A High-Field EPR Study of P700 +• in Wild-Type and Mutant Photosystem I from Chlamydomonas reinhardtii

• Published in: Biochemistry (Easton) 2004-02, Vol.43 (7), p.1781-1786
• Main Authors: Petrenko, Alexander, Maniero, Anna Lisa, van Tol, Johan, MacMillan, Fraser, Li, Yajing, Brunel, Louis-Claude, Redding, Kevin
• Format: Article
• Language: English
• Subjects: Algal Proteins - chemistry; Algal Proteins - genetics; Amino Acid Substitution - genetics; Animals; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Chlamydomonas reinhardtii - chemistry; Chlamydomonas reinhardtii - genetics; Chlorophyll - chemistry; Chlorophyll - genetics; Crystallography, X-Ray; Electron Spin Resonance Spectroscopy - methods; Ligands; Photosystem I Protein Complex - chemistry; Photosystem I Protein Complex - genetics; Protozoan Proteins - chemistry; Protozoan Proteins - genetics; Quantum Theory
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi035466j

High-frequency, high-field EPR at 330 GHz was used to study the photo-oxidized primary donor of photosystem I (P700 +•) in wild-type and mutant forms of photosystem I in the green alga Chlamydomonas reinhardtii. The main focus was the substitution of the axial ligand of the chlorophyll a and chlorophyll a‘ molecules that form the P700 heterodimer. Specifically, we examined PsaA-H676Q, in which the histidine axial ligand of the A-side chlorophyll a‘ (PA) is replaced with glutamine, and PsaB-H656Q, with a similar replacement of the axial ligand of the B-side chlorophyll a (PB), as well as the double mutant (PsaA-H676Q/PsaB-H656Q), in which both axial ligands were replaced. We also examined the PsaA-T739A mutant, which replaces a threonine residue hydrogen-bonded to the 131-keto group of PA with an alanine residue. The principal g-tensor components of the P700 +• radical determined in these mutants and in wild-type photosystem I were compared with each other, with the monomeric chlorophyll cation radical (Chlz +•) in photosystem II, and with recent theoretical calculations for different model structures of the chlorophyll a + cation radical. In mutants with a modified PB axial ligand, the gzz component of P700 +• was shifted down by up to 2 × 10-4, while mutations near PA had no significant effect. We discuss the shift of the gzz component in terms of a model with a highly asymmetric distribution of unpaired electron spin in the P700 +• radical cation, mostly localized on PB, and a deviation of the PB chlorophyll structure from planarity due to the axial ligand.