The quaternary structure of wheat-germ aspartate transcarbamoylase

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Abstract
The MW of aspartate transcarbamoylase purified from wheat germ was found to be 101 kDa [kilodaltons] by sucrose-density-gradient centrifugation, 103 kDa by gel-filtration chromatography and 108 kDa by polyacrylamide-gel electrophoresis. A mean value of 104 .+-. 11 kDa was obtained by pooling several replicate results from each method. Polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulfate indicated a single size of polypeptide chain of mean MW 37 .+-. 4 kDa. The ratio of the mean MW of the active and denatured enzymes is 2.8. When the active enzyme was covalently cross-linked at a low protein concentration by dimethyl suberimidate, and then examined electrophoretically under denaturing conditions, 3 size species were observed to predominate, of apparent MW 36, 77 and 106 kDa, respectively. The intact, fully regulatory enzyme is apparently a simple trimer, slightly larger than the trimeric catalytic subunit of the aspartate transcarbamoylase from Escherichia coli. The prevalence of trimeric structures among carbamoyltransferase enzymes is discussed.