In chemistry, control coefficients are used to describe how much influence (i.e., control) a given reaction step has on the steady-state flux or species concentration level. In practice, this can be accomplished by changing the expression level of a given enzyme and measuring the resulting changes in flux and metabolite levels. Control coefficients form a central component of metabolic control analysis.
There are two primary control coefficients:
* Flux Control Coefficients
* Concentration Control Coefficients. 
The simplest way to look at control coefficients is as the scaled derivatives of the steady-state change in an observable with respect to a change in enzyme activity. For example, the flux control coefficients can be written as:
while the concentration control coefficients can be written as:
Control coefficients can have any value that includes negative and positive values. A negative value indicates that the observable in question decreases as a result of the change in enzyme activity. 
In theory, other observables, such as growth rate, or even combinations of observables, can be defined using a control coefficient. But flux and concentration control coefficients are by far the most commonly used.
The approximation in terms of percentages makes control coefficients easier to measure and more intuitively understandable. 
Control coefficients are useful because they tell us how much influence each enzyme or protein has in a biochemical reaction network.
It is important to note that control coefficients are not fixed values but will change depending on the state of the pathway or organism. If an organism shifts to a new nutritional source, then the control coefficients in the pathway will change.