An enzyme is a biological macromolecule that acts as a catalyst, accelerating chemical reactions without being permanently changed. Enzymes are overwhelmingly classified as proteins, the biomolecule class responsible for nearly all catalytic activity in living systems. These catalysts are necessary for life, allowing thousands of complex metabolic reactions inside cells to occur fast enough to sustain the organism. Without enzymes, these biochemical processes would happen too slowly to be useful at normal body temperatures.
Enzymes are Proteins
The vast majority of enzymes are complex globular proteins, characterized by their compact, spherical three-dimensional shape. The fundamental structure of any protein begins as a linear chain of smaller units called amino acids, linked together by peptide bonds. The specific order of these amino acids is determined by the genetic code, and this sequence is known as the primary structure.
This long, flexible chain then spontaneously folds into a highly specific three-dimensional architecture, referred to as the tertiary structure. This unique folding pattern creates the functional pocket on the enzyme’s surface known as the active site. The chemical properties of the amino acid side chains lining this active site determine the enzyme’s ability to bind to its target molecule and facilitate a reaction.
How Enzymes Speed Up Chemical Reactions
Enzymes function by significantly lowering the activation energy, the minimum amount of energy required to start a chemical reaction. They accomplish this by temporarily binding to a reactant molecule, known as the substrate, at their active site. This binding forms a transient enzyme-substrate complex that provides an alternative, lower-energy pathway for the reaction to proceed.
The interaction between the enzyme and substrate is best explained by the induced-fit model. This model proposes that the enzyme’s active site is not perfectly rigid, but undergoes a slight change in its three-dimensional shape when the substrate binds. This conformational shift helps position the substrate optimally, straining their existing chemical bonds and bringing them closer to the transition state needed for the reaction. Once the reaction is complete and the product is released, the enzyme reverts to its original shape and is ready to catalyze another reaction cycle.
RNA Molecules That Act as Enzymes
While nearly all biological catalysts are proteins, a small but important exception exists in the form of RNA molecules that possess catalytic capabilities. These molecules are called ribozymes, and their discovery demonstrated that RNA can function as both genetic material and a biological catalyst. Ribozymes fold into complex structures that create an active site, much like proteins, allowing them to catalyze specific reactions.
A notable example of a ribozyme is the large ribosomal RNA found within the ribosome, the cellular machinery responsible for building proteins. In this role, the ribosomal RNA catalyzes the formation of peptide bonds that link amino acids into a protein chain. Ribozymes also play roles in processes such as the splicing of RNA molecules within the nucleus.