Small-molecule metabolism: an enzyme mosaic

Escherichia coli has been a popular organism for studying metabolic pathway s. In an attempt to find out more about how these pathways are constructed, the enzymes were analysed by defining their protein domains. Structural as signments and sequence comparisons were used to show that 213 domain famili es constitute similar to 90% of the enzymes in the small-molecule metabolic pathways. Catalytic or cofactor-binding properties between family members are often conserved, while recognition of the main substrate with change in catalytic mechanism is only observed in a few cases of consecutive enzymes in a pathway. Recruitment of domains across pathways is very common, but t here is little regularity in the pattern of domains in metabolic pathways. This is analogous to a mosaic in which a stone of a certain colour is selec ted to fill a position in the picture.