Human Myeloperoxidase:  Structure of a Cyanide Complex and Its Interaction with Bromide and Thiocyanate Substrates at 1.9 Å Resolution

Abstract

The 1.9 A X-ray crystal structure of human myeloperoxidase complexed with cyanide (R = 0.175, R(free) = 0.215) indicates that cyanide binds to the heme iron with a bent Fe-C-N angle of approximately 157 degrees, and binding is accompanied by movement of the iron atom by 0.2 A into the porphyrin plane. The bent orientation of the cyanide allows the formation of three hydrogen bonds between its nitrogen atom and the distal histidine as well as two water molecules in the distal cavity. The 1.85 A X-ray crystal structure of an inhibitory complex with thiocyanate (R = 0.178, R(free) = 0.210) indicates replacement of chloride at a proximal helix halide binding site in addition to binding in the distal cavity in an orientation parallel with the heme. The thiocyanate replaces two water molecules in the distal cavity and is hydrogen bonded to Gln 91. The 1.9 A structures of the complexes formed by bromide (R = 0.215, R(free) = 0.270) and thiocyanate (R = 0.198, R(free) = 0.224) with the cyanide complex of myeloperoxidase show how the presence of bound cyanide alters the binding site for bromide in the distal heme cavity, while having little effect on thiocyanate binding. These results support a model for a single common binding site for halides and thiocyanate as substrates or as inhibitors near the delta-meso carbon of the porphyrin ring in myeloperoxidase.

Thiocyanate Modulates the Catalytic Activity of Mammalian Peroxidases

“…11). It was thought that halides binding at the distal cavity site inhibit the enzyme by interfering with the deprotonation of H 2 O 2 by the adjacent distal His-95 (16,38,39,73), a mechanism consistent with previous reports indicating that inhibition by halides is competitive with respect to H 2 O 2 (56 -59). Thus, the saturation in a bell-shaped curve occurs, at a high concentration of SCN Ϫ (Ͼ300 M), when the two sites are occupied.…”

“…Indeed, the plot of the second-order combination rate constant (k on ) of NO binding to MPO-Fe(III) as a function of SCN Ϫ concentration displayed a bell-shaped curve with a negative slope over the range where SCN Ϫ concentrations were Ͻ60 M and a positive slope over the range where SCN Ϫ concentrations were Ͼ60 M, with saturation Ͼ300 M. This behavior may have a broader link effect on MPO activity, because the initial decrease in the second-order rate constant of NO combination (k on ) occurs within the range where SCN Ϫ binds to a distal cavity located in MPO Compound I and allows the direct contact with the oxyferryl oxygen. Under these circumstances, Compound I appears to act favorably in triggering electron transfer to the heme with subsequent conversion to hypothiocyanate as a final reaction product (16,38,39,73). This decrease was reversed and reaches saturation at higher SCN Ϫ concentrations, where SCN Ϫ binds to MPO at both the distal cavity and the proximal helix sites and forms an inactive complex (SCN-E-Fe(III) (inh) ).…”

“…This decrease was reversed and reaches saturation at higher SCN Ϫ concentrations, where SCN Ϫ binds to MPO at both the distal cavity and the proximal helix sites and forms an inactive complex (SCN-E-Fe(III) (inh) ). Indeed, Blair-Johnson et al (73) have shown that the inhibitory complex with thiocyanate indicates replacement of chloride at the proximal helix halide binding site in addition to binding in the distal cavity in an orientation parallel with the heme (Fig. 11).…”

Ciona intestinalis peroxinectin is a novel component of the peroxidase–cyclooxygenase gene superfamily upregulated by LPS

“…The homology modeling process was performed on the basis of the known crystal structure of human myeloperoxidase isoform C (1mhlD) (Blair-Johnson et al, 2001). Fig.…”

Disruption of the Aspartate to Heme Ester Linkage in Human Myeloperoxidase

“…The same tendency was also observed with nitrite and indicates a decreased affinity of anions to the Asp 94 variants at pH 5. Apparently, the loss of the ester bond hampers to some extent the hydrogen bonding between the anion and the distal site histidine, which in its protonated form has been suggested to stabilize halide or nitrite complexes of MPO (15,44).…”