Studies on the symmetry of quaternary structure of PEP carboxylase from sorghum leaves were carried out by means of chemical crosslinking with bifunctional reagents (diimidoesters) and subsequent SDS polyacrylamide gel electrophoresis.Results showed that the four identical subunits of the enzyme are associated isologously: i.e. the quaternary structure of the enzyme has a D_2 symmetry.The patterns of PEP carboxylase crosslinked with suberic dimethylimidate varied in the presence of various ligands.Substrate PEP, Mg~(2+) (required for catalysis of the enzyme), PEP plus Mg~(2+) and the inhibitor malate increased the extent of crosslinking of the enzyme in varying degrees, indicating that binding of these ligands to the enzyme gave rise to changes in the subunit interactions.However,the extent of crosslinking of PEP carboxylase with suberic dimethylimidate decreased greatly in the presence of the activator G6P. Meanwhile, the enzyme remains a tetrameric form either in the absence or in the presence of G6P. Studies on the symmetry of quaternary structure of PEP carboxylase from sorghum leaves were carried out by means of chemical crosslinking with bifunctional reagents (diimidoesters) and subsequent SDS polyacrylamide gel electrophoresis. Results showed that the four identical subunits of the enzyme are associated isologously: ie the quaternary structure of the enzyme has D_2 symmetry. The patterns of PEP carboxylase crosslinked with suberic dimethylimidate varied in the presence of various ligands. Substrate PEP, Mg 2+ (required for catalysis of the enzyme), PEP plus Mg ~ ( 2+) and the inhibitor malate increased the extent of crosslinking of the enzyme in varying degrees, indicating that binding of these ligands to the enzyme gave rise to changes in the subunit interactions. Although, the extent of crosslinking of PEP carboxylase with suberic dimethylimidate greatly in the presence of the activator G6P. Meanwhile, the enzyme remains a tetrameric form either in the absence or in th e presence of G6P.