Sulfite-oxidizing enzymes

Abstract

Sulfite oxidizing enzymes are essential mononuclear molybdenum (Mo) proteins involved in sulfur metabolism of animals, plants and bacteria. There are three such enzymes presently known: (1) sulfite oxidase (SO) in animals, (2) SO in plants, and (3) sulfite dehydrogenase (SDH) in bacteria. X-ray crystal structures of enzymes from all three sources (chicken SO, Arabidopsis thaliana SO, and Starkeya novella SDH) show nearly identical square pyramidal coordination around the Mo atom, even though the overall structures of the proteins and the presence of additional cofactors vary. This structural information provides a molecular basis for studying the role of specific amino acids in catalysis. Animal SO catalyzes the final step in the degradation of sulfur-containing amino acids and is critical in detoxifying excess sulfite. Human SO deficiency is a fatal genetic disorder that leads to early death, and impaired SO activity is implicated in sulfite neurotoxicity. Animal SO and bacterial SDH contain both Mo and heme domains, whereas plant SO only has the Mo domain. Intraprotein electron transfer (IET) between the Mo and Fe centers in animal SO and bacterial SDH is a key step in the catalysis, which can be studied by laser flash photolysis in the presence of deazariboflavin. IET studies on animal SO and bacterial SDH clearly demonstrate the similarities and differences between these two types of sulfite oxidizing enzymes. Conformational change is involved in the IET of animal SO, in which electrostatic interactions may play a major role in guiding the docking of the heme domain to the Mo domain prior to electron transfer. In contrast, IET measurements for SDH demonstrate that IET occurs directly through the protein medium, which is distinctly different from that in animal SO. Point mutations in human SO can result in significantly impaired IET or no IET, thus rationalizing their fatal effects. The recent developments in our understanding of sulfite oxidizing enzyme mechanisms that are driven by a combination of molecular biology, rapid kinetics, pulsed electron paramagnetic resonance (EPR), and computational techniques are the subject of this review.

Adaptation of Candida albicans to Reactive Sulfur Species

“…In C. albicans, there are no known enzymes that convert toxic sulfite to nontoxic sulfate, such as sulfite oxidase, an enzyme that occurs in humans, bacteria, and plants (Kappler and Enemark 2015). Therefore, C. albicans controls the levels of excess toxic sulfite produced in a different way.…”

Scorpionate Complexes as Models for Molybdenum Enzymes

“…Many workers enjoyed and profited from exchanges, sabbaticals, collaborations and conference interactions with Enemark and these greatly assisted the ongoing development of the field. Enemark and co‐workers went on to make major contributions to our understanding of SO and related enzymes , …”

Consensus structures of the Mo(v) sites of sulfite-oxidizing enzymes derived from variable frequency pulsed EPR spectroscopy, isotopic labelling and DFT calculations

“…The corresponding structures deduced from electron spin echo (ESE) experiments on these forms, in conjunction with isotopic labeling, density functional theory (DFT) calculations, 9,10 and Please do not adjust margins Please do not adjust margins studies of model compounds, are shown on the right hand side of Figure 3. 11 Elucidation of the specific structures associated with the various CW EPR spectra has been especially challenging because the Mo(V) coordination sphere involves only O and S atoms, whose primary isotopes ( 16 O and 32 S) are magnetically silent (I = 0). Determining whether the solvent-exposed, exchangeable equatorial ligand is an O, OH − , H 2 O, SO 3 2− , or SO 4 2− group, has required isotopic labeling with 17 O (I = 5/2) or 33 S (I =3/2) in order to obtain experimental hyperfine interaction (hfi) and nuclear quadrupole interaction (nqi) constants for these coordinated nuclei using variable-frequency high resolution pulsed EPR spectroscopy.…”