Stability increased disulfide bridges

Covalent bridging of biopolymers is one of the widely occurring prindples in nature for increasing the stability of the tertiary structure, for example the disulfide bridges in keratin and ribonuclease. 

Similarly, human fibroblast interferon p contains three cysteines at positions 17, 31, and 141. Of these, cysteines at positions 31 and 141 appear to be involved in a disulfide bridge, and are required for antiviral activity of the molecule. Replacement of the free cysteine residue at position 17 with serine resulted in a 10-fold increase in the specific activity of interferon p expressed and purified from E. coU, which also showed improved stability during storage. These improvements are attributed to elimination of intermolecular aggregation and incorrect disulfide bonds caused by the free cysteine 17 (Mark et al, 1984). 

A sulfur moiety introduces a hydroperoxide deactivating effect into the stabilizer molecule. Moreover, the condensation of low molecular weight compounds with halides of sulfur represents an easy synthetic approach to the increase of the molecular weight and consequently to the improvement of the physical persistency of stabilizers. Efficient AO were prepared by this way from alkylphenols, aromatic amines or phenothiazine [21,174]. Most of them contain two active units connected with a sulfide/disulfide bridge and should be listed rather among high molecular weight stabilizers. Antioxidant 136 (n > m), prepared from pyrocatechol and... 

Rational methods are based on experimental evidence that the effect of single amino acid substitutions on protein stability can be well approximated as additive, distributed, and large independent interactions. Moreover, by comparison of homologous enzymes from thermophilic and nonthermophilic microorganisms, it is known that introduction of disulfide bridges as well as increased numbers of proline residues increase protein stability. 

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