Abstract
BackgroundCarboxylesterases (CES) perform diverse metabolic roles in mammalian organisms in the detoxification of a broad range of drugs and xenobiotics and may also serve in specific roles in lipid, cholesterol, pheromone and lung surfactant metabolism. Five CES families have been reported in mammals with human CES1 and CES2 the most extensively studied. Here we describe the genetics, expression and phylogeny of CES isozymes in the opossum and report on the sequences and locations of CES1, CES2 and CES6 'like' genes within two gene clusters on chromosome one. We also discuss the likely sequence of gene duplication events generating multiple CES genes during vertebrate evolution.ResultsWe report a cDNA sequence for an opossum CES and present evidence for CES1 and CES2 like genes expressed in opossum liver and intestine and for distinct gene locations of five opossum CES genes,CES1, CES2.1, CES2.2, CES2.3 and CES6, on chromosome 1. Phylogenetic and sequence alignment studies compared the predicted amino acid sequences for opossum CES with those for human, mouse, chicken, frog, salmon and Drosophila CES gene products. Phylogenetic analyses produced congruent phylogenetic trees depicting a rapid early diversification into at least five distinct CES gene family clusters: CES2, CES1, CES7, CES3, and CES6. Molecular divergence estimates based on a Bayesian relaxed clock approach revealed an origin for the five mammalian CES gene families between 328–378 MYA.ConclusionThe deduced amino acid sequence for an opossum cDNA was consistent with its identity as a mammalian CES2 gene product (designated CES2.1). Distinct gene locations for opossum CES1 (1: 446,222,550–446,274,850), three CES2 genes (1: 677,773,395–677,927,030) and a CES6 gene (1: 677,585,520–677,730,419) were observed on chromosome 1. Opossum CES1 and multiple CES2 genes were expressed in liver and intestine. Amino acid sequences for opossum CES1 and three CES2 gene products revealed conserved residues previously reported for human CES1 involved in catalysis, ligand binding, tertiary structure and organelle localization. Phylogenetic studies indicated the gene duplication events which generated ancestral mammalian CES genes predated the common ancestor for marsupial and eutherian mammals, and appear to coincide with the early diversification of tetrapods.
Highlights
Carboxylesterases (CES) perform diverse metabolic roles in mammalian organisms in the detoxification of a broad range of drugs and xenobiotics and may serve in specific roles in lipid, cholesterol, pheromone and lung surfactant metabolism
Six human CES genes have been reported on chromosome 16: CES1, encoding the major liver enzyme which is found in lung epithelia, macrophages and other tissues [16,17]; CES2, encoding the major intestinal enzyme and expressed in liver, kidney, heart and skeletal muscle [18,19]; CES3, expressed in liver, colon and small intestine [20,21]; and CES4 [22,23], CES6 [24] and CES7 [25,26], the products of which are less well characterized as proteins
We found that opossum CES genes are located in two regions on chromosome 1, with a CES1like gene ~231 million bp upstream of a CES2/CES6 gene cluster, which comprises three CES2like genes and a CES6like gene
Summary
Carboxylesterases (CES) perform diverse metabolic roles in mammalian organisms in the detoxification of a broad range of drugs and xenobiotics and may serve in specific roles in lipid, cholesterol, pheromone and lung surfactant metabolism. We describe the genetics, expression and phylogeny of CES isozymes in the opossum and report on the sequences and locations of CES1, CES2 and CES6 'like' genes within two gene clusters on chromosome one. The enzyme catalyses reactions in cholesterol and fatty acid metabolism, including fatty acyl CoA hydrolase [4], acyl CoA: cholesterol acyl transferase [5], cholesterol:ester hydrolase [6], acyl carnitine hydrolase [7], fatty acyl: ethyl ester synthase [8] and triacylglycerol hydrolase [9], and may serve specific roles in lung surfactant [10] and pheromone [11] metabolism. Tertiary and quaternary structures for several human CES1 complexes have been determined at high resolution (2.8Å) which are consistent with three functional domains: the catalytic domain containing the active site 'triad' and the carbohydrate binding site; the αβ domain supporting the majority of the hydrophobic internal structure and the subunit-subunit binding sites; and the regulatory domain which facilitates substrate binding, product release and the trimer-hexamer equilibrium [3,15,29,30]
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