Upward curvatures: a reversible coenzyme or activator is part of the enzyme preparation. v is proportional to the square of [E]tot, until the enzyme is saturated with coenzyme or activator at high [E]tot, whereupon a linear rate dependence obtains (Fig. 4, curve c). This behavior is observed with proteinases known to need some activator, such as in benzoyl-arginineamide hydrolysis by ficin [20]. Suboptimal amounts of thiols in the enzyme preparation give upward curvature with or without cyanide activator. When thiogylcollate is added, the curvature disappears (e.g., Fig. 4, curve a). Another example is phosphofructokinase, which is active only in its aggregated form [20], so velocity increases as some power of the concentration of the enzyme (Fig. 4, curve d). AMP binds to the aggregated active form, whereas ATP favors the dissociated form [21], so as ATP is hy-drolyzed the enzyme becomes increasingly active. However, the behavior is more complicated with ATP, as ATP binds to both aggregated and dissociated forms of the enzyme, acting both as an activator of activity and a modulator of aggregation state. Its profile is a displaced activation curve with a different linear terminal rate at high enzyme concentration (Fig. 4, curve e).

Downward curvatures: are a more common occurrence. They can result

Figure 5 Examples of initial rate versus enzyme concentration plots with downwards curvature. Normal (a), limiting coupling enzyme or assay component (b), addition of more limiting enzyme or assay component (c), and time-dependent inactivation or presence of inhibitor in enzyme preparation (d).

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