Investigation into the Role of A0 Axial Ligand MET Residue in Photosystem I of Cyanobacteria

Open Access
Hao, Sijie
Graduate Program:
Biochemistry, Microbiology, and Molecular Biology
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 05, 2013
Committee Members:
  • John H Golbeck, Thesis Advisor
  • Donald Ashley Bryant, Thesis Advisor
  • Lasse Jensen, Thesis Advisor
  • Photosystem I
  • A0
  • Protein cofactor interaction
This study investigates the protein-cofactor interaction around A0 site in the cyanobacterial photosystem I (PS I). In particular, we study the role of methionine ligand (MET) in fine-tuning the redox potential of A0 chlorophyll a (Chl a). A total of 96 Chl a molecules are non-covalently bonded to PS I. The amino nitrogen of HIS residue is a preferred ligand. One pair of Chl a molecules serves as the primary electron acceptors A0 along the PS I electron transfer chain. Unusually, a methionine residue (MET) provides the axial ligand to the central Mg2+ ion, MET688 to eC-A3 Chl a in the A-branch and MET668 to eC-B3 Chl a in the B-branch. We constructed the M688HPsaA and M668HPsaB variants in Synechocystis sp. PCC 6803, where the A0 MET ligand is changed to HIS in A- and B-branch, respectively. The molecular dynamics simulation of the A-branch variant PS I implies that the amino nitrogen of HIS688 binds to the A0 Mg2+ ion. This prediction is confirmed by the hyperfine sublevel correlation (HYSCORE) spectroscopy and Q-band EPR measurement. The strong interaction between HIS and its protein environment raises the redox potential of A0–/A0 pair, and possibly that of A1–/A1 pair as well. We conclude that the Met residue function as a space holder for the ligand to A0 Chl a, increasing its redox potential in order to optimize the electron transfer rate through A0 site. We also discuss two possible conformations of the HIS residue in Met to HIS variants in cyanobacteria vs. that in algae.