Enzyme Kinetics

[garytse86]

Enzyme kinetics often follow both zero and first order kinetics.

 

Zero order kinetics are followed when the substrate S has saturated the enzyme.

First order kinetics are followed when the enzyme is not saturated.

 

Why is this?

[Skye]

I've only started kinetics this week, so if any of this is wrong, you know why Anyway...

 

This is only with respect to the substrate concentration. With respect to the enzyme concentration the reaction is first order in both cases.

 

The order refers to the power you raise the concentrations of the reactacts to in an equation that determines the rate of a reaction. If a reactants concentration has no effect on the rate of the reaction, you can assign it an order of zero. Remembering that a number raised to zero is equal to one, then it follows that that reactants concentration will have no effect on the rate.

 

If an increase in a reactants concentration leads to a corresponding increase in reaction rate (i.e. doubling it's concentration makes the reaction proceed twice as fast), then it is assigned an order of one. If it has a nonlinear relationship then you assign it other orders, but that's not important here.

 

Now when the enzyme is saturated with the substrate it doesn't matter if you add any further substrate. There is already so much that the enzymes are constantly catalysing the reaction, and can't go any faster. So in this case the concentration of the substrate is no longer important to the reaction rate and we can say it has an order of zero. But if we doubled the number of enzymes, then they could do twice as much catalysing, so the enzyme is first order.

 

When the enzyme is unsaturated there are times when it isn't catalysing a reaction because it doesn't have a substrate available to it. If you were to increase the concentration of the substrate then this would mean the enzyme would be more likely to come in contact with a substrate and to catalyse the reaction. So in this case the the substrate is a factor in the reaction rate, and has an order of one. The enzyme concentration is still important and also has an order of one.