The Enzyme-Substrate Complex (ES complex) is formed when enzyme is catalysing the substrates. Formation of the complex offers alternative transition state that requires less activation energy than the previous transition state. Hence, the enzyme can accelerate the rate of the reaction. There are four mechanisms of how the formationof the complex can alter the rate of the reaction. Firstly, the formation of the complex allows the molecules to come close together, lowering the effort required to collide in correct orientation at the active site. Secondly, the formation of the complex gives physical strain on the substrate so that the bonds within the substrate can be broken more easily. Thirdly, when the complex is formed, there is an internal environment formed in which can be altered differently from the surrounding environment. This allows the complex to be present in different pH, which makes breakdown of the bonds in substrates easier. Lastly, the formation of temporary covanlent bonds when the complex is formed alters the rate of reaction, by lowering the activation energy of the reaction.