Comparing Enzyme Affinities

This lesson covers:

  1. What enzyme affinity means
  2. The definition of Vmax
  3. How to derive Km from a curve 

Enzyme affinity

When comparing enzymes, we often talk about the affinity of each enzyme.


Here, affinity refers to the strength of the attraction between an enzyme and its substrate:

  • An enzyme with a high affinity has a strong attraction to its substrate.
  • An enzyme with a low affinity has a weak attraction to its substrate.

Vmax

Increasing the substrate concentration increases the rate of an enzyme-catalysed reaction, but only up to a certain point. Eventually a saturation point is reached, where all active sites are occupied by a substrate.


This maximum rate of reaction is known as Vmax.

Graph showing the relationship between substrate concentration and the rate of an enzyme-catalysed reaction, highlighting the Vmax point where the rate levels off.

Vmax can be used as an indicator for the efficiency of an enzyme. However, the type of curve shown above never flattens out in practice, only in theory.

Michaelis-Menten constant (Km)

Instead of using Vmax, we can use a value known as Km to measure the affinity of an enzyme for its substrate.


Km is the substrate concentration at which half the enzyme's active sites are bound to a substrate.

We can calculate Km from a graph by following these three steps:

Graph showing the relationship between substrate concentration and rate of reaction with Vmax indicated.

Calculate Vmax from the curve - This is the maximum rate.  

Interpreting Km values

The higher the affinity of an enzyme for its substrate, the lower the substrate concentration needed to reach 12\frac{1}{2}Vmax. 


Therefore, the higher the affinity of an enzyme for its substrate, the lower its Km value will be.