple photosynthetic bacteria. Thus, we will be concerned here with the events which begin with excitation of the primary electron donor P, a dimer of bacteriochlorophyll (BChl) molecules, and which culminate, less than a nanosecond later, in the formation of the stable (relatively long-lived) state P+Q~. Subsequent electron transfer from the reduced
226 primary quinone (Q£) to the secondary quinone (QB), leading ultimately to the formation and utilization of QBH2, has been recently reviewed [31], and will not be described here. Nor will we describe in detail the interesting and often novel photochemistry observed in reaction centers in which electron transfer to QA is prevented by its removal or chemical reduction (see [15, 56, 101, 112] and references therein), except insofar as such experiments reveal information about the nature and energetics of states which take part in the normal sequence of events leading to P+ QA formation. This review will especially focus on progress made during the last three to five years, with the intent of highlighting current efforts to understand better the relationship between the structure of the reaction center and how, at a molecular level, it functions so efficiently. The reaction center structure and absorption spectroscopy
All of the purple bacterial reaction centers that have been isolated and analyzed contain six pigment molecules (four BChls and two bacteriopheophytins (BPh)), two quinones (QA and QB), and a divalent nonheme iron atom [26, 36, 143, 150] (in one case manganese [126]). These p
BChI~/\ BChl BChI'M~ .~/BChlL
BPhM~
BPhL QB F 0e
