Crystal structure of human branched-chain alpha-ketoacid dehydrogenase andthe molecular basis of multienzyme complex deficiency in maple syrup urinedisease

Background: Mutations in components of the extraordinarily large alpha-keto acid dehydrogenase multienzyme complexes can lead to serious and often fata l 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 dehydr ogenase (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 reveale d essentially the full alpha and beta polypeptide chains of the tightly pac ked heterotetramer. The position of two important potassium (K+) ions was d etermined. One of these ions assists a loop that is close to the cofactor t o 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 k nown MSUD mutations affect the functioning of E1b by interfering with the c ofactor and K+ sites, the packing of hydrophobic cores, and the precise arr angement of residues at or near several subunit interfaces. The Tyr-->Asn m utation at position 393-alpha occurs very frequently in the US population o f Mennonites and is located in a unique extension of the human E1b alpha su bunit, contacting the beta' subunit, Conclusions: Essentially all MSUD mutations in human E1b can be explained o n the basis of the structure, with the severity of the mutations for the st ability and function of the protein correlating well with the severity of t he disease for the patients. The suggestion is made that small molecules wi th high affinity for human E1b might alleviate effects of some of the milde r forms of MSUD.