Thioesterase; part of the pkg gene cluster that mediates the biosynthesis of dihydrocitreoisocoumarin and 6,8-dihydroxy-3-(2-oxopropyl)-isocoumarin (PubMed:22510154). The non-reducing polyketide synthase pkgA performs the condensation of one acetyl-CoA starter unit with 6 and 5 malonyl-CoA units, respectively (PubMed:22510154). As pkgA lacks a releasing domain, the thioesterase pkgB is necessary to break the thioester bond and release dihydrocitreoisocoumarin and 6,8-dihydroxy-3-(2-oxopropyl)-isocoumarin from pkgA (PubMed:22510154). 3,5,7,9,11,13-hexaoxotetradecanoyl-[ACP] = dehydrocitreoisocoumarin + H2O + holo-[ACP] 3,5,7,9,11-pentaoxododecanoyl-[ACP] = 6,8-dihydroxy-3-(2-oxopropyl)-isocoumarin + H2O + holo-[ACP] Binds 2 Zn(2+) ions per subunit. Belongs to the metallo-beta-lactamase superfamily.