Commitment factors

The terms partitioning ratio and commitment factor are also found. 

An approach to studying transition states in enzyme-catalyzed reactions using solvent isotope effects. In this treatment, very useful in isotope effect experiments, the relative rates of contributing steps in a multistep reaction are grouped into a fraction referred to as the commitment factor ... 

According to the Equation (30) the experimental isotope effect depends not only on the intrinsic isotope effects a,-, but also on the rate constants k2 and k. The intrinsic isotope effects describe the structure of the transition states and the commitment reflects the relative heights of energetic barriers of competitive reactions. If k2i. k -[li(x l), the formation of intermediate B is the rate-limiting step and experimental isotope effect is equal to ai(aexp = i). When intermediate B returns to substrate much faster than forms the product k2, k it (x 1), the experimental isotope effect is aexp = (a1a2)/a 1. For more complex multistep reactions the analysis of isotope effects is analogous, however the commitment factor become a complex collection of kinetic terms.54... 

Mixed labeling experiments with specifically isotopically substituted 4R- and 4S-NADPH cofactors established the primary and secondary kinetic isotope effects and their temperature dependence for the hydride transfer reaction. Indeed, resulting data could be rationalized only by a reaction model featuring an extensive tunneling contribution that is environmentally coupled. The difference in the observed and calculated intrinsic kinetic isotope effects requires a commitment factor arising from dissecting the pre-steady state hydride step into kinetic steps, one the actual hydride transfer step itself and the other a motion of the protein and/or nicotinamide associated with the hydride transfer step [17]. 

If ODCase in fact uses the 02 protonation mechanism, a solution of Eq. 5 must yield plausible values for the individual kinetic constants. Specifically, the values for k and (k5/k4)(l+k3/k2)> the commitment factor... 

For the ordered mechanism shown in reaction (17.18) expressions for the commitment to catalysis have the form of a series of ratios of rate constants from adjacent steps. For example, at low concentrations of substrate A and fixed levels of substrate B, the commitment factor Cfa is ... 

Commitment factors vary in different experiments this is an important distinction between deuterated substrates where the noncompetive method is... 

For the Random Bi Bi mechanism, expressions for commitment factors are more complex due to the branch points the expression forward through the common sequence is divided by the net rate constant away through the separate branches. 

If either substrate is rapidly released from the ternary complex, that is, either k4 or ks is large, the forward commitment factor simplifies to... 

It is important to note that commitment factors may depend on the level of other reactants present, and this variation can be used to determine the kinetic mechanism. For an ordered bisubstrate mechanism, the commitment of B (the second substrate to add) is independent of A (the first substrate to add), and depends only on how fast B is released from the enzyme relative to the forward rate constant for the bond-breaking step. Conversely, the commitment for A varies from infinity at saturating B to the value for B at near zero B, and thus the actual rate constant for release of A from the EA complex does not affect the commitment of B, even if it is quite small. 

Figures. Dependence ofthe isotope effect, (V// a)app, on the concentration of B. The data points are calculated according to Eq. (17.41), assuming that all the rate constants in commitment factors CpA and Cr are equal to unity, except that and °Kec(=°k / ka=5.

At very low [B], the expression approaches the Mmiting isotope effect on V/K, that is, tends to (V/Kb), where the forward commitment factor reduces to... 

Hence, Eq. (17.41), describing the limiting isotope effect on V/K, is independent of the concentration of A. In practice, what we do is to vary [B] at different levels of labeled or unlabeled [A] and determine (V/JCb) from the slope ratio this minimizes any external commitment factors. 

The commitment factors do not contain isotopic steps therefore, the parallel tritium expression for Eq. (17.49) has the same denominator, leading to a cancellation of commitment factors in the ratio... 

Examination of the general equation for the kinetic isotope effect on V/K (Eq. (17.19)) shows that one must lower the conomitment factors to obtain a reliable estimate of the intrinsic isotope effect, The value of commitment factors can... 

Some emphasis should be added here. Factor 1 is not management s commitment to the safety process, it is management s demonstration of commitment. Factor 3 is not the quality of the supervisor, it is the quality of the supervisory process. A supervisory process can exist with or without the existence of first-line supervisors. Self-directed work teams provide an example of a supervisory process that does not use first-line snpervisors. 

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