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Crystal structure of a putative acyl‐CoA thioesterase from Xanthomonas campestris (XC229) adopts a tetrameric hotdog fold of ϵγ mode

Introduction. Thioesterases are widespread in eukaryotes, bacteria, and archaea, and are involved in the thioester hydrolysis in fatty acid metabolism to degradation of environmental pollutant 4-chlorobenzoate. These acyl-coenzyme A (CoA)-binding enzymes are found to adopt a core “hotdog” domain comprising mainly a long central -helix and an enclosing antiparallel -sheet, in various oligomeric states. Recently, several crystal structures of such enzymes in tetrameric state were solved, including 4-hydroxybenzoyl-CoA thioesterases from Pseudomonas sp. strain CBS-3 (PsHTE) and Arthrobacter sp. strain SU (ArHTE), thioesterase II from Escherichia coli (EcTEII), and phenylacetate degradation protein PaaI from Thermus thermophilus HB8 (TtPaaaI). Although all these enzymes form quaternary structures of dimer of dimers (as defined in Kunishima et al.), with active site residues located in a deep crevice formed between the subunit interface, the orientation of the two dimers in the tetramer can adopt opposite direction; they can either associate on the -sheet side (so-called mode), as those observed in EcTEII, ArHTE, TtPaaaI, or associate on the helical side (so-called mode), as that observed in PsHTE. Interestingly, these enzymes involved in the degradation of environmental pollutants contain a unique nucleophilic Asp residue in the active site, contrary to those using active site Ser/Cys residues commonly observed in the thioesterase family of fatty acid metabolism. XC229 (gi|21112185) from a soil-dwelling plant pathogen Xanthomonas campestris pv. Campestris strain 17 (Xcc) is classified as a conserved hypothetical protein from a BLAST database search (http://xcc.life.nthu.edu.tw/). It contains 134 amino acids, and is annotated as a putative thioesterase in the Pfam database. We have determined the crystal structure of XC229 to a resolution of 1.8 Å and found that it adopts a unique tetramer of mode with extensive subunit H-bonding/salt bridge interactions. A sequence alignment and model study with a 4-hydroxybenzoyl-CoA ligand suggest that XC229 very likely belongs to the novel type thioesterase, and is the second member belonging to such a thioesterase superfamily observed to date.