Is There Consistency Working Within the Context of a Kinetic Model

5 IS THERE CONSISTENCY WORKING WITHIN THE CONTEXT OF A KINETIC MODEL  

In this final section we examine a set of observations that may be interpreted in alternative ways the point being that interpretation should be made within the context of any model that is evoked to represent enzyme kinetic behavior. The simplest and most commonly applied model, the 

Michaelis-Menten or hyperboKc kinetics model, is used here to illustrate how a model can be employed to guide interpretations and conclusions. 

Substrate inhibition in studies on enzyme kinetics is a property observed more often than perhaps one would anticipate. An example of an enzyme reaction subject to substrate inhibition is illustrated in Fig. 14.9. A conclusion that may be reached upon the presentation of such data is ... the enzyme reaction was subject to substrate inhibition at [S] of greater than 2 mM. This would be a naive conunent a more a precise comment would be that ... the enzyme reaction was subject to substrate inhibition and reaction rates started to decline at [S] of greater than 2 mM. The difference between these statements lies much deeper than simply semantics. 

To make an appropriate assessment of the pattern of inhibition, one need only compare the pattern of reaction velocity versus [S] observed relative to the pattern predicted from an application of the hyperbolic kinetics model. This requires making an estimate of V ax and from the data available. Transforming the original data to a Lineweaver-Burke plot (despite the aforementioned limitations) indicates that only four data points (at low [S]) can be used to estimate Vmax and Km (as 3.58 units and 0.48 mM, respectively. Fig. 14.10). The predicted (uninhibited) behavior of the enzyme activity can now be calculated by applying the rectangular hyperbola [Eq. (14.5)] (yielding the upper curve in Fig. 14.11), and it becomes clear that inhibition was obvious at [S] 1 mM. The degree of inhibition is expressed appropriately as the difference between observed and predicted activity at any [S] value, if one makes interpretations within the context of the Michaelis-Menten model. 

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