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The starvation sensing protein RspA from E.coli and its homologs are lactonizing enzymes whose putative targets are homoserine lactone (HSL)-derivative. They are part of the mandelate racemase (MR)-like subfamily of the enolase superfamily. Enzymes of this subfamily share three conserved carboxylate ligands for the essential divalent metal ion (usually Mg2+), two aspartates and a glutamate, and catalytic residues, a partially conserved Lys-X-Lys motif and a conserved histidine-aspartate dyad.
Jiyao W et al.(2023). "The conserved domain database in 2023.",