Structural, redox, and mechanistic parameters for cysteine-sulfenic acid function in catalysis and regulation

A primary objective of this review is to facilitate the application of the chemical and structural approaches that are currently being employed in the identification of Cys-SOH, as both transient intermediates and stable redox forms, in biochemical systems where these derivatives are suspected of playing key roles in redox catalysis or regulation. These range from high-resolution crystallographic analyses benefiting from recent technological advances in rapid data collection at cryogenic temperatures to 13C NMR investigations of [3-(13)C]Cys-labeled proteins and chemical modification protocols that can be integrated with both UV-visible and fluorescence spectroscopic as well as mass spectrometric (especially ESI, MALDI-TOF, and even FT ion-cyclotron-resonance) analyses. In summarizing the diversity of biological functions currently identified with Cys-SH reversible Cys-SOH redox cycles (Fig. 17), it should also be [figure: see text] emphasized that in at least one protein (nitrile hydratase) stable Cys-SOH and Cys-SO2H derivatives play important structural roles while also modulating the electronic properties of the iron center; in neither case is the Cys-SOH residue itself involved in reduction and oxidation. The somewhat incomplete structural descriptions of the oxidized Cys forms involved in redox regulation of some transcription factors (e.g., BPV-1 E2 protein and activator protein-1) indicate that there is ample room for the application of the types of investigations employed, for example, with NADH peroxidase and the AhpC peroxiredoxin, with a view toward defining the potential roles of Cys-SOH in these very important contexts of intracellular redox signaling. These advances will also build on the recent progress in defining sulfenic acid stabilization and properties in small molecule model systems, as evidenced in the work of Okazaki, Goto, and others. When viewed in the perspective of Allison's 1976 review on the subject of sulfenic acids in proteins, the reader will hopefully come to appreciate the conclusion that the concept of protein-sulfenic acids has now become a very well-defined and established principle of biochemistry, with current efforts in this and other laboratories being directed to bring about still more detailed understanding of Cys-SOH function in both redox and nonredox modes of enzyme catalysis and regulation of protein function.