A common ancestor for oxygenic and anoxygenic photosynthetic systems: a comparison based on the structural model of photosystem I.
The 4 A structural model of photosystem I (PSI) has elucidated essential features of this protein complex. Inter alia, it demonstrates that the core proteins of PSI, PsaA and PsaB each consist of an N-terminal antenna-binding domain, and a C-terminal reaction center (RC)-domain. A comparison of the RC-domain of PSI and the photosynthetic RC of purple bacteria (PbRC), reveals significantly analogous structures. This provides the structural support for the hypothesis that the two RC-types (I and II) share a common evolutionary origin. Apart from a similar set of constituent cofactors of the electron transfer system, the analogous features include a comparable cofactor arrangement and a corresponding secondary structure motif of the RC-cores. Despite these analogies, significant differences are evident, particularly as regards the distances between and the orientation of individual cofactors, and the length and orientation of alpha-helices. Inferred roles of conserved amino acids are discussed for PSI, photosystem II (PSII), photosystem C (PSC, green sulfur bacteria) and photosystem H (PSH, heliobacteria). Significant sequence homology between the N-terminal, antenna-binding domains of the core proteins of type-I RCs, PsaA, PsaB, PscA and PshA (of PSI, PSC and PSH respectively) with the antenna-binding subunits CP43 and CP47 of PSII indicate that PSII has a modular structure comparable to that of PSI.
Schubert
W D
WD
Institut für Kristallographie, Freie Universität Berlin, Takustr. 6, Berlin, D-14195, Germany.
Klukas
O
O
Saenger
W
W
Witt
H T
HT
Fromme
P
P
Krauss
N
N
D000595:Amino Acid Sequence; D001419:Bacteria / Q000378:metabolism; D016227:Benzoquinones / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry; D004579:Electron Transport; D007501:Iron / Q000737:chemistry; D008961:Models, Structural; D008969:Molecular Sequence Data; D010100:Oxygen / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D011758:Pyrroles / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D045725:Tetrapyrroles
D016227:Benzoquinones; D045322:Photosynthetic Reaction Center Complex Proteins; D011758:Pyrroles; D045725:Tetrapyrroles; D002734:Chlorophyll; C004532:quinone; D007501:Iron; D010100:Oxygen