Biochemical basis of type IB (E1β) mutations in maple syrup urine disease. A prevalent allele in patients from the druze kindred in Israel

Maple syrup urine disease (MSUD) is a metabolic disorder associated with often-fatal ketoacidosis, neurological derangement, and mental retardation. In this study, we identify and characterize two novel type IB MSUD mutations in Israeli patients, which affect the E1β subunit in the decarboxylase (E1) component of the branched-chain α-ketoacid dehydrogenase complex. The recombinant mutant E1 carrying the prevalent S289L-β (TCG → TTG) mutation in the Druze kindred exists as a stable inactive αβ heterodimer. Based on the human E1 structure, the S289L-β mutation disrupts the interactions between Ser-289-β and Glu-290-β′, and between Arg-309-& and Glu-290-β′, which are essential for native α₂β₂ heterotetrameric assembly. The R133P-β (CGG → CCG) mutation, on the other hand, is inefficiently expressed in Escherichia coli as heterotetramers in a temperature-dependent manner. The R133P-β mutant E1 exhibits significant residual activity but is markedly less stable than the wild-type, as measured by thermal inactivation and free energy change of denaturation. The R133P-β substitution abrogates the coordination of Arg-133-β to Ala-95-β, Glu-96-β, and Ile-97-β, which is important for strand-strand interactions and K ⁺ ion binding in the β subunit. These findings provide new insights into folding and assembly of human E1 and will facilitate DNA-based diagnosis for MSUD in the Israeli population.