Proteins are large biological molecules formed as a linear chain, by the successive addition of hundreds of individual building blocks called amino acids, which fold in a complex three-dimensional pattern. It is the specific sequence, defined by the encoding gene, of the 20 different amino acids along the chain that determines the final folded structure adopted by a protein. This in turn enables the unique and specific function of each of the thousands of distinct proteins inside a living cell.

One of the many roles of proteins is in the catalysis of chemical reactions. Enzymes are large biological molecules, generally proteins, which catalyze biochemical reactions, converting the starting `substrate` to a specific `product` under the physiological conditions of the living cell. These remarkable biocatalysts are capable of selectively increasing the rate of a specific chemical reaction by as much as 17 orders of magnitude. By analogy, if the rate of approach of the Asian and North American continental plates (assuming 5 cm per year) were increased by the same rate, it would take much less than one second for Vancouver and Tokyo to come together. Without the ability of enzymes to catalyze the vast array of chemical reactions routinely required by living cells, life as we know it would not be possible.