Crystal structure of human branched-chain α-ketoacid dehydrogenase and the molecular basis of multienzyme complex deficiency in maple syrup urine disease

Background: Mutations in components of the extraordinarily large alpha-ketoacid dehydrogenase multienzyme complexes can lead to serious and often fatal disorders in humans, including maple syrup urine disease (MSUD). In order to obtain insight into the effect of mutations observed in MSUD patients, we determined the crystal structure of branched-chain alpha-ketoacid dehydrogenase (E1), the 170 kDa alpha(2)beta(2) heterotetrameric E1b component of the branched-chain alpha-ketoacid dehydrogenase multienzyme complex. Results: The 2.7 Angstrom resolution crystal structure of human E1b revealed essentially the full alpha and beta polypeptide chains of the tightly packed heterotetramer. The position of two important potassium (K+) ions was determined. One of these ions assists a loop that is close to the cofactor to adopt the proper conformation. The second is located in the beta subunit near the interface with the small C-terminal domain of the a subunit. The known MSUD mutations affect the functioning of E1b by interfering with the cofactor and K+ sites, the packing of hydrophobic cores, and the precise arrangement of residues at or near several subunit interfaces. The Tyr-->Asn mutation at position 393-alpha occurs very frequently in the US population of Mennonites and is located in a unique extension of the human E1b alpha subunit, contacting the beta' subunit, Conclusions: Essentially all MSUD mutations in human E1b can be explained on the basis of the structure, with the severity of the mutations for the stability and function of the protein correlating well with the severity of the disease for the patients. The suggestion is made that small molecules with high affinity for human E1b might alleviate effects of some of the milder forms of MSUD.