Evidence for redox cooperativity between c-type hemes of MauG which is likely coupled to oxygen activation during tryptophan tryptophylquinone biosynthesis

MauG is a novel 42 kDa diheme protein which is required for the biosynthesis of tryptophan tryptophylquinone, the prosthetic group of methylamine dehydrogenase. The visible absorption and resonance Raman spectroscopic properties of each of the two c-type hemes and the overall redox properties of MauG are described. The absorption maxima for the Soret peaks of the oxidized and reduced hemes are 403 and 418 nm for the low-spin heme and 389 and 427 nm for the high-spin heme, respectively. The resonance Raman spectrum of oxidized MauG exhibits a set of marker bands at 1503 and 1588 cm(-1) which exhibit frequencies similar to those of the nu(3) and nu(2) bands of c-type heme proteins with bishistidine coordination. Another set of marker bands at 1478 and 1570 cm(-1) is characteristic of a high-spin heme. Two distinct oxidation-reduction midpoint potential (E-m) values of -159 and -244 mV are obtained from spectrochemical titration of MauG. However, the two nu(3) bands located at 1478 and 1503 cm(-1) shift together to 1467 and 1492 cm(-1), respectively, upon reduction, as do the Soret peaks of the low- and high-spin hemes in the absorption spectrum. Thus, the two hemes with distinct spectral properties are reduced and oxidized to approximately the same extent during redox titrations. This indicates that the high- and low-spin hemes have similar intrinsic E-m values but exhibit negative redox cooperativity. After the first one-electron reduction of MauG, the electron equilibrates between hemes. This makes the second one-electron reduction of MauG more difficult. Thus, the two E-m values do not describe redox properties of distinct hemes, but the first and second one-electron reductions of a diheme system with two equivalent hemes. The structural and mechanistic implications of these findings are discussed.