Explain how enzymes speed up the rate of biochemical reactions by lowering the energy needed to start reaction ?

Enzymes are catalysts that speed up the rate of a reaction by providing an alternate pathway that has a much lower activation energy, as compared to the original (non-catalyzed) pathway. Activation energy is the energy barrier that must be overcome for the reaction to take place. Note that enzymes do not lower the energy barrier, rather they provide an alternate route to the reaction. Enzymes do so by two mechanisms: It brings the reactants close to each other and in orientations that facilitate the formation of bonds at lower activation energy. This is facilitated by provision of active sites by the enzymes. Another mechanism is easing out the transformation of reactants to the transition state, by stretching the bonds in the reactant molecules. The reactants, then find it easier and less energy intensive to convert to transition state and then to final products. 

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