The cytosolic environment of most well-studied organisms is chemically reducing. As a result, stabilizing disulfide bonds are generally absent from cytosolic proteins, though they are abundant in extracellular proteins, including those that are secreted and those that reside in the bacterial periplasmic space. However, this simplistic textbook view of protein disulfide bonding is apparently violated by certain organisms. Based upon a crystal structure of a hyperthermophilic protein he determined as a student in 2000, Eric Toth predicted that certain thermophiles are able to use disulfide bonding to stabilize their proteins against extreme conditions. This claim was supported by Parag Mallick in 2002 using computational approaches, and has since been validated by multiple subsequent studies. The various studies included: a simple cysteine-counting exercise (which showed that hyperthermophilic archaea show a clear abundance of proteins having an even number of cysteine residues), genome-wide sequence-structure mapping calculations (which showed a striking tendency of cysteine residues to be near other cysteine residues in these organisms), and 2D oxidized-reduced SDS gels (which showed an abundance of proteins and protein-complexes held together by disulfide bonds.
The widespread occurrence of disulfide bonds in these organisms helps explain the puzzle of how their proteins are stabilized under such extreme conditions. It also paints a picture of protein disulfide bonding and cellular redox state that is more complex than previously anticipated. Our future efforts aim to exploit the expected presence of disulfide bonds in proteins from these organisms to boost protein structure prediction algorithms.