NMR SOLUTION STRUCTURE OF THE OXIDIZED FORM OF MERP, A MERCURIC ION-BINDING PROTEIN INVOLVED IN BACTERIAL MERCURIC ION RESISTANCE

Mercuric ions are toxic to living organisms because of their strong af finity for cysteine residues in proteins. Some bacteria have developed a resistance mechanism whereby Hg2+ is transported into the cytoplasm and reduced to Hg-0. One of the proteins involved in the transport of mercuric ion is the periplasmic binding protein MerP, which can exist both as oxidized (disulfide) and as reduced (dithiol) forms, Only the reduced form with Cys-17 and Cys-14 residues as free thiols is a pote nt receptor for mercuric ion. In this work the solution structure of t he oxidized form of MerP has been determined by multidimensional NMR s pectroscopy and compared to the NMR structures of the previously publi shed structures of the reduced and mercury-bound forms of MerP, The me rcury-bound and oxidized forms have similar tertiary structures. where as in the reduced form then is a large rearrangement of the mercuric i on binding loop and the nearby loop comprising residues 38-41, The str uctural arrangement of the latter loop seems to be important for the s tabilization of the surface location of the cysteine-containing loop. In the reduced form at low pH the cysteine-containing loop adopts a co nformation similar to what is observed in the oxidized and mercury-bou nd forms, The oxidized form also differs with respect to the other two forms in the relative positions of some of the alpha-helices and beta -strands. Structural differences between the oxidized and reduced form s may help explain why the reduced form is stable in the periplasm, wh ich is considered to be an oxidizing environment.