|MoBio||Catalytic Mechanisms of Enzymes||Chapter 2|
From the energetic point of view, the reason why an enzyme can accelerate a reaction is because it can lower the energy barrier (activation energy) separating the substrate and the reaction product. For example, the covalent bond energy between two atoms ranges from 50 to 200 kcal/mol, which is far greater than the thermal energy (0.6 kcal/mol) at room temperature. Therefore, the covalent bond is unlikely to break in the absence of external interactions. Enzymes can provide a proper environment to lower the energy barrier.
The exact mechanism of lowering the energy barrier depends on individual systems. RNase A is a very interesting example. This enzyme can cleave an RNA molecule through hydrolysis reaction, but has no effect on DNA. In Chapter 3, we shall see that DNA and RNA differ by only one oxygen atom, which happens to play a critical role in the catalytic mechanism of RNase A.