A substance that decreases the velocity of an enzyme-catalyzed reaction is called an inhibitor. There are two main categories of inhibitors:

  1. Irreversible Inhibitors: These bind covalently to the enzyme, and therefore permanently, causing the enzyme to lose its activity and it can never be used again.
  2. Reversible InhibitorsThese bind non-covalently to the enzyme. Therefore, the enzyme activity can be regained when the inhibitor is removed.

There are four types of reversible inhibition. We shall look at them in summary.

  • Competitive Inhibition

This is when the inhibitor is similar in shape to the substrate and it binds only to the active site on the enzyme. Upon binding, however, no product can be formed. The Vmax of the enzyme remains the same, and the Km is increased. Increasing the substrate concentration overcomes the effects of this inhibitor.




  • Non-Competitive Inhibition

This is where the inhibitor binds at either the free enzyme or at the enzyme-substrate complex, thereby changing the shape of the enzyme and therefore changing the shape of the active site and preventing the reaction from occurring. The Vmx for the reaction decreases but the Km remains unchanged.



  • Uncompetitive Inhibition

The inhibitor in this case, binds only to the enzyme-substrate complex and prevents product formation. Both the Vmax and the Km is reduced by the same amount.



  • Mixed Inhibition

This inhibition is similar to non-competitive inhibition, in that the inhibitor bind to either to the enzyme-substrate complex or the free enzyme, except that little no product will be formed. The Km may be increased or decreased and the Vmax will always be reduced.

Hurry Up Enzyme!


Enzymes already speed up chemical reactions. But there are factors that can also help speed up the rate of a reaction when an enzyme is used.

  1. Substrate Concentration.


As seen on this graph, as substrate concentration increases, the rate of an enzyme catalyzed reaction increases. The flat area at the top of the graph represents the point of high substrate concentration at which saturation occurs, i.e. the point where all the available binding sites of the enzyme are occupied with substrate.

2. Enzyme Concentration

The same occurs with increasing enzyme concentration; as this increases, the rate of the reaction increases.

3. Temperature

The graph clearly shows that increasing the temperature, the rate of enzyme activity increases, However, only up until a certain point of high temperature, this is called the optimum temperature of the enzyme (i.e. the highest temperature at which the enzyme can function at). Beyond this temperature, the enzyme is denatured and, as seen on the graph, the react of enzyme activity immediately decreases.

4. pH

The catalytic process may require that the enzyme and substrate have specific chemical groups in either an ionized or unionized state in order to interact. Also, at extreme pH’s, an enzyme can be denatured (at pH’s beyond or below the optimum pH of the enzyme).

Enzymes are Very Fussy About Who They Are Friends With


Enzymes are specific, i.e. they bind with only certain, selected substrates through molecular recognition. This reversible binding occurs at an enzyme’s active site via hydrophobic interactions, electrostatic interactions, hydrogen bonding and Van Der Waal’s interactions.


There are two hypotheses that describe enzyme specificity:

  1. The Lock And Key Hypothesis: This describes how a substrate fits EXACTLY into the active site of the enzyme. However, this hypothesis is too rigid, because it says the active site had a specific shape and it never changed.


2. The Induced-Fit Hypothesis: This describes how the active site of the enzyme is similar in shape to the substrate that binds with it, however when the substrate actually binds, it induces a change in the enzyme’s conformation (i.e. the active site is moulded into a precise conformation). 


Enzymes- What Are They?



Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy of the reaction. To be an enzyme, a substance has to:

  • be substrate specific
  • enhance reaction rate
  • come out of the reaction unchanged

There are six different classes of enzymes that catalyze different types of reactions, as seen in the following table: