Enzyme catalysis, activation energy nucleophilic reaction

Reactions can be accelerated by the formation of intermediates, provided that such intermediates are rapidly formed and broken down. This then provides a new reaction pathway of lower activation energy. Many enzyme-catalyzed reactions proceed via covalent catalysis, involving side chains such as Cys, Ser, Eys. or His acting as nucleophiles in the formation of intermediates such as acyl-enzymes or Schiff bases. Changes in environment... 

As discussed, AdoMet is a high energy compound, and therefore, AdoMet-dependent methylation reactions are known to be irreversible. However, enzymatic catalysis is often required to enhance the rate of the reaction. Enzymes employ a variety of ways to enhance the nucleophilicity on the attacking atom in a substrate. Often, the reaction results in a proton exchange for the methyl group. The proton can be removed before, in concert with, or after the methyl transfer this step usually requires the presence of a general base in the active site. 

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