A low-resolution crystallographic study of porcine heart lactate dehydrogenase

The H 4 isoenzyme of porcine heart lactate dehydrogenase complexed with coenzyme and a substrate analog has been found to have 222 molecular symmetry. The three molecular 2-fold axes have been related to the corresponding P, Q and R 2-fold axes in the dogfish M 4 isoenzyme. The orientation of these axes, as well as the position of the molecular center in the asymmetric unit, has been determined. An electron density map to a resolution of 6 Å based on the measured pig H 4 LDHase‡ structure amplitudes and phases, calculated for a properly oriented and positioned tetramer of the dogfish M 4 isoenzyme in the pig H 4 cell, has been calculated. In order to avoid the effect of the assumed LDHase model, the electron density was then averaged over the four subunits. The only observable difference in the porcine H 4 LDHase molecule with respect to the dogfish M 4 LDHase structure was a displacement in the vicinity of the essential histidine 195.