Structure of the Cytochrome b ₆ f Complex: n-Side Donor Pathway to the Plastoquinone Pool

Abstract

Three Prosthetic Groups, Heme C_N, Chl A, And β-Carotene Were Identified In Crystal Structures Of The Cytochrome B ₆ F Complex In The Thermophilic Cyanobacterium, M. Laminosus (Kurisu Et Al. 2003) And The Green Alga, C. Reinhardtii (Stroebel Et Al. 2003). The Functions Of These Groups Are Still Not Understood. A Native Structure Of The Cytochrome B ₆ F Complex From The Thermophilic Cyanobacterium, M. Laminosus, Was Obtained From Crystals Grown With Divalent Cadmium (Pdb Accession: 2E74; (Yamashita Et Al. 2007) ). One Cd²⁺ Binding Site Bridges His143 Of Cytochrome F And The Acidic Residue, Glu75, Of Cyt B ⁶; (Ii) A Second Site Has Three Identified Ligands, Asp58 (Subunit Iv), Glu3 (Petg Subunit) And Glu4 (Petm Subunit). Binding Sites Of Quinone Analogue Inhibitors Map The Transfer Pathway Of The Lipophilic Quinone Across The Complex. Two Sites Were Found For The Chromone Ring Of The Tridecyl-Stigmatellin (Tds) Quinone Analogue Inhibitor, One Near The P-Side [2Fe-2S] Cluster (Pdb: 2E76). A Second Tds Site Faces The Quinone Exchange Cavity As An Axial Ligand Of Heme C _N. A Similar Binding Site As An Axial Ligand To Heme C _N Was Found For The N-Side Quinone Analogue Inhibitor, Nqno (Pdb: 2E75). Binding Of These Inhibitors Required Their Addition Before That Of The Lipid Used To Facilitate Crystallization. Binding Of Nqno And Tds As Axial Ligands To Heme C _N Implies That C _N Utilizes Plastoquinone As A Natural Ligand, Thus Defining An N-Side Electron Transfer Compleand Pq In Thex Consisting Of Hemes B _N, C _N, And Pq In The Reduction Pathway Of Pq In The Cavity. Strong Coupling Of Hemes Bn And Cn Suggests A Mechanism For 2 Electron Reduction Of Pq, Thus Avoiding The Generation Of Plastosemiquionone And Reactive Oxygen Species. The Nqno Binding Site Explains Several Experimental Observations Associated With Its Inhibitory Action: A Negative Shift In The Heme C _N E_M (Alric Et Al. 2005), Increased Amplitude Of Light-Induced Reduction Of Heme B _N (Jones And Whitmarsh 1988; Furbacher Et Al. 1989), And G Value Shifts In The Epr Spectrum Attributed To Interaction Between Hemes C _N And B _N (Zatsman Et Al. 2006; Baymann Et Al. 2007). These Structures Suggest Pathways For H+ Uptake And Potential Site(S) Of Ferredoxin Binding.