High Torsional Energy Disulfides: Relationship Between Cross-Strand Disulfides and Right-Handed Staples

Abstract

Redox-active disulfides are capable of being oxidized and reduced under physiological conditions. The enzymatic role of redox-active disulfides in thiol-disulfide reductases is well-known, but redox-active disulfides are also present in non-enzymatic protein structures where they may act as switches of protein function. Here, we examine disulfides linking adjacent beta-strands (cross-strand disulfides), which have been reported to be redox-active. Our previous work has established that these cross-strand disulfides have high torsional energies, a quantity likely to be related to the ease with which the disulfide is reduced. We examine the relationship between conformations of disulfides and their location in protein secondary structures. By identifying the overlap between cross-strand disulfides and various conformations, we wish to address whether the high torsional energy of a cross-strand disulfide is sufficient to confer redox activity or whether other factors, such as the presence of the cross-strand disulfide in a strained beta-sheet, are required.

“Forbidden” Disulfides: Their Role as Redox Switches

“…Most aCSDs adopt the right-handed staple conformation while pCSDs tend to adopt a highly strained left-handed hook conformation. pCSDs generally have higher torsional energies than aCSDs with modal torsional energies of 20 and 15 kJ.mol -1 respectively [24].…”

“…Quantum chemical calculations on the model compound diethyl disulfide show that the staple conformation commonly adopted by the aCSD sits on a saddle point on the potential energy surface. Structurally-stabilizing disulfides on the other hand, which typically adopt low energy spiral and hook conformations, sit in deep potential wells in the surface [23,24]. Most aCSDs adopt the right-handed staple conformation while pCSDs tend to adopt a highly strained left-handed hook conformation.…”

“…Three of the pCSDs identified to date function in the cell cycle: a process which is known to be redox-regulated [27]. These include the eukaryotic cell cycle regulator CDC25 phosphatase [28], and two enzymes involved in peptidoglycan synthesis in E. coli: GlmU and MurD [24,29,30].…”

“…These data suggest CSDs are employed in a diverse range of biological redox control processes. Our own work has studied the presence of CSDs in protein structures with the ultimate aim of being able to differentiate a redox-active disulfide from a structurally-stabilizing one [12,23,24].…”

The PAPS-Independent Aryl Sulfotransferase and the Alternative Disulfide Bond Formation System in Pathogenic Bacteria

“…Interestingly, bioinformatic analyses revealed that strained, high energy disulfides, like the Cys 418 -Cys 424 in ASST, often play a regulatory role in protein function, triggering a conformational change when they break or form (17). Such disulfides typically have a high torsional strain energy (23) and are often found on adjacent b-strands (24,74). A well-studied example is the -RHStaple disulfide in the tissue factor involved in the regulation of hemostasis, and several other similar disulfides have been described (11,12,26,66).…”