The effect of succinate on the translational diffusion coefficient of aspartate transcarbamylase

Employing a differential optical mixing spectrometer, we have determined that the translational diffusion coefficient (DT) of aspartate transcarbamylase (AT-Case) decreases by (4.1 plus or minus 0.6)% in the presence of succinate and carbamyl phosphate. This result, combined with the change in the sedimentation coefficient determined by Gerhart and Schachman (1968) and repeated by us in the present work indicates that ATCase experiences an increase in frictional coefficient of approximately 4% due to succinate and carbamyl phosphate, and that any change in the enzyme's partial specific volume (v) under these conditions is less than about 0.3%. We have also measured (DT)20,w for ATCase as (3.75 plus of minus 0.11) x 10-7 cm2/sec. Combining this with our measured value of s20,2-o for ATCase of (11.7 plus or minus 0.2) x 10-13 sec and the calculated value of v of 0.738 cm3/g (Rosenbusch and Weber, 1971), we have determined the molecular weight of ATCase as (2.9 plus of minus 0.1) x 10-5. We have also observed the ATCase dimer and find that at a dimer concentration of 0.6 mg/ml the value of s20,w for the dimer is 15.8 x 10-13 sec and that this value decreases by (4.0 plus or minus 0.5)% upon the addition of succinate and carbamyl phosphate, a behavior essentially identical with that of the monomer.