Evidence for Hydrogen Bond Formation to the PsaB Chlorophyll of P700 in Photosystem I Mutants of Synechocystis sp. PCC 6803

• Published in: Biochemistry (Easton) Vol. 44; no. 14; pp. 5402 - 5408
• Main Authors: Breton, Jacques, Chitnis, Parag R, Pantelidou, Maria
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 12-04-2005
• Subjects: Hydrogen Bonding; Models, Molecular; Mutagenesis, Site-Directed; Photosystem I Protein Complex - chemistry; Photosystem I Protein Complex - genetics; Spectroscopy, Fourier Transform Infrared; Synechocystis; Synechocystis - chemistry
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi0473612

P700, the primary electron donor of photosystem I, is an asymmetric dimer made of one molecule of chlorophyll a‘ (PA) and one of chlorophyll a (PB) that are bound to the homologous PsaA and PsaB polypeptides. While the carbonyl groups of PA are involved in hydrogen-bonding interactions with several surrounding amino acid side chains and a water molecule, PB does not engage hydrogen bonds with the protein. Notably, the residue Thr A739 is donating a strong hydrogen bond to the 9-keto CO group of PA and the homologous residue Tyr B718 is free from interaction with PB. Light-induced FTIR difference spectroscopy of the photooxidation of P700 has been combined with a site-directed mutagenesis attempt to introduce hydrogen bonds to the carbonyl groups of PB in Synechocystis sp. PCC 6803. The FTIR study of the Y(B718)T mutant provides evidence that the 9-keto CO group of PB and PB + engages a relatively strong hydrogen-bonding interaction with the surroundings in a significant fraction (40 ± 10%) of the reaction centers. Additional mutations on the two PsaB residues homologous to those involved in the main interactions between the PsaA polypeptide and the 10a-carbomethoxy groups of PA affect only marginally the vibrational frequency of the 10a-ester CO group of PB. The FTIR data on single, double, and triple mutants at these PsaB sites indicate a plasticity of the interactions of the carbonyl groups of PB with the surrounding protein. However, these mutations do not perturb the hydrogen-bonding interactions assumed by the 9-keto and 10a-ester CO groups of PA and PA + with the protein and have only a limited effect on the relative charge distribution between PA + and PB +.