Resonance Raman spectroscopic characterization of the molybdopterin active site of DMSO reductase

Resonance Raman spectra are compared for Rhodobacter sphaeroides dimethyl sulfoxide reductase, an enzyme containing a molybdopterin cofactor, and two model compounds, I and II, which have pterin and quinoxaline, respectively, attached to a Cp2Mo[IV]-dithiolene chelate [Cp = cyclopentadienyl]. The effect of 34S incorporation was also determined. Several bands in the 200-500 cm-1 region show remarkably similar patterns of frequencies and isotope shifts between protein and models: a band at 351 cm-1 shifts 6-8 cm-1, and bands at lower and higher frequencies show smaller shifts upon 34S substitution. A normal coordinate analysis on II indicates the 351 cm-1 mode to be the symmetric Mo-S[dithiolene] stretch and the remaining low-frequency modes to contain contributions from deformations of the quinoxaline ring as well as from Mo-S stretching. The similarity in the low-frequency spectra between the model compounds and the enzyme strongly supports a dithiolene chelate as the mode of Mo-pterin interaction in the cofactor. Resonance enhancement of both high- and low-frequency quinoxaline or pterin modes is observed for both model compounds, implicating the heterocyclic rings as part of the electronic system involved in the Mo-dithiolene charge transfer transitions. RR spectra of 6-methylpterin and biopterin are reported and used to identify the pterin and quinoxaline high-frequency bands in the model compound spectra. The dithiolene C = C stretch is tentatively assigned to bands at 1506 cm-1 in I and 1515 cm-1 in II.(ABSTRACT TRUNCATED AT 250 WORDS)