- Any of numerous proteins or conjugated proteins produced by living organisms and functioning as biochemical catalysts.
- A protein produced by a living organism, capable of catalyzing a chemical reaction. Almost all processes in living organisms require some form of enzyme to cause the reactions to occur at a rate sufficient to support life. There are a very wide variety of enzymes, each specifically catalyzing a different chemical reaction, the sum of which cause the bulk of the physiological changes observed as life processes. Enzymes, like most proteins, are synthesized by the protein-synthetic mechanism of the living cell, at special sites on ribosomes, using the genetic information in messenger RNA transcribed from the genetic instructions stored as nuleotide sequences in the DNA (or in some viruses, the RNA) of the genome. Some examples of enzymes are: pepsin, diastase, rennet, DNA polymerase, invertase, glucose oxidase, protease, and ribonuclease. There are many other types of enzyme.
- A globular <xref>protein</xref> that <xref>catalyses</xref> a <xref>biological</xref> <xref>chemical reaction</xref>.
- any of several complex proteins that are produced by cells and act as catalysts in specific biochemical reactions