Biochemical Activity of Strobilurins and Oudemansins

Experiments with Ehrlich-Ascites carcinoma cells led to the conclusion that inhibition of mitochondrial respiration is the mode of action of strobilurin and 

Four protein complexes, three ofthem function as proton pumps, are embedded in the inner mitochondrial membrane and constitute essential components of the electron transport chain. Every complex consists of a different set of proteins with a variety of redox-active prosthetic groups. AU in all, through oxidation of NADH, a proton gradient between the matrix and the intermembrane space is created, which eventually drives the ATP synthase-complex. The correspondingly released electrons are consumed in the reduction of oxygen to water. Both, the NADH oxidation and the oxygen reduction, as well as the ATP synthesis, take place in the matrix of the mitochondrion (Fig. 8.14). 

14 Strobilurins halt the production of ATP by blocking the electron transport at the level of the be,-complex, located within the mitochondrial Inner membrane, which separates the matrix from the intermembrane space (transmission electron microscope image). In a process, the so-called Q cycle, which was first proposed by the British biochemist Peter Dennis Mitchell (1920-1992), ubihydroquinone (also known as ubiquinol) is oxidized to ubiquinone, thereby transfering an electron to each, the Rieske iron-sulfur complex and the bi heme. While this cycle operates twice, four protons in total are pumped into the intermembrane space, and generate a proton gradient. 

The cytochrome-bci-complex (also called coenzyme Q or Complex 111 ) is composed of multiple subunits, containing two cytochromes and a [2Fe-2S] cluster (Rieske cluster). This complex passes on the electrons from ubihydroqui-none (QH2) to cytochrome c. Cytochromes are families of redox-active proteins with a haem (heme) moiety as prosthetic group, that is linked via cystein residues sequence and length of the polypeptidic chain are species dependent. 

Cytochrome c is only loosely attached to the inner mitochondrial membrane, and shuttles the electrons within the intermembrane space in a rotating mode from the cytochrome-bci complex to the cytochrome-oxidase complex. Repeated electron transfer cycles within the cytochrome-oxidase complex ultimately reduce oxygen to water. 

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