Mechanism of metal activation of human hematopoietic prostaglandin D synthase

Abstract

Here we report the crystal structures of human hematopoietic prostaglandin (PG) D synthase bound to glutathione (GSH) and Ca²⁺ or Mg²⁺. Using GSH as a cofactor, prostaglandin D synthase catalyzes the isomerization of PGH₂ to PGD₂, a mediator for allergy response. The enzyme is a homodimer, and Ca²⁺ or Mg²⁺ increases its activity to ∼150% of the basal level, with half maximum effective concentrations of 400 μM for Ca²⁺ and 50 μM for Mg²⁺. In the Mg²⁺-bound form, the ion is octahedrally coordinated by six water molecules at the dimer interface. The water molecules are surrounded by pairs of Asp93, Asp96 and Asp97 from each subunit. Ca²⁺ is coordinated by five water molecules and an Asp96 from one subunit. The Asp96 residue in the Ca²⁺-bound form makes hydrogen bonds with two guanidium nitrogen atoms of Arg14 in the GSH-binding pocket. Mg²⁺ alters the coordinating water structure and reduces one hydrogen bond between Asp96 and Arg14, thereby changing the interaction between Arg14 and GSH. This effect explains a four-fold reduction in the K_m of the enzyme for GSH. The structure provides insights into how Ca²⁺ or Mg²⁺ binding activates human hematopoietic PGD synthase.