Initiator dissociation rate constant

All enzymatic reactions are initiated by formation of a binary encounter complex between the enzyme and its substrate molecule (or one of its substrate molecules in the case of multiple substrate reactions see Section 2.6 below). Formation of this encounter complex is almost always driven by noncovalent interactions between the enzyme active site and the substrate. Hence the reaction represents a reversible equilibrium that can be described by a pseudo-first-order association rate constant (kon) and a first-order dissociation rate constant (kM) (see Appendix 1 for a refresher on biochemical reaction kinetics) ... 

In a typical SPR experiment real-time kinetic study, solution flows over the surface, so desorption of the guest immobilized on the surface due to this flow must be avoided.72 In the first stage of a typical experiment the mobile reactant is introduced at a constant concentration ([H]0) into the buffer flowing above the surface-bound reactant. This favors complex association, and the progress of complex formation at the surface is monitored. The initial phase is then followed by a dissociation phase where the reactant is removed from the solution flowing above the surface, and only buffer is passed over the surface to favor dissociation of the complex.72 74 The obtained binding curves (sensograms) contain information on the equilibrium constant of the interaction and the association and dissociation rate constants for complex formation (Fig. 9). 

If the method used to initiate the dissociation affects the measured dissociation rate constant, allosteric interactions within the ligand-receptor complex may be implicated. 

Figure 5 Kinetic mechanism of aminoacyl-tRNA selection by the ribosome. The aminoacyl-tRNAs are delivered to the ribosome in the form of a ternary complex with EF-Tu-GTP. Incorrect aminoacyl-tRNAs can either dissociate as a ternary complex in the initial selection phase or later as free aminoacyl-tRNA in the proofreading phase. The two selection phases are separated through the irreversible GTP hydrolysis by EF-Tu. Discrimination against incorrect tRNAs is achieved by increased dissociation rate constants (/r 2 and kj) as well as decreased forward rate constants (ks and ks) compared to cognate tRNAs.

A further characteristic of ligand-target interaction is the dissociation rate constant. Dissociation experiments are not only an important criterion for the establishment of binding assays, they also can show the reversibility of the specific marker binding at the target [16], which is usually verified by dissociation experiments (see also Section 7.2.1). In the MS binding assays presented here, GABA was used as a competitor to initiate dissociation. The experiment was then termi-... 

As would be predicted from Equation 1, the rate of dissociation of free radical initiators is decreased by the application of pressure. Thus azobisisobuty-ronitrile dissociates with a rate constant equal to 4.47 X 1(H sec." at 1500 atm. but at 1 atm. the dissociation rate constant is 5.5 X 10 sec. (8). Studies concerning the effect of pressure on the decomposition of benzoyl peroxide reveal that the rate of this reaction also decreases with increasing pressure (II, 18). The extent to which the radical-induced decomposition of this peroxide at high pressures affects the rate is not clear, but it appears that some complications arise from this cause. 

A plot of the left side of Eq. (6.99), which is equivalent to the expression In [(ln[M]oo — ln[M]) / ln[M]oo — ln[M],)], versus time permits evaluation of kd (see Problem 6.15). Since kd is determined, / can be obtained from either Eq. (6.26) or (6.98) if the ratio kp/kf is known from other studies. The thermal dissociation rate constants and activation energy values for several commonly used initiators are listed in Table 6.6. 

Czeisler and Hollis (302) suggested from an NMR study that the NADH binding and dissociation is a two-step process A rapid initial binding is followed by a slow isomerization step. They found strong evidence for a dissociation rate constant of 100 sec for the adenine moiety of the coenzyme and, in combination with other studies (281,303), indica-... 

Excited triplet xanthone was initially employed as a probe molecule to study the dynamics of surfactant aggregated to polyelectrolytes [186]. Triplet-triplet absorption spectra were obtained 40 ns and 1 ps after the laser pulse, and a blue shift was observed. These two spectra were ascribed respectively to xanthone within the macroassembly and in the aqueous phase. Quenching experiments were carried out where the probe residing in the aqueous phase was quenched, whereas that in the self-assembly was protected. The xanthone dissociation rate constant from the aggregates was 1.1 x 10 s" [186]. 

The initiation step involves the dissociation of an initiator (I) with the formation of two radicals (R ) with a dissociation rate constant kj ... 

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