It can be a complicated life being an enzyme

It is becoming increasingly apparent that many well-known enzymes have alternative non-enzymic functions. Similarly, several proteins that were identified as having non-catalytic functions were subsequently found to have enzyme activities. Some examples are considered to illustrate the diversity of alternative functions. The semicarbazide-sensitive amine oxidase (EC 1.4.3.6) is considered in more depth as an example. It was originally believed to be a detoxifying enzyme, but the reaction products may have important signalling functions. Furthermore, this enzyme, from some sources, also behaves as a vascular-adhesion protein. Finally, the challenges posed by such multiplicity of functions for the interpretation of genetic deletion, in vivo inhibition and the development of functional protein databases are briefly considered.