Encyclopedia
In
chemistry, a
disulfide bond is a single
covalent bond between two
sulfur atoms that are themselves not bonded to sulfur. Disulfide bonds are usually formed from the
oxidation of sulfhydryl groups, as depicted formally in Figure 1.
Disulfide bonds in proteins
Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium. Since most cellular compartments are a reducing environment, disulfide bonds are generally unstable in the
cytosol .
Disulfide bonds in proteins are formed only between the thiol groups of
cysteine residues; the other sulfur-containing residue,
methionine, has not been observed in intra-protein disulfide bonds. A disulfide bond is typically denoted by hyphenating its cysteines, e.g., the "Cys26-Cys84 disulfide bond" or, more simply, the "26-84 disulfide bond". The prototype of a protein disulfide bond is the two-amino-acid peptide,
cystine, which is composed of two
cysteine amino acids joined by a disulfide bond . The structure of a disulfide bond can be described by its
dihedral angle between the atoms, which is usually close to ±90°.
The disulfide bond stabilizes the folded form of a protein in several ways.
First, it holds two portions of the protein together, biasing the protein towards the folded topology. Expressed differently, the disulfide bond
destabilizes the unfolded form of the protein by lowering its entropy.
Second, the disulfide bond may form the nucleus of a hydrophobic core of the folded protein, i.e., local hydrophobic residues may condense around the disulfide bond and onto each other through hydrophobic interactions.
Third, by linking
two segments of the protein chain, the disulfide bond
increases the effective local concentration of protein residues and
lowers the effective local concentration of water molecules. Since water molecules attack amide-amide
hydrogen bonds and break up
secondary structure, a disulfide bond stabilizes secondary structure in its vicinity. For example, researchers have identified several pairs of peptides that are unstructured in isolation, but adopt stable secondary and tertiary structure upon forming a disulfide bond between them.
Disulfide bonds in proteins are formed by
thiol-disulfide exchange reactions.
A
disulfide species is a particular pairing of cysteines in a disulfide-bonded protein and is usually depicted by listing the disulfide bonds in parentheses, e.g., the " disulfide species". A
disulfide ensemble is a grouping of all disulfide species with the same number of disulfide bonds, and is usually denoted as the 1S ensemble, the 2S ensemble, etc. for disulfide species having one, two, etc. disulfide bonds. Thus, the disulfide species belongs to the 1S ensemble, whereas the species belongs to the 2S ensemble. The single species with no disulfide bonds is usually denoted as R for "fully reduced". Under typical conditions,
disulfide reshuffling is much faster than the formation of new disulfide bonds or their reduction; hence, the disulfide species within an ensemble equilibrate more quickly than between ensembles.
The native form of a protein is usually a single disulfide species, although some proteins may cycle between a few disulfide states as part of their function, e.g., thioredoxin. In proteins with more than two cysteines, non-native disulfide species may be formed, which are almost always unfolded. As the number of cysteines increases, the number of nonnative species increases factorially. The number of ways of forming
p disulfide bonds from
n cysteine residues is given by the formula
For example, an eight-cysteine protein such as
ribonuclease A has 105 different four-disulfide species, only one of which is the native disulfide species. Isomerases have been identified that catalyze the interconversion of disulfide species, accelerating the formation of the native disulfide species.
Disulfide species that have only native disulfide bonds are denoted by
des followed by the lacking native disulfide bond in square brackets. For example, the des[40-95] disulfide species has all the native disulfide bonds except that between cysteines 40 and 95. Disulfide species that lack one native disulfide bond are frequently folded, particularly if the missing disulfide bond is exposed to solvent in the folded, native protein.
In prokaryotes
Disulfide bonds play an important protective role for
bacteria as a reversible switch that turns a protein on or off when bacterial cells are exposed to
oxidation reactions.
Hydrogen peroxide in particular could severely damage
DNA and kill the
bacterium at low concentrations if it weren't for the protective action of the SS-bond.
In rubber
Disulfide bonds also play a significant role in the
vulcanization of
rubber.
In eukaryotes
In
eukaryotic cells, disulfide bonds are generally formed in the lumen of the
RER but not in the
cytosol. This is due to the oxidative environment of the
ER and the reducing environment of the cytosol . Thus disulfide bonds are mostly found in secretory proteins, lysosomal proteins, and the exoplasmic domains of membrane proteins.
There are notable exceptions to this rule. A number of cytosolic proteins have cysteine residues in proximity to each other that function as oxidation sensors; when the reductive potential of the cell fails, they oxidize and trigger cellular response mechanisms. Vaccinia virus also produces cytosolic proteins and peptides that have many disulfide bonds; although the reason for this is unknown presumably they have protective effects against intracellular proteolysis machinery.
Disulfide bonds are also formed within and between protamines in the
sperm chromatin of many mammalian species.
In hair
Hair is a biological polymer, with over 90% of its dry weight made of proteins called
keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held together by disulfide bonds, from the amino acid cysteine. These links are very robust: for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material. Breaking and making disulfide bonds governs the phenomenon of wavy or frizzy hair. It is breaking and remaking of the disulfide bonds which is the basis for the
permanent wave.
References
- Sela M and Lifson S. "On the Reformation of Disulfide Bridges in Proteins", Biochimica et Biophysica Acta, 36, 471-478.
- Stark GR. "Cleavage at cysteine after cyanylation", Methods in Enzymology, 11, 238-255.
- Thornton JM. "Disulphide Bridges in Globular Proteins", Journal of Molecular Biology, 151, 261-287.
- Thannhauser TW, Konishi Y and Scheraga HA. "Sensitive Quantitative Analysis of Disulfide Bonds in Polypeptides and Proteins", Analytical Biochemistry, 138, 181-188.
- Wu J and Watson JT. "Optimization of the Cleavage Reaction for Cyanylated Cysteinyl Proteins for Eficient and Simplified Mas Mapping", Analytical Biochemistry, 258, 268-276.
- Futami J, Tada H, Seno M, Ishikami S and Yamada H. "Stabilization of Human RNase 1 by Introduction of a Disulfide Bond between Residues 4 and 118", J. Biochem., 128, 245-250.
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