The machinery positioning chlorophyll in Cytochrome b6f

Maskineriet som posisjonerer klorofyll i Cytochrome b6f - Klorofyll er nøkkelmolekylet for fotosyntese på jorden. I en m2 av et plantebladareal omtrent 10ˆ18 klorofyll molekyler er bundet til fotosystemproteinene og hver klorofyll må finne en nøyaktig posisjon og orientering for å høste og trekke ut energi fra lys. Vi vet ikke hvordan denne prosessen er regulert. Nøkkelproteinet som regulerer den lysavhengige elektron- og protonstrømmen i fotosyntesen er Cytochrome b6f-komplekset. Proteinet binder kun ett klorofyllmolekyl. Vi karakteriserer proteinmaskineriet som plasserer dette enkelte klorofyllet inn i enzymets struktur for å lære hvordan dette viktige fotosyntetiske proteinkomplekset er satt sammen.

In oxygenic photosynthesis, the cytochrome b6f complex is the plastoquinol—plastocyanin reductase. It regulates electron and proton flow and binds a unique Chlorophyll molecule per complex monomer. This chlorophyll is evolutionarily conserved in oxygenic photosynthetic organisms, but its function is unknown. In angiosperms grown in darkness, chlorophyll can´t be synthesized. We found in this developmental stage that the complex still assembles; however, here protochlorophyll is bound. This finding confirmed the importance of the pigment. We showed that chlorophyll stabilizes the structure of cytochrome b6f and that it binds to cytochrome b6. We identified that the light-harvesting protein LIL3 is binding chlorophyll in parallel with the accumulation of chlorophyll in the cytochrome b6f dimer. This finding prompted us to isolate, characterize, and determine the atomic structure of the protein machinery that delivers and regulates the binding of chlorophyll to the cytochrome b6f complex. We differentiate between chlorophyll-binding to Cyt b6 and subunit IV and the assembly of the complex during or after the completion of translation and membrane integration. The cytochrome b6f complex is isolated in both the protochlorophyll and chlorophyll-bound state and the atomic structures are determined. For atomic structure determination and imaging, we continue to focus on cryo-electron microscopy and extend our efforts using the free-electron x-ray laser sources at the XFEL in Hamburg.