Carboxylesterases: General detoxifying enzymes

Abstract

Carboxylesterases (CE) are members of the esterase family of enzymes, and as their name suggests, they are responsible for the hydrolysis of carboxylesters into the corresponding alcohol and carboxylic acid. To date, no endogenous CE substrates have been identified and as such, these proteins are thought to act as a mechanism to detoxify ester-containing xenobiotics. As a consequence, they are expressed in tissues that might be exposed to such agents (lung and gut epithelia, liver, kidney, etc.). CEs demonstrate very broad substrate specificities and can hydrolyze compounds as diverse as cocaine, oseltamivir (Tamiflu), permethrin and irinotecan. In addition, these enzymes are irreversibly inhibited by organophosphates such as Sarin and Tabun. In this overview, we will compare and contrast the two human enzymes that have been characterized, and evaluate the biology of the interaction of these proteins with organophosphates (principally nerve agents).

Keywords: Carboxylesterase; Expression; Hydrolysis; Organophosphorus compounds; Structure.

Figure 1. Carboxylesterase substrates

The bond that is subject to hydrolysis is indicated by the arrow.

Figure 2. The mechanism of substrate hydrolysis by CEs

The essential catalytic amino acids are shown (Ser, His and Glu) and the mechanism depicts the hydrolysis of a generic ester (RCOOR₁). It should be noted that an intermediate serine ester is generated (red circle) in this process, and that with molecules that contain poor leaving groups (e.g., organophosphates), the efficiency of the second hydrolysis step is markedly reduced leading to enzyme inhibition.

Figure 3. Structure of hCE1

A – A cross section view of the protein. The entrance to the active site gorge is shown on the left hand side with the catalytic amino acids on the right. The distance from the active site serine to the entrance to the gorge in 20.5Å. The essential histidine and glutamic acid residues are also displayed. B. The same image as A with o-nitrophenyl acetate docked into the active site. The distance from the serine Oγ atom to the carbonyl carbon atom is 3.58 Å.

Figure 4. Expression of CEs in zebrafish

The left panel shows a bright-field image of a zebrafish larvae at 5 days postfertilization; the middle panel indicates CE-mediated hydrolysis of 4-methylumbelifferyl acetate to the blue fluorescent product 4-methylumbelifferone; and the right panel shows an overlay of the images. L – liver; G – gut; E – ear.

Figure 5. The complex formed between the reaction of hCE1 and cyclosarin

A – A close up view of the active site demonstrating the cyclohexyl phosphonyl adduct (yellow/orange) bound to the serine Oγ atom. B – The same view as in A except that the ribbon structure has been removed. C – Same view as in B except that the residues capable of directing a water molecule for attack of the phosphonyl adduct (V146H and L363E) are indicated.