The crystal structure of methylglyoxal synthase from Escherichia coli

Background: The reaction mechanism of methylglyoxal synthase (MGS) is belie ved to be similar to that of triosephosphate isomerase (TIM). Both enzymes utilise dihydroxyacetone phosphate (DHAP) to form an ene-diol(ate) phosphat e intermediate as the first step of their reaction pathways. However, the s econd catalytic step in the MGS reaction pathway is characterized by the el imination of phosphate and collapse of the ene-diol(ate) to form methylglyo xal instead of reprotonation to form the isomer glyceraldehyde 3-phosphate. Results: The crystal structure of MGS bound to formate and substoichiometri c amounts of phosphate in the space group P6(5)22 has been determined at 1. 9 Angstrom resolution, This structure shows that the enzyme is a homohexame r composed of interacting five-stranded beta/alpha proteins, rather than th e hallmark odp barrel structure of TIM, The conserved residues His19, Asp71 ,and His98 in each of the three monomers in the asymmetric unit bind to a f ormate ion that is present in the crystallization conditions. Differences i n the three monomers in the asymmetric unit are localized at the mouth of t he active site and can be ascribed to the presence or absence of a bound ph osphate ion. Conclusions: In agreement with site-directed mutagenesis and mechanistic en zymology, the structure suggests that Asp71 acts as the catalytic base. Fur ther, Asp20 and Asp101 are involved in intersubunit salt bridges. These sal t bridges may provide a pathway for transmitting allosteric information.