How catalysts and enzymes work

Step 2 - if the energy of collision is greater than the activation energy, the old bonds break. 

There have been many theories of the exact mechanism of the action of an enzyme because so little of it is needed to affect the rate of a reaction. Some enzymes can speed up a reaction as much as 20 000 times. Some negative catalysts (or inhibitors) slow down the rate of a reaction. 

One explanation for the mechanism is as follows in a reaction the two reactants need to collide with each other many times for a reaction to occur. In doing this only a few of the molecules are in the correct orientation to react with their counterpart. Owing to the randomness of motion, some of the orientations will be suitable and the reaction can take place. However, in many thousands of collisions the orientation is not correct and no reaction takes place. When an enzyme or catalyst is present, one reactant fits onto the enzyme molecule in the correct specific 

Note only A will fit into some holes and only B into others due to their shapes 

Usually catalysts and enzymes are very specific and will only speed up one particular reaction. For example, one enzyme in the blood will only catalyse one type of reaction and will not affect other reactions. Therefore a blood clotting enzyme reacting at the site of a cut will not be affected by a clot dissolving enzyme at a different site. The reactions are very specific. 

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