Everything about Disulfide Bridge totally explained
In
chemistry, a
disulfide bond is a single
covalent bond derived from the coupling of
thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore C-S-S-C. The terminology is almost exclusively used in biochemistry, bioinorganic and bioorganic chemistry. Formally the connection is called a persulfide, in analogy to its
congener, a
peroxide (R-O-O-R), but this terminology is rare.
Three sulfur atoms singly bonded in a sequence are sometimes called a trisulfide bond, although there are in fact two S-S bonds. Disulfide bonds are usually formed from the
oxidation of
sulfhydryl (-SH) 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 (with some exceptions noted below).
Disulfide bonds in proteins are formed between the
thiol groups of
cysteine residues. The other sulfur-containing amino acid,
methionine, can't form disulfide bonds. A disulfide bond is typically denoted by hyphenating the abbreviations for cysteine, for example, the "Cys26-Cys84 disulfide bond", or the "26-84 disulfide bond", or most simply as "C26-C84" where the disulfide bond is understood and doesn't need to be mentioned. 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 (shown in Figure 2 in its unionized form). The structure of a disulfide bond can be described by its
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 (but not all of them) are denoted by
des followed by the lacking native disulfide bond(s) 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 order to analyze the structure of proteins, it's often necessary to break disulfide bonds. This reduction of disulfide bonds can be accomplished by treatment with
2-Mercaptoethanol.
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 (
H2O2) in particular could severely damage
DNA and kill the
bacterium at low concentrations if not 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 (rough endoplasmic reticulum) but not in the
cytosol. This is due to the oxidative environment of the
ER and the reducing environment of the cytosol (see
glutathione). 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've 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 and feathers
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, and the disulfide links also cause hair (and
feathers which have similar keratins) to be extremely resistant to protein digestive enzymes. Different parts of the hair and feather 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 in hairstyling.
In
feathers, the high disulfide content dictates the high sulfur content of bird eggs, which need to contain enough sulfur to feather the chick.
In both hair and feathers, the high sulfur content due to the high number of disulfides causes the disagreeable smell of the material when it's burned.
In organic chemistry
The
Zincke disulfide cleavage is a classic
organic reaction in which a disulfide is converted to a
sulfur halide R-S-X with X = Br, Cl by reaction with
bromine or
chlorine. It was named after
Theodor Zincke.
General references
- Sela M and Lifson S. (1959) "On the Reformation of Disulfide Bridges in Proteins", Biochimica et Biophysica Acta, 36, 471-478.
- Stark GR. (1977) "Cleavage at cysteine after cyanylation", Methods in Enzymology, 11, 238-255.
- Thornton JM. (1981) "Disulphide Bridges in Globular Proteins", Journal of Molecular Biology, 151, 261-287.
- Thannhauser TW, Konishi Y and Scheraga HA. (1984) "Sensitive Quantitative Analysis of Disulfide Bonds in Polypeptides and Proteins", Analytical Biochemistry, 138, 181-188.
- Wu J and Watson JT. (1998) "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. (2000) "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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