The enzyme-substrate complex forms when a substrate binds to the active site of an enzyme, according to the University of California at Davis. The complex exists before the enzyme converts the substrate into a product.
An enzyme is a protein that increases the rate of a chemical reaction, notes UC Davis. Enzymes form a bond with molecules, referred to as a substrate. The substrate binds a special pocket on the enzyme, called the active site. The bond forms the enzyme-substrate complex.
In the lock and key model, an enzyme and a substrate only form a complex if the sites are an exact match, informs Study.com. Under this theory, only a single type of substrate can bind at the active site of a given enzyme. The binding induces the chemical reaction and forms a product. The product’s confirmation differs from that of the original substrate, causing the molecules to separate and freeing the enzyme to bind a new substrate.
In the induced-fit model, the fit between an enzyme and a weakly-matched substrate is improved as the molecules interact, explains Study.com. The substrate’s proximity to the enzyme induces a conformational change in the enzyme, causing its shape to shift until it properly fits the substrate. This theory explains why some enzyme-substrate complexes form but do not undergo a chemical reaction.