The sulfonium ion linkage in myeloperoxidase - Direct spectroscopic detection by isotopic labeling and effect of mutation

The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutat ion of Met(243) into Thr, Gin, and Val, which are the corresponding residue s of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respec tively, and into Cys was performed. The Soret band in the optical absorbanc e spectrum in the oxidized mutants is now found at approximately 411 nm, Bo th the pyridine hemochrome spectra and resonance Raman spectra of the mutan ts are affected by the mutation. In the Met(243) mutants the affinity for c hloride has decreased 100-fold, All mutants have lost their chlorination ac tivity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specif ically detect the (CD3)-C-13-labeled methionyl sulfonium ion linkage. We co nclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsibl e for the lower symmetry of the heme group and distortion from the planar c onformation normally seen in heme-containing proteins.