Structural and functional study of Alternative Complex III in the photosynthetic electron transfer chain of Chloroflexus aurantiacus.

摘要

his dissertation focuses on the structural and functional study of a newly found electron transfer protein complex in Chloroflexus aurantiacus. The lack of the cytochrome bc complex in the green photosynthetic bacterium Chloroflexus aurantiacus, which belongs to the phylum of filamentous anoxygenic phototrophs, suggests that a functional replacement exists to link the reaction center photochemistry to cyclic electron transfer as well as respiration. Earlier work identified a potential substitute of the cytochrome bc complex, the menaquinol: auracyanin oxidoreductase (also named alternative complex III, or ACIII), which has been purified, identified and characterized from C. aurantiacus. AC III is an integral-membrane protein complex of around 300 kDa that consists of 7 different subunits, including a 113 kDa iron-sulfur cluster-containing polypeptide, a 25 kDa multi-heme c -containing subunit and two copies of a 23 kDa mono-heme c-containing subunit. In this work, the number of copies of each subunit and the numbers of heme c in the multi-heme-containing subunit have been studied by HPLC combined with ESI-MS, potentiometric titration and intensity analysis of heme-stained SDS-PAGE. A preliminary structural model of the ACIII complex is proposed based on the transmembrane and signal peptide analysis and chemical cross-linking combined with MALDI-TOF. The measurement of menaquinol:auracyanin oxidoreductase activity strongly supports the view that the ACIII functions as an electron carrier in the electron transfer chain of C. aurantiacus. The lack of sensitivity to the common inhibitors of the cytochrome bc complex suggests a different catalytic mechanism of the ACIII complex. The contribution of individual subunits to the overall function of the ACIII complex was also investigated in this work. The monoheme subunit ActE is suggested to be a critical component to the electron transfer function of ACIII by utilizing biochemical and molecular biological methods. This research was supported by grant