Fevzi Daldal

photosynthetic and respiratory electron transfer pathways and structure, function and biogenesis of cytochrome complexes

figure 1

A schematic drawing of the photosynthetic and respiratory electron transport pathways and components of the facultative phototrophic bacterium Rhodobacter capsulatus. Download full (high-quality) PDF drawing.

We are interested in understanding the molecular basis of biological electron transfer during cellular energy transduction in photosynthesis and respiration. These basic metabolic pathways contain several multisubunit, membrane-bound protein complexes with various redox-active prosthetic groups. They are vital components for important cellular functions ranging from ATP synthesis to secretion, solute transport, motility and thermogenesis. Their dysfunction severely compromises cellular energy production, and leads to low crop yields in plants (photosynthesis), or neurological and muscular diseases in humans (respiration). A detailed understanding of how these evolutionarily well-conserved energy producing molecular machines perform their functions is of considerable biological significance and of general interest. Our studies aim to define the structure, function, assembly, biogenesis and regulation of these proteins in response to environmental signals, such as light and oxygen. Cytochrome bc1 complex and cytochrome cbb3 oxidase, which are membrane-associated proton pumps, and their physiological electron carriers the cytochromes c2 and cy are currently under study.

figure 2 - Ccm-system I components for c-type cytochrome maturation in purple bacteria. 

All components of the c-type cyt maturation, except the thiol-disulfide oxidoreductase DsbA, are located in the cytoplasmic membrane. Both apocyt c and heme follow different routes to the heme ligation core complex, composed of CcmI, CcmH and CcmF. Apocyt c is translocated via the Sec pathway, its cysteine thiols in the conserved CXYCH motif are first oxidized by the DsbA- DsbB pathway, and then reduced by the cyt c maturation specific CcdA - CcmG and/or CcmH thio-reductive pathway. CcmI is involved in delivering apocyt c to the core heme ligation complex via its different domains. Heme is translocated across the membrane, possibly via ABC­type transporter CcmABCD, and is covalently attached to the conserved His residue of the heme chaperone CcmE. CcmC is involved in attaching heme to CcmE, and CcmD enhance holo-CcmE production. CcmA and CcmB promote the release of holo-CcmE from CcmC and CcmD. Upon formation of the thioether bonds between the apocyt c and heme vinyls, catalyzed by CcmH, CcmI and CcmF complex mature holocyt c is released. Download full (high-quality) PDF drawing.

We use molecular genetic and genomic approaches combined with biochemical, biophysical and structural techniques. As a model system, we use the purple non-sulfur photosynthetic bacterium Rhodobacter capsulatus instead of mitochondria of eukaryotes or chloroplasts of plants which are more refractory to multidisciplinary analyses. Current work is focused on the 1) structure and function of the functional sites of the cytochrome complexes and the assembly of their subunits and 2) biogenesis of c-type cytochromes including cyt cy and cyt cbb3 which are novel membrane-associated electron carriers recently discovered in our group.