Structure-function studies of the cytochrome b6f complex of oxygenic photosynthesis

Syed Saif Hasan, Purdue University

Abstract

The cytochrome b6f complex of oxygenic photosynthesis is a hetero-oligomeric membrane protein complex that functions in the photosynthetic electron transfer chain in thylakoid membranes. The b6f complex, along with the cytochrome bc1 complex of anoxygenic photosynthetic bacteria and the mitochondrial respiratory chain, belongs to the family of cytochrome bc complexes. Cytochrome bc complexes act as quinone oxidoreductases that catalyze the generation of a transmembrane proton electrochemical gradient linked to quinone redox reactions. The electrochemical gradient is discharged by the membrane bound ATP synthase for ATP synthesis. Crystal structures of the b6f complex have been obtained from the prokaryotic filamentous cyanobacteria, Mastigocladus laminosus and Nostoc PCC 7120, and the eukaryotic green alga, Chlamydomonas reinhardtii, to a resolution of 3.0-3.1 Å. The structures revealed the presence of ordered, well-defined lipid, detergent, and photosynthetic pigment binding sites within the hydrophobic b6f transmembrane domain, a narrow portal that leads to the substrate oxidation (Qp) site on the electrochemically positive (p) side of the membrane, and the presence of a unique heme cn, on the electrochemically negative (n) side of the membrane. In the present study, crystallographic analyses of the cytochrome b6f complex were performed to understand the role of lipids in mediating structural and functional stability of the b6f complex. The first sub-3.0 Å resolution crystal structures, solved from the M. laminosus and Nostoc PCC 7120 cytochrome b6f complex, provided information on lipid-mediated conformational changes and the proton transfer pathways through the b6f transmembrane hydrophobic domain. The conclusions reached from the present study are: (i) conservation of structure-function in cytochrome bc complexes extends beyond polypeptide subunits and redox prosthetic groups to well-defined lipid binding sites; (ii) charged lipids associated with the b 6f complex mediate long range conformational changes within the complex, (iii) the unique chlorophyll-a molecule within the Qp-site portal may be involved in transmembrane signaling, (iv) proton transfer pathways on the n and p-side of the complex contrast in the degree of hydration, and (v) the Qp -site portal may be engineered to accelerate photosynthetic electron transfer.

Degree

Ph.D.

Advisors

CRAMER, Purdue University.

Subject Area

Biology|Biochemistry|Biophysics

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