Abstract
Aspartate transcarbamylase (EC 2·1·3·2) purified from mung bean seedlings was used as a model to understand the mechanism of allosteric regulation. The enzyme exhibited homotropic interactions with carbamyl phosphate. Preincubation of the enzyme with aspartate abolished the sigmoidicity of the carbamyl phosphate saturation curve. UMP was the most potent inhibitor of the reaction and was noncompetitive with respect to aspartate. The sigmoidicity of carbamyl phosphate saturation curves increased with increase in UMP concentration. These results were analysed by an iterative least squares procedure. There was no change inV max values with increase in the UMP concentration, although theK 0·5 values (concentration of carbamyl phosphate required to reach half maximal velocity) increased. This implied that the effect of UMP was on the binding of carbamyl phosphate only and not on the catalytic function of the enzyme. The allosteric properties of the enzyme could be explained in terms ofK system of the symmetry model. The values of the allosteric constantsn, L andc calculated for mung bean enzyme, making use of the Monod equation accounted for all the observed properties. The enzyme appeared to be a tetramer (n=4) and in the absence of ligands was predominantly in theT form (L o= 2·25). Carbamyl phosphate bound preferentially to theR form (c= 10−3), while UMP bound preferentially to theT form and hence these two ligands exhibited the typical heterotropic interactions as expected of antagonistic ligands.
Similar content being viewed by others
References
Achar B S, Savithri H S, Vaidyanathan C S and Appaji Rao N 1974Eur. J. Biochem. 47 15
Blackburn M N and Schachman H K 1976Biochemistry 15 1316
Changeux J P and Rubin M M 1968Biochemistry 7 553
Frieden C 1967J. Biol. Chem. 242 4045
Gerhart J C 1970Curr. Top. Cell. Regul. 2 276 eds B L Horecker and E R Stadtman, (Academic Press)
Gerhart J C and Pardee A B 1962J. Biol. Chem. 237 891
Horn A and Bornig H 1969FEBS Lett. 3 325
Jacobson G R and Stark G R 1973 inThe Enzymes ed. P D Boyer 3rd ed.9 225 (Academic Press)
London E R and Schmidt P G 1972Biochemistry 11 3136
Lowry C H, Rosenbrough M J, Fars A L and Randall R J 1951J. Biol. Chem. 193 265
Monad J, Wyman J and Changeux J P 1965J. Mol. Biol. 12 88
Ong B L and Jackson J F 1972Biochem. J. 129 571
Ta-Yuan Chang and Jones M E 1974Biochemistry 13 629
Wieker H J, Johannes K J and Hess B 1970FEBS Lett. 8 178
Yon R J 1972Biochem. J. 128 311
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Savithri, H.S., Vaidyanathan, C.S. & Appaji Rao, N. Studies onplant aspartate transcarbamylase: Regulatory properties of the enzyme from mung bean (Phaseolus aureus) seedlings. Proceedings of the Indian Academy of Sciences - Section B 87, 67–79 (1978). https://doi.org/10.1007/BF03178975
Received:
Published:
Issue Date:
DOI: https://doi.org/10.1007/BF03178975