Structures of PmSOD1 and PmSOD2, two superoxide dismutases from the protozoan parasite Perkinsus marinus

Abstract

Perkinsus marinus, a facultative intracellular parasite of the eastern oyster Crassostrea virginica, is responsible for mass mortalities of oyster populations. P. marinus trophozoites survive and proliferate within oyster hemocytes, invading most tissues and fluids, thus causing a systemic infection that eventually kills the host. The phagocytosis of P. marinus trophozoites lacks a respiratory burst, suggesting that the parasite has mechanisms that actively abrogate the host's oxidative defense responses. One mechanism and the first line of defense against oxidative damage is the dismutation of superoxide radical to molecular oxygen and hydrogen peroxide by superoxide dismutases (SODs). P. marinus possesses two iron-cofactored SODs, PmSOD1 and PmSOD2. Here, the crystallization and X-ray structures of both PmSOD1 and PmSOD2 are presented.

Figure 1

(a) Ribbon diagram of the PmSOD1 dimer showing Fe ions (purple spheres) at the active sites with their ligands. At the center of the figure, Glu165 interacts across the dimer interface with the metal ligand His166. (b) Close-up of the active site of PmSOD1 shows the Fe ion (represented as a large yellow sphere) coordinated by three histidine residues, an aspartic acid residue and a water molecule. The water molecule, whose O-atom location is represented as a small red sphere, is coordinated by Gln76.

Figure 2

An overlay of the PmSOD1 (green) and PmSOD2 (blue) structures reveals key mutations in a highly conserved triad (Phe-Try-Trp) of Fe-SOD. PmSOD1 has the unusual sequence Phe-Phe-Phe. The Tyr84 to Phe83 change is structurally compensated by the Ile23 to Met23 change owing to the larger volume of Met relative to Ile. Overall, PmSOD1 displays a more hydrophobic second shell of amino-acid residues than PmSOD2 around the metal site, i.e. Phe83, Phe84 and Tyr183.

Figure 3

(a) The PmSOD1 dimer interface reveals several polar interactions, notably the Glu26–Arg173 salt bridges, that are conserved in most Fe/Mn-SODs. (b) The PmSOD2 dimer interface is more hydrophobic and lacks the Glu26–Arg173 salt bridge; the equivalent amino-acid residues are Gln180 and Ala26. The absence of such salt bridges is very unusual in both Mn-SODs and Fe-SODs.