IB Biology Topic C1.1.6: Role of Molecular Motion and Substrate-active Site Collisions in Enzyme Catalysis Notes

Catalysis Starts With a Random Collision Between Substrate and Active Site

1. Enzymes and substrates are not static.
2. In solution, such as the cytoplasm, they are in constant random motion.
   1. This random movement of particles in a fluid is called Brownian motion.
3. For a reaction to begin, the substrate must collide with the active site.

A Collision Only Works if It Has the Correct Orientation

1. Not every collision leads to binding.
2. The substrate must hit the active site in the correct orientation.
3. The substrate and active site must also be chemically compatible so that binding can occur.

More Motion Means More Collisions Per Second

1. Anything that makes molecules move faster or brings them closer together raises how often collisions happen.
2. Higher temperature
   1. Molecules gain kinetic energy and move faster, so they collide with the active site more often.
3. Higher substrate concentration
   1. With more substrate molecules present, a substrate is more likely to meet an active site.

Collision Theory Applied to Enzymes

1. Collision theory explains why only some substrate encounters lead to a reaction.
2. A productive collision needs the substrate to reach the active site in the correct orientation and be able to bind there.

Sometimes the Enzyme Moves to the Substrate Instead

1. Motion does not always mean the substrate is the one that moves.
2. When a substrate is very large, the enzyme moves to meet it instead.