The active site of pyruvate decarboxylase from Saccharomyces uvarum (Brewer's yeast) in complex with its thiamine pyrophosphate (TPP) cofactor. The enzyme's identical 563 residue subunits form a tightly associated dimer, two of which associate loosely to form a tetramer (not shown here). The dimer binds two copies of TPP in two symmetrically related cavities. The TPPs and side chains of Glu 51 in each monomer are shown in skeletal form with C green, N blue, O red, S yellow, and P orange. The TPP binds in a cavity situated between the dimer's two subunits (cyan and magenta) where it hydrogen bonds to Glu 51. Choose 'Show' from the 'H-Bonds' menu to see the hydrogen bond between the protein and the cofactor. [X-ray structure by William Furey and Martin Sax, Veterans Administration Medical Center and University of Pittsburgh. PDBid 1PYD.]