Structure of the catalytic chain of Methanococcus jannaschii aspartate transcarbamoylase in a hexagonal crystal form: insights into the path of carbamoyl phosphate to the active site of the enzyme.

Crystals of the catalytic chain of Methanococcus jannaschii aspartate transcarbamoylase (ATCase) grew in the presence of the regulatory chain in the hexagonal space group P6(3)22, with one monomer per asymmetric unit. This is the first time that crystals with only one monomer in the asymmetric unit have been obtained; all known structures of the catalytic subunit contain several crystallographically independent monomers. The symmetry-related chains form the staggered dimer of trimers observed in the other known structures of the catalytic subunit. The central channel of the catalytic subunit contains a sulfate ion and a K(+) ion as well as a glycerol molecule at its entrance. It is possible that it is involved in channeling carbamoyl phosphate (CP) to the active site of the enzyme. A second sulfate ion near Arg164 is near the second CP position in the wild-type Escherichia coli ATCase structure complexed with CP. It is suggested that this position may also be in the path that CP takes when binding to the active site in a partial diffusion process at 310 K. Additional biochemical studies of carbamoylation and the molecular organization of this enzyme in M. jannaschii will provide further insight into these points.