Temperature jump kinetic study of the stability of apo-calmodulin.
Temperature-jump relaxation spectrometry has been used to study the unfolding properties of Ca(2+)-free Drosophila calmodulin from 278 to 336 K, monitored by absorption of Tyr-138. The T-jump amplitude data are well fitted throughout with a melting temperature T(m) = 315.7 K, deltaH(o)(m) = 140.5 kJ mol(-1) and deltaC(p)(o) = 3.28 kJ K(-1) mol(-1), giving deltaG(o)(293) = 7.36 kJ mol(-1) for the C-domain, in good agreement with other data. The relaxation rate observed (time range 1 micros-1 ms) obeys a simple two-state kinetic mechanism throughout. The activation energy for unfolding is nearly temperature-independent, in contrast to that for refolding, and hence the transition state is relatively compact, resembling the folded state, and the relaxation time, tau, shows complex temperature dependence. The domain unfolding is a two-state process occurring with tau of approximately 100 micros at the T(m). At 296 K, when the C-domain is approximately 6% unfolded, k(unfolding) approximately 305 s(-1), k(refolding) approximately 4660 s(-1) and tau approximately 200 micros. This closely resembles the rate and extent of a reported C-domain exchange process, inferred from NMR line-broadening at 296 K. The inherent instability of the apo-C-domain of calmodulin indicates that the unfolded form significantly contributes to the physical properties of apo-calmodulin at normal temperatures, and this instability is enhanced by low ionic strength conditions.
Rabl
Carl-Roland
CR
Faculty of Chemistry, University of Bielefeld, PO Box 100130, D-33501 Bielefeld, Germany.
Martin
Stephen R
SR
Neumann
Eberhard
E
Bayley
Peter M
PM
D000818:Animals; D002147:Calmodulin / Q000737:chemistry*; D004330:Drosophila; D007700:Kinetics; D009994:Osmolar Concentration; D011994:Recombinant Proteins / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
D002147:Calmodulin; D011994:Recombinant Proteins