Structure of Enzymes
The structure of enzymes
- Enzyme catalysis involves molecular motion and the collision of substrates with the active site
- For an enzyme-catalysed reaction to take place, substrates collide at random with the enzyme's active site
- This must happen at the correct orientation and speed in order for a reaction to occur
- Unsuccessful collisions can occur when the molecules are not correctly aligned with each other at the moment of collision
- The molecules 'bounce' off each other and no reaction takes place
- Some enzymes have two substrates that must each collide with a separate active site at the same time
- Substrates bind to enzymes, forming a temporary enzyme-substrate complex
- The active site of an enzyme has a specific shape and chemical properties to bind with a specific substrate
- The reaction occurs within the enzyme-substrate complex which leads to changes in the chemical structure of the substrate
- Products are formed, which detach and move away from the active site, which can be re-used
Enzyme action diagram
The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)
- The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
- The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex 3D shape of the protein that makes up the enzyme
- The active site is made of only a few amino acids but the interaction of these amino acids within the 3D shape of the enzyme ensures that catalysis can occur
- This is achieved by:
- Binding to the substrate molecule
- Holding it in position for a chemical reaction to occur
- Lowering the energy needed for the reaction to occur
Enzyme specificity diagram
An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not
Formation of enzyme-substrate complex diagram
The temporary formation of an enzyme-substrate complex
Examiner Tip
Don't forget that both enzymes and their substrates are highly specific to each other – this is known as enzyme-substrate specificity.