HIGH-RESOLUTION CRYSTAL-STRUCTURE OF PYRUVATE DECARBOXYLASE FROM ZYMOMONAS-MOBILIS - IMPLICATIONS FOR SUBSTRATE ACTIVATION IN PYRUVATE DECARBOXYLASES

The crystal structure of tetrameric pyruvate decarboxylase from Zymomo nas nobilis has been determined at 1.9 Angstrom resolution and refined to a crystallographic R-factor of 16.2% and R-free of 19.7%, The subu nit consists of three domains, all of the alpha/beta type. Two of the subunits form a tight dimer with an extensive interface area. The thia min diphosphate binding site is located at the subunit-subunit interfa ce, and the cofactor, bound in the V conformation, interacts with resi dues from the N-terminal domain of one subunit and the C-terminal doma in of the second subunit, The S-fold symmetry generates the second thi amin diphosphate binding site in the dimer, Two of the dimers form a t ightly packed tetramer with pseudo 222 symmetry. The interface area be tween the dimers is much larger in pyruvate decarboxylase from Z, mobi lis than in the yeast enzyme, and structural differences in these part s result in a completely different packing of the subunits in the two enzymes. In contrast to other pyruvate decarboxylases, the enzyme from Z, mobilis is not subject to allosteric activation by the substrate. The tight packing of the dimers in the tetramer prevents large rearran gements in the quaternary structure as seen in the yeast enzyme and lo cks the enzyme in an activated conformation. The architecture of the c ofactor binding site and the active site is similar in the two enzymes . However, the x-ray analysis reveals subtle but significant structura l differences in the active site that might be responsible for variati ons in the biochemical properties in these enzymes.