Three-dimensional structure of transketolase, a thiamine diphosphate dependent enzyme, at 2.5 Angstrom resolution

The crystal structure of Saccharomyces cerevisiae transketolase, a thiamine diphosphate dependent enzyme, has been determined to 2.5 angstrom resolution. The enzyme is a dimer with the active sites located at the interface between the two identical subunits. The cofactor, vitamin B sub(1) derived thiamine diphosphate, is bound at the interface between the two subunits. The enzyme subunit is built up of three domains of the alpha / beta type. The diphosphate moiety of thiamine diphosphate is bound to the enzyme at the carboxyl end of the parallel beta -sheet of the N-terminal domain and interacts with the protein through a Ca super(2+) ion. The thiazolium ring interacts with residues from both subunits, whereas the pyrimidine ring is buried in a hydrophobic pocket of the enzyme, formed by the loops at the carboxyl end of the beta -sheet in the middle domain in the second subunit.