Editing Protein (section)

===Enzymes===

{{Main|Enzyme}}

The best-known role of proteins in the cell is as [[enzyme]]s, which [[catalysis|catalyse]] chemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in [[metabolism]], as well as manipulating DNA in processes such as [[DNA replication]], [[DNA repair]], and [[transcription (genetics)|transcription]]. Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification. About 4,000 reactions are known to be catalysed by enzymes.<ref name=EXPASY2000/> The rate acceleration conferred by enzymatic catalysis is often enormous—as much as 10<sup>17</sup>-fold increase in rate over the uncatalysed reaction in the case of [[orotate decarboxylase]] (78 million years without the enzyme, 18 milliseconds with the enzyme).<ref name=Radzicka1995/>

The molecules bound and acted upon by enzymes are called [[Substrate (biochemistry)|substrates]]. Although enzymes can consist of hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with the substrate, and an even smaller fraction—three to four residues on average—that are directly involved in catalysis.<ref name="urlEBI"/> The region of the enzyme that binds the substrate and contains the catalytic residues is known as the [[active site]].<ref name=Biochemistry2010/>{{rp|389}}

[[Dirigent protein]]s are members of a class of proteins that dictate the [[stereochemistry]] of a compound synthesized by other enzymes.<ref name="Pickel 2013"/>