A single protein may have

two or more EC numbers if it catalyses two or more reactions. This is the case, for example, for two proteins in Escherichia coli, each of which catalyses the reactions both of aspartate kinase (EC 2.7.2.4) and of homoserine dehydrogenase (EC 1.1.1.3). It may also happen that two or more proteins with no detectable evidence of homology 8 catalyse the same reaction. For example, various different proteins catalyse the superoxide dismutase reaction, and share a single EC number, EC 1.15.1.1. This latter case is relatively rare, but it is almost universal that proteins catalysing the same reaction in different organisms, or sets of isoenzymes in one organism, are homologous, with easily recognisable similarities R428 in sequence. The Nomenclature Committee of IUBMB discussed ways of incorporating structural information in the enzyme list in a systematic way, i.e. going beyond what are little more than anecdotal notes in the Comments. Nothing was ever agreed or implemented, however, but fortunately the web-based list includes links to databases such as EXPASY, thus allowing structural information to be combined with reaction information. The original classification scheme remains very satisfactory

find protocol for the enzymes of central metabolism, but there have always been some problem groups, most notably the peptidases, and the wholesale reorganization of group 3.4 in 1972 reflected the difficulties. The primary problem is that although the enzymes of central metabolism have sufficient specificity for reaction to be defined with some precision, many peptidases have broad and overlapping specificity. In addition, the fact that the peptidases constituted a much higher proportion in 1961 than now of the enzymes that had been studied meant that numerous enzymes that differ mainly in being derived from different organisms have been

classified as different enzymes with different EC numbers. For example, papain (now EC 3.4.22.2), ficain (EC 3.4.22.3), asclepain (EC 3.4.22.7), actinidain Morin Hydrate (EC 3.4.22.14) and stem bromelain (EC 3.4.22.32) all have very similar catalytic properties. Classifying the overlapping specificity of peptidases (many more of which are known today than there were at the time of the original Report (IUB, 1961)) is now more efficiently covered by a dedicated database (Rawlings et al. 2012).9 At the other extreme are the enzymes of the restriction-modification systems. For example, EC 1.1.1.113 contains the enzymes collectively known as site-specific DNA-methyltransferase (cytosine-N4-specific). This is actually a large group of enzymes, each clearly distinct, that recognize specific sequences of DNA.