Siroheme-containing sulfite reductase: A density functional investigation of the mechanism

Siroheme-containing sulfite reductases (SiR) catalyze the six-electron reduction of sulfite to sulfide via a mechanism involving sulfite binding at the heme iron. The exact sequence in which the required electrons and protons are delivered to the heme-bound sulfite has received little attention to date. Here, a detailed account is given of these steps, based on density functional theory, thus providing data for the first attempt to draw a detailed picture of sulfite reduction in SiR by theoretical methods. Parallels are shown with reduction of other small molecules at heme centers: dioxygen (including generation of sulfide high-valent iron centers akin to hemoproteins Compounds I and II), nitrite (including linkage isomerism akin to the nitro/nitrito known for nitrite reducing proteins), or nitric oxide. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011