On the origins of esterases

Comparisons among the primary sequences of five cloned eukaryotic esterasesreveal two distinct lineages, neither bearing any significant overallsequence similarity to the functionally related serine protease multigenefamily. We have not eliminated the possibility that the esterases may haveresidual conformational similarities to the serine proteases. However, ourprofile analysis and analyses of the predicted conformations of theesterases reveal little similarity to the serine proteases. Four of theesterase proteins share 27%-53% overall sequence similarity and evidence ofa catalytic mechanism involving the same Arg- Asp-Ser or His-Asp-Ser chargerelay. We propose that these four esterases, three of them cholinesterases,form part of a multigene family essentially separate from the serineproteases.