The Velocity of Reactions
Rate of Reaction: The amount of product per unit of time or the amount the substrate consumed per unit time.
How to measure the reaction velocity?
In some reactions the products change color, such as with ONPG->ONP (yellow), providing a type of colorimetric assay, which can then be measured via a spectrophotometer (a spectrophotometer measures the intensity of the color). Thus with a given amount of substrate and a given amount of enzyme, one can measure the intensity of the assay. This is but one example.
Graphing the concentration of product against time yields a hyperbolic curve. On this curve, the velocity at any point in time is just the slope at that point on the curve: V = d[P]/dt
The initial portion of this curve is almost linear. This allows us to determine a parameter that characterizes the enzyme called V0, the initial velocity.
Next, if [E] is held constant while increasing [S], we can measure V0 by plotting V0 as a function of the [S]. What we see is that as [S] increases, V0 increases as well. But after some time, any increase in [S] does not affect V0. It approaches an asymptote.
Why? When an E reacts with an S it forms an ES. Once the E is consumed, there can be no more substrate complex, ES. Thus E becomes the limiting factor, which means changes in [S] will longer have an effect.
From this we define Vmax