Higher eukaryotes have many distinct esterases. Among the different types are those which act on carboxylic esters (EC 3.1.1.-). Carboxyl-esterases have been classified into three categories (A, B and C) on the basis of differential patterns of inhibition by organophosphates. The sequence of a number of type-B carboxylesterases indicates [1,2,3] that the majority are evolutionary related. This family currently consists of the following proteins:

• Acetylcholinesterase (EC 3.1.1.7) (AChE) from vertebrates and from Drosophila.
• Mammalian cholinesterase II (butyryl cholinesterase) (EC 3.1.1.8). Acetylcholinesterase and cholinesterase II are closely related enzymes that hydrolyze choline esters [4].
• Mammalian liver microsomal carboxylesterases (EC 3.1.1.1).
• Drosophila esterase 6, produced in the anterior ejaculatory duct of the male insect reproductive system where it plays an important role in its reproductive biology.
• Drosophila esterase P.
• Culex pipiens (mosquito) esterases B1 and B2.
• Myzus persicae (peach-potato aphid) esterases E4 and FE4.
• Mammalian bile-salt-activated lipase (BAL) [5], a multifunctional lipase which catalyzes fat and vitamin absorption. It is activated by bile salts in infant intestine where it helps to digest milk fats.
• Insect juvenile hormone esterase (JH esterase) (EC 3.1.1.59).
• Lipases (EC 3.1.1.3) from the fungi Geotrichum candidum and Candida rugosa.
• Caenorhabditis gut esterase (gene ges-1).
• Duck acyl-[acyl-carrier protein] hydrolase, medium chain (EC 3.1.2.14), an enzyme that may be associated with peroxisome proliferation and may play a role in the production of 3-hydroxy fatty acid diester pheromones.
• Membrane enclosed crystal proteins from slime mold. These proteins are, most probably esterases; the vesicles where they are found have therefore been termed esterosomes.

So far two bacterial proteins have been found to belong to this family:

• Phenmedipham hydrolase (phenylcarbamate hydrolase), an Arthrobacter oxidans plasmid-encoded enzyme (gene pcd) that degrades the phenylcarbamate herbicides phenmedipham and desmedipham by hydrolyzing their central carbamate linkages.
• Para-nitrobenzyl esterase from Bacillus subtilis (gene pnbA).

The following proteins, while having lost their catalytic activity, contain a domain evolutionary related to that of carboxylesterases type-B:

• Thyroglobulin (TG), a glycoprotein specific to the thyroid gland, which is the precursor of the iodinated thyroid hormones thyroxine (T4) and triiodo thyronine (T3).
• Drosophila protein neurotactin (gene nrt) which may mediate or modulate cell adhesion between embryonic cells during development.
• Drosophila protein glutactin (gene glt), whose function is not known.

As is the case for lipases and serine proteases, the catalytic apparatus of esterases involves three residues (catalytic triad): a serine, a glutamate or aspartate and a histidine. The sequence around the active site serine is well conserved and can be used as a signature pattern. As a second signature pattern, we selected a conserved region located in the N-terminal section and which contains a cysteine involved in a disulfide bond.