Catalysts reduce the amount of energy that a chemical reaction needs in order to happen, so that the reaction can occur more easily. However, the catalyst itself remains chemically unchanged and is not consumed at all in the reaction. A catalyst lowers the activation energy needed for a reaction to take place, and it will change the rate of both directions of the reaction.
Catalysts function by one of two main methods. The first is adsorption, where particles stick to the surface of the catalyst and move around, increasing their likelihood of collision. A more complicated method is the creation of intermediate compounds which are unstable and then break down into other substances, leaving the catalyst in its original state.
If the rate of a chemical reaction can be increased, it can also be decreased. Inhibitors are essentially the opposite of catalysts, and they act to slow down the reaction rate or even stop the reaction altogether. Inhibitors are used for various reasons, including giving scientists more control over reactions. Both inhibitors and catalysts naturally play significant roles in the chemical reactions that occur in human bodies.
Enzymes are large, soluble protein molecules that serve to speed up chemical reactions in cells. Cellular respiration, DNA replication, digestion, protein synthesis, and photosynthesis are common processes, all essential for life, that are catalyzed with enzymes.
Like other types of catalysts, enzymes take part in a reaction to provide an alternative pathway with a smaller activation energy, but they remain unchanged themselves. However, enzymes only alter the reaction rate; they do not actually change the equilibrium point of a reaction. Also, unlike most chemical catalysts, enzymes are very selective, which means that they only catalyze certain reactions.
This particular aspect of enzyme behavior is referred to as the lock and key model. This alludes to the fact that not all keys can open all locks; most keys can only open specific locks. Similarly, the shape of any one enzyme only matches the shape of the molecule it reacts with, called a substrate. The active site is the place on the enzyme that directly contacts the substrate, or the place where the two “puzzle pieces” fit together facilitating the actual reaction.
Enzymes have a characteristic optimum temperature at which they function best and require a sufficient substrate concentration. The reason for these restrictions is that variables like temperature and pH affect the shape of an enzyme’s active site. In fact, if the temperature is increased too much, usually past 60 degrees Celsius, an enzyme can become denatured. This means that the active site has undergone a permanent change in shape, so it can no longer serve its purpose as a catalyst.
As suggested by the lock and key model, enzymes are typically highly specific to the reaction they catalyze. In a cellular context, why would it be detrimental if enzymes universally catalyzed any reaction?