Enzymes are proteins that have a specific function. They speed up the rate of chemical reactions in a cell or outside a cell. Enzymes act as catalysts; they do not get consumed in the chemical reactions that they accelerate.
Why do cells need a catalyst? Cells use a lot of energy! There are thousands of reactions that take place in cells and these require energy. Since energy is always limiting in a living cell, cells have adopted enzymes as a way to conserve energy. Insufficient energy is a barrier to initiating the reaction. Only when there is a sufficient amount of energy, can the reactant overcome the energy barrier and proceed to form a product. This is called the activation energy.
How do catalysts speed up chemical reactions? In biological systems, the energy required to make a reaction go is stored primarily in the bonds that make up adenosine triphosphate (ATP). Specifically, the energy is stored in the bonds between phosphate groups and the nucleotide, adenosine. Energy (~7 kcal/mole) is released when one of the phosphate bonds in ATP to form adenosine diphosphate (ADP) is broken. This is equivalent to the amount of energy in a peanut. A catalyst lowers the barrier for the activation energy. For example, ethanol is metabolized into acetaldehyde by the enzyme, alcohol dehydrogenase. In the absence of ADH, the rate of the reaction would be less than 0.000006 (or 6 x 10-6) µmoles/L per minute. While in the presence of ADH, the reaction rate is 2700 µmoles/L per minute. That’s an acceleration of more than 4500 million times!