What is the relationship between activation energy and reaction mechanism?

A reaction mechanism means "how a reaction takes place", it outlines the exact way in which electrons and ions move from one compound to another. Activation energy is the minimum amount of energy required to initiate the reaction. Consider the reaction between methane and air(oxygen). When you look at thermodynamic data for this reaction it will be spontaneous, but will not start until a little energy is supplied to it such as a spark or heat. This energy is the activation energy for this reaction. This means that these compounds are kinetically inert. When you use a catalyst it lowers this activation energy by finding an alternative route to the reaction, so the proportion of molecules with energy greater or equal to activation energy increase. This will not alter the reaction mechanism in anyway as reaction mechanism and activation energy are not related in any way. The activation energy will only determine the initial rate of the reaction and not the way it occurs. If you want to understand what a reaction mechanism is, google SN1 and SN2 reaction mechanisms they are the simplest types or reaction mechanism.