Natural Osmolyte Trimethylamine N-Oxide Corrects Assembly Defects of Mutant Branched-chain α-Ketoacid Decarboxylase in Maple Syrup Urine Disease

Maple syrup urine disease is caused by deficiency in the mitochondrial branched-chain α-ketoacid dehydrogenase (BCKD) complex. The clinical phenotype includes often fatal ketoacidosis, neurological derangement, and mental retardation. The type IA mutations Y393N-α, Y368C-α, and F364C-α, which occur in the E1α subunit of the decarboxylase (E1) component of the BCKD complex, impede the conversion of an αβ heterodimeric intermediate to a native α₂β₂ heterotetramer in the E1 assembly pathway. In the present study, we show that a natural osmolyte trimethylamine N-oxide (TMAO) at the optimal 1 M concentration restores E1 activity, up to 50% of the wild type, in the mutant E1 carrying the above missense mutations. TMAO promotes the conversion of otherwise trapped mutant heterodimers to active heterotetramers. This slow step does not involve dissociation/reassociation of the mutant heterodimers, which are preformed in the presence of chaperonins GroEL/GroES and Mg-ATP. The TMAO-stimulated mutant E1 activity is remarkably stable upon removal of the osmolyte, when cofactor thiamine pyrophosphate and the transacylase component of the BCKD complex are present. The above in vitro results offer the use of chemical chaperones such as TMAO as an approach to mitigate assembly defects caused by maple syrup urine disease mutations.