Enzymes, as specific proteins, act as catalysts, lowering activation energy and accelerating reactions. While not altering the reaction equilibrium, they enhance the rate significantly. Without enzymes, reactions still occur but at a much slower pace. Their vital role in our bodies involves specific metabolic reactions, showcasing substrate selectivity akin to a key and door match. Initially proposed as the "lock and key" hypothesis by Emil Fischer in 1894, enzymes aren't static but embrace the "induced fit" model. This implies conformational changes when binding to substrates, essential for efficient catalysis. Enzymes typically boost reaction rates by 105 to 107 ten to the five to ten to the seven times.[1,2].

A graph illustrating the reaction from a substrate to a product. On the horizontal x-axis is the reaction progress and on the vertical y-axis is the free energy. Two reactions are illustrated with blue lines, moving from left to right, transitioning from the substrate to product. During the transition, the blue lines peak at what is referred to as the transition state. After the transition state, the blue line starts to drop and eventually becomes fully transitioned from substrate to product. The two blue lines represent two different reactions; one that is catalyzed by enzymes and one that is uncatalyzed. The reaction that has been catalyzed reaches the transition state earlier than the uncatalyzed reaction.

Figure 1: A reaction from a substrate to product is a transition from one energy state to another. A transition state exists between the substrate and product. This state has a higher energy level than both the substrate and product. A catalyst will lower this energy level so that the transition energy is reached more easily, and this results in a faster reaction.

How does an enzyme lower the activation energy?

Enzymes facilitate specific reactions through various mechanisms. In cases where two substrates yield multiple products, the enzyme binds them together, ensuring proximity and proper orientation. Without the enzyme, the substrates must collide at the correct angle and sufficient energy to surmount a higher activation energy barrier. Enzyme structures overcome these challenges. Crucially, enzymes, like catalysts in general, remain unchanged after catalyzing a reaction and can be used repeatedly. Once an enzyme catalyzes a reaction, it stands prepared to convert another set of substrates[1].


  1. Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2008). Principles of Biochemistry (5th ed.). New York, NY: W.H. Freeman and Company. ISBN 978-0-7167-7108-1.

  2. Atkins, Peter W.; de Paula, Julio; Friedman, Ronald (2009). Quanta, Matter, and Change: A molecular approach to physical chemistry. Oxford University Press. ISBN 978-0-19-920606-3.