Enzymes

Enzymes are globular proteins with a specific three-dimensional structure, maintained by:

• Hydrogen bonds.

• Ionic bonds.

• Disulfide bridges (covalent bonds).

Enzymes are biological catalysts—they speed up biochemical reactions without being consumed. To do this they:

• Lower activation energy, allowing reactions to proceed faster.

• Remain unchanged after the reaction and can be reused.

The active site is a region where the substrate binds, forming an enzyme-substrate complex. Only a complementary substrate can fit into the active site, known as substrate specificity.

Reactions depend on random collisions between enzymes and substrates. Successful binding requires:

• Correct orientation of the substrate.

• Sufficient kinetic energy (higher temperature increases movement).

Once the enzyme-substrate complex forms, the reaction occurs, and the products are released. The enzyme is free to catalyse more reactions.

Theories of enzyme action

1. Lock and Key Theory

• The active site is a perfect fit for the substrate, like a key fitting into a lock.

• The enzyme-substrate complex forms instantly.

• Once the reaction occurs, the products are released, and the enzyme remains unchanged.

The lock and key model of enzyme action.

2. Induced Fit Model

• The active site is flexible and adjusts its shape to fit the substrate.

• As the substrate binds, the enzyme undergoes a conformational change to become complementary.

• This interaction helps position the substrate correctly, weakening bonds and making the reaction easier.

The induced fit model is now widely accepted because it explains enzyme flexibility and specificity better.

The induced fit model of enzyme action.

Factors affecting enzyme action

1. Temperature

• Low temperature → Enzymes and substrates have less kinetic energy so move slowly resulting in fewer successful collisions.

• Increasing temperature → Bothe enzymes and substrates have more kinetic energy so so there are more successful collisions and a higher reaction rate.

• The optimum temperature →is the temperature at which there is the highest rate of reacvtion. This is a balance between giving the enzymes a lot of kinetic energy but avoiding denaturation. 

• Above optimum temperature: the enzyme denatures as hydrogen bonds break. The active site loses its shape, making it non-functional since the shape of the substrate is no longer complementary to the active site. This denaturation is irreversible.

2. pH

• Enzymes have an optimal pH, where their structure is most stable and the shape of the active site is most complementary to the shape of the substrate.

• Too high or too low pH disrupts hydrogen and ionic bonds, leading to denaturation.

• Example:

  • Pepsin (stomach enzyme) works best at pH 2 (acidic).

  • Amylase (saliva enzyme) works best at pH 7 (neutral).

3. Substrate Concentration

• More substrate → More collisions so a faster rate of reaction.

• When all enzyme active sites are occupied, the reaction reaches its maximum rate.

• Beyond this point, adding more substrate has no effect (enzyme concentration is the limiting factor).

4. Enzyme Concentration

• More enzymes → More active sites available so a higher reaction rate (if enough substrate is present).

• When all substrates are being processed, adding more enzymes has no further effect.

Enzyme inhibitors

1. Competitive Inhibitors

• Have a similar shape to the substrate so compete for the active site.

• Prevents substrate binding, slowing down the reaction.

• Effect can be overcome by increasing substrate concentration, making it more likely the substrate binds instead of the inhibitor.

2. Non-Competitive Inhibitors

• Bind to an allosteric site (not the active site) on the enzyme.

• Change the shape of the active site, preventing substrate binding since they no longer have complementary shapes.

• Effect cannot be overcome by adding more substrate, as the enzyme is no longer functional.

Definitions