Modern medicine offers no cure for mitochondrial disorders such as lipoamide dehydrogenase (LAD) deficiency. LAD is the E3 subunit shared by α-ketoacid dehydrogenase complexes in the mitochondrial matrix, and these complexes are crucial for the metabolism of carbohydrates and amino acids. We propose a novel concept for restoring the activity of an immense multicomponent enzymatic complex by replacing one mutated component, the LAD subunit. Our approach entails the fusing of LAD with the transactivator of transcription (TAT) peptide, which is capable of rapidly crossing biological membranes, thereby allowing TAT-LAD to be delivered into cells and their mitochondria where it can replace the mutated endogenous enzyme. Our results show that TAT-LAD is indeed delivered into the cells and their mitochondria, where it is processed, restoring LAD activity to normal values and, most importantly, increasing the activity of pyruvate dehydrogenase complex. We report here, for the first time, that TAT-mediated replacement of one mutated component restores the activity of an essential mitochondrial multicomponent enzymatic complex in cells of patients with enzyme deficiencies. We believe that this approach involving TAT-mediated enzyme replacement therapy (ERT) can be applied to the treatment of LAD deficiency as well as to other mitochondrial and metabolic disorders.