Discovery of Native Metal Ion Sites Located on the Ferredoxin Docking Side of Photosystem I

• Published in: Biochemistry (Easton) 2008-03, Vol.47 (12), p.3671-3676
• Main Authors: Utschig, Lisa M, Chen, Lin X, Poluektov, Oleg G
• Format: Article
• Language: English
• Subjects: Absorptiometry, Photon; Binding Sites; Copper - chemistry; Electron Spin Resonance Spectroscopy; Ferredoxins - chemistry; Photosystem I Protein Complex - chemistry; Synechococcus; Synechococcus - chemistry; Zinc - chemistry
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi800038d

Photosystem I (PSI) is a large membrane protein that catalyzes light-driven electron transfer across the thylakoid membrane from plastocyanin located in the lumen to ferredoxin in the stroma. Metal analysis reveals that PSI isolated from the cyanobacterial membranes of Synechococcus leopoliensishas a near-stoichiometric 1 molar equiv of Zn2+ per PSI monomer and two additional surface metal ion sites that favor Cu2+ binding. Two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopy reveals coupling to the so-called remote nitrogen of a single histidine coordinated to one of the Cu2+ centers. EPR and X-ray absorption fine structure (XAFS) studies of 2Cu−PSI complexes reveal the direct interaction of ferredoxin with the Cu2+ centers on PSI, establishing the location of native metal sites on the ferredoxin docking side of PSI. On the basis of these spectroscopic results and previously reported site-directed mutagenesis studies, inspection of the PSI crystal structure reveals a cluster of three highly conserved residues, His(D95), Glu(D103), and Asp(C23), as a likely Cu2+ binding site. The discovery of surface metal sites on the acceptor side of PSI provides a unique opportunity to probe the stromal region of PSI and the interactions of PSI with its reaction partner, the soluble electron carrier protein ferredoxin.