# 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 **V _{0}**, the initial velocity.

Next, if [E] is held constant while increasing [S], we can measure V_{0} by plotting V_{0} as a function of the [S]. What we see is that as [S] increases, V_{0} increases as well. But after some time, any increase in [S] does not affect V_{0}. 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 **V _{max}**