Hydrophilic metformin and hydrophobic biguanides inhibit mitochondrial complex I by distinct mechanisms.

Metformin is the only antihyperglycemic biguanide targeting type 2 diabetes mellitus with proven safety. Although a mechanism of action involving tight inhibition of the respiratory complex I has been proposed for hydrophobic biguanides, it remains elusive for the hydrophilic metformin, whose excellent pharmacological tolerance depends on weak complex I inhibition without competitive nature. Here we solved cryo-electron microscopy structures of the metformin-bound porcine respirasome. Our structural and kinetic data are consistent with a model in which metformin enters complex I only in its open state and becomes trapped at the ubiquinone redox site by ubiquinone-induced conformational closing of the enzyme. By contrast, the hydrophobic proguanil alone occupies both the entrance and the redox site of the ubiquinone channel in open and closed complex I and is kinetically consistent with competitive inhibition with conformation-dependent affinities. Our data provide the molecular basis for metformin's well-known superior properties, such as a wide therapeutic window and positive ubiquinone cooperativity, leading to its clinical success and facilitating future therapeutic developments.