Regulation of L-Threonine Dehydrase Via Ligand-Induced Oligomerization
The biodegradative L-threonine dehydrase of Escherichia coli is activated by AMP through structural changes which decrease the Km for L-threonine 23-fold without affecting Vmax. AMP also causes a drastic increase in sedimentation velocity under conditions of catalytic assays. These are equivalent to at least a monomerdimer conversion. Thus, the changes in both kinetic and sedimentation behavior are mutually relevant. Oligomers formed at high protein concentration in the absence of AMP do not show a decreased Km for threonine. In addition, the activation caused by AMP is second order in protein concentration. Therefore, oligomerization is not a sufficient condition but is necessary for activation of the dehydrase.
The twelve equilibria for all combinations of random bindings involving AMP-dehydrase, dehydrase-threonine, and dehydrase (monomer-dimer) interactions can be represented by a cube composed of six binding cycles. Consideration of three individual cycles in conjunction with steady-state kinetic predicts that: (1) the Km for L-threonine and the Ka for AMP should depend on protein concentration; (2) the Km for L-threonine should depend on AMP concentration as well as the converse; and (3) threonine binding favors dissociation into monomers as well as the converse. Some of these predictions are verified in the data presented. Use of Hill coefficients in expressing the oligomeric state is also shown.
KeywordsSedimentation Velocity Hill Coefficient Reaction Cycle Oligomeric State Sedimentation Coefficient
Unable to display preview. Download preview PDF.
- Moss, J. Yamagishi, M., Kleinschmidt, A.K., and Lane, M.D. (1972) Biochemistry 11, 3779.Google Scholar
- Phillips, A.T., and Wood, W.A. (1965) J. Biol. Chem. 240, 4703.Google Scholar
- Umbarger, H.E., and Brown, B. (1957) J. Bacterial. 73, 105.Google Scholar
- Whanger, P.D., Phillips, A.T., Rabinowitz, K.W., Piperno, J.R., Shada, J.D., and Wood, W.A. (1968) J. Biol. Chem. 243, 167.Google Scholar
- Wood, W.A., and Gunsalus, I.C. (1949) J. Biol. Chem. 181, 171.Google Scholar
- Wood, W.A. (1969) Current Topics in Cellular Regulation 1, 161.Google Scholar