No abstract

The interactions between silver ion and DNA were studied kinetically by the stopped flow method. The experimental results show that the reaction between silver ion and GC base pair of DNA proceeds by two steps and that between silver ion and AT base pair of DNA with one step.

The quaternary structural change of Escherichia coli aspartate transcarbamylase (ATCase) was studied by time-resolved X-ray solution scattering following the binding of carbamoyl phosphate and of succinate, a competitive inhibitor of the natural substrate L-aspartate. Stopped-flow experiments at sub-zero temperatures in the presence of 30% ethylene glycol allowed us to monitor the evolution of the scattering pattern, including the characteristic scattering peak in an s (=2 sin theta/lambda) range of 0.01-0.06 A-1. The inhibitor binding promotes a quaternary structure change from the T state toward the R state, and as expected for a simple ligand binding process, ATCase remains in the R state, unlike the physiological enzyme reaction [Tsuruta, H., et al. (1990) FEBS Lett. 263, 66-68]. After equilibrium had been established, the final scattering pattern was recorded. When the succinate concentration was sufficiently high, this pattern was the same as that given by ATCase saturated with the bisubstrate analogue N-phosphonoacetyl-L-aspartate (PALA). This implies that, under cryogenic conditions, succinate and carbamoyl phosphate promote the same quaternary structure change as PALA, which is in good agreement with the crystallographic studies of Gouaux and Lipscomb [Gouaux, J.E., & Lipscomb, W.N. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 4205-4208]. Scattering patterns recorded during the course of the structural transition were satisfactorily reproduced by a linear combination of the initial and final patterns, suggesting that there is no significant concentration of quaternary structure intermediates between the T and R states. This is consistent with a concerted structural transition of ATCase.(ABSTRACT TRUNCATED AT 250 WORDS)

We have studied the kinetics of the qu&rnary structure change associated witb tke allosteric transition of aspartate tra~~~~y~e (ATCase) (E. co&), inducing this change by expastzre to the. naturaS substrates &arbamyl phasphate and L-aspartate]. The presence of 3O$ etkytene g&o1 slowed the quaternary structure cbaqe ~~~~~~ faz it tcs kc f~ibwcd by &~p~-~~w X-ray s&&ring at -5°C. After adding substrates to the enzyme, tke change occurred, with a k&f-life of a fw seconds* yiedmg a mixtnre of&e tw5 standard ~~~~~ slfuclures for, conceivab~y~ a state intermediate between them). This mixture per&ted until fke enzyme red& the substrate ~~n~~n below a ~~~~ value.