Thiol oxidation of mitochondrial FO-c subunits: a way to switch off antimicrobial drug targets of the mitochondrial ATP synthase

A primary goal in antimicrobial drug design is to find molecules which inhibit key proteins in bacteria without affecting mammalian homologues. To this aim, structural differences between eukaryotic and prokaryotic enzyme proteins involved in life processes are widely exploited. The membrane-bound enzyme complex ATP synthase synthesizes the energy currency molecule of the cell. Due to its bioenergetic role, it represents “the enzyme of life” of all living beings. The enzyme complex has the unique bi-functional property of exploiting either the electrochemical transmembrane gradient to make ATP or, conversely, the free energy of ATP hydrolysis to build an electrochemical gradient across the membrane. The catalytic mechanism of ATP synthesis/hydrolysis, based on the coupling between the two rotary sectors FO and F1 is shared by eukaryotes and prokaryotes. However slight structural differences distinguish prokaryotic ATP synthases, embedded in cell membrane, from eukaryotic ones localized in the mitochondrial inner membrane. In spite of its fundamental task in living organisms, up to now the ATP synthase has been poorly exploited as target in antibacterial therapy, mainly due to harmful effects on patients. Recent advances shoulder the use of drugs targeting the ATP synthase to fight mycobacteria and treat human tuberculosis. Macrolide antibiotics and other antimicrobial drugs specifically bind to the c-ring of the membrane-embedded FO domain, thus blocking ion translocation through FO which is essential for both ATP synthesis and ATP hydrolysis. Our findings show that, once bound to the ATP synthase, probably through different binding sites on a common binding region on FO, the macrolide antibiotics oligomycin, venturicidin and bafilomycin behave as enzyme inhibitors. Interestingly, the c subunits of mitochondrial ATP synthase contain conserved cysteine residues which are absent in bacteria. We pointed out that when these crucial cysteine thiols are oxidized, the common drug binding site of the enzyme is somehow destabilized, thus weakening the enzyme-drug interactions and making the ATP synthase insensitive to drug inhibition. On these bases we hypothesize that the selective oxidation of these cysteine thiols can be exploited to desensitize the mitochondrial ATP synthase to drugs which target FO and maintain their inhibitory potency on bacterial ATP synthases. According to our hypothesis, this strategy could represent an intriguing tool to prevent adverse effects of antimicrobial drugs in mammals, thus enhancing the number of natural and synthetic compounds which can be used in therapy. To this aim studies should be addressed to the identification and formulation of compounds and/or treatments able to selectively oxidize the crucial cysteine thiols of c-subunits without affecting the overall functionality of the mitochondrial ATP synthase and other thiol containing proteins.