Crystallization and structure determination of the catalytic trimer of Methanococcus jannaschii aspartate transcarbamoylase

Aspartate transcarbamoylase (ATCase) catalyzes the first step in the pyrimi dine biosynthetic pathway, the reaction between carbamoyl phosphate and L-a spartate to form N-carbamoyl-L-aspartate and phosphate. The structural anal ysis of the ATCase catalytic trimer from Methanococcus jannaschii, a unicel lular thermophilic archaea-bacterium, has been undertaken in order to gain insight into the structural features that are responsible for the thermosta bility of the enzyme. As a first step, the catalytic trimer was crystallize d in space group R32, with unit-cell parameters a = b = 265.3, c = 195.5 An gstrom and two trimers in the asymmetric unit. Its structure was determined using molecular replacement and Patterson methods. In general, structures containing multiple copies of molecules in the asymmetric unit are difficul t to determine. In this case, the two trimers in the asymmetric unit are pa rallel to each other and use of the Patterson function greatly simplified t he structure solution.