Molecular Motion and Substrate-Active Site Collisions in Enzyme Catalysis
- Enzyme catalysis is not only dependent on the enzyme's structure and the interaction between the enzyme and substrate but also on the molecular motion of both the enzyme and substrate molecules.
- Enzymes and substrates are not static. They are in constant motion, especially in liquid environments like the cytoplasm.
- This motion is random and unpredictable, driven by the kinetic energy of molecules.
- For a reaction to occur, the substrate must collide with the enzyme’s active site.
The more frequent the collisions, the higher the probability that the substrate will bind to the enzyme, forming an enzyme-substrate complex and resulting in catalysis.
Tip- The more kinetic energy molecules have, the faster they move.
- This is why increasing temperature often increases the rate of enzyme-catalysed reactions as molecules collide more frequently.
What Makes a Collision Successful?
- For catalysis to occur, the substrate must collide with the enzyme’s active site.
- The frequency of these collisions is directly related to the rate of reaction.
- Not every collision between a substrate and an enzyme leads to a reaction. For a collision to be successful:
- The substrate must collide with the active site at the correct orientation.
- The substrate and active site must be chemically compatible to allow binding.
- Imagine trying to fit a key into a lock. If the key is not aligned correctly, it won’t fit, even if it’s the right key.
- Similarly, a substrate must approach the active site in the right orientation for binding to occur.
Factors Affecting Collision Rates
- Several factors influence how often substrates collide with active sites:
- Concentration: Higher concentrations of substrates or enzymes increase the likelihood of collisions.
- Temperature: Higher temperatures increase molecular motion, leading to more frequent collisions.
- Enzyme and Substrate Size: Smaller molecules move faster and collide more often.
- In many reactions, the substrate is smaller and more mobile than the enzyme.
- Enzymes are often much larger than their substrates. However, only a small region of the enzyme, the active site, is involved in binding and catalysis.
- When enzyme saturation occurs, all the enzyme active sites are occupied, and increasing the substrate concentration further will not increase the reaction rate.
Collision Theory
Definition
Collision theory
Collision theory states that in order for a reaction to occur, the reacting molecules must collide with sufficient energy and in the correct orientation.
