Increasing the substrate concentration
In an experimental situation, the concentration of substrate can be varied for a fixed concentration of enzyme molecules. If there is no substrate, the reaction cannot proceed, because enzyme-substrate complexes cannot form.
As the concentration of substrate increases, collisions between enzyme and substrate molecules occur more often. More enzyme-substrate complexes form, so more product is formed. The reaction rate increases.
If the concentration of substrate increases further, a point will be reached where the reaction rate reaches a maximum value.
At this point, all the enzyme molecules present are forming enzyme-substrate complexes as fast as possible. In effect, all of the active sites are occupied at all times, so any further increase a substrate concentration will have no effect on the reaction rate.

Increasing the enzyme concentration
Reversing the situation above, so that the concentration of the enzyme is varied for a fixed concentration of substrate molecules, follows a similar pattern:

as the enzyme concentration increases, more active sites become available
more enzyme-substrate complexes form, so the reaction rate increases
if the enzyme concentration increases further, a point will be reached where all substrate molecules are occupying enzyme active sites
the reaction rate is the maximum possible for the fixed substrate concentration
repeating the experiment with even more enzyme cannot give a higher reaction rate

Initial Reaction Rate

in an experimental situation where enzyme and substrate are mixed together, the rate of the enzyme-controlled reaction will be highest at the point when enzymes and substrate are mixed
as the reaction proceeds, product molecules are formed and increase in number
at the same time, substrate molecules are used up and decrease in number
this means the frequency of collisions between enzyme and substrate goes down as the enzyme controlled reaction proceeds
the highest reaction rate, known as the intial reaction rate, gives the maximum possible reaction rate for an enzyme under a particular experimental situation.

Limiting factors
The levelling off of a reaction occurs because either enzyme concentration or substrate concentration prevents any further increase in the reaction rate. They are limiting the reaction, and are descirbed as limiting factors. If the concentration of the limiting factor is increased, the reaction rate increases.

Enzymes and substrate concentration in cells
Enzyme concentrations in cells are usually maintained at a relatively low level. This is partly because enzymes, being catalysts, work over and over again, driving the same reaction. The control of metabolism, as we shall see later, is based on the control of enzyme activity. One way of regulating enzyme activity is by adjusting the concentrations of enzymes and/or substrates.

	Created by siobhan.quirk over 11 years ago

Undergraduate Biology (Biological Molecules) Note on Enzymes at Work - Concentration Effects, created by siobhan.quirk on 30/05/2013.

Enzymes at Work - Concentration Effects