Oxygen cysteine residues

Fig. 6.3.9 An illustration showing the mechanism of the reconstitution and luminescence of symplectin. The binding of dehydrocoelenterazine with the SH group of a cysteine residue of aposymplectin (left) results in the formation of active symplectin (center). Symplectin is oxygenated at the C2 position, resulting in the formation of coelenteramide bound to aposymplectin (right), accompanied by the emission of light. From Isobe et al., 2002, with permission from Elsevier.

Protection against cell damage due to oxidative stress is provided, amongst others, by glutathione (GSH), a cellular tripeptide with a thiol function in a cysteine residue. GSH is deprotonated to GS, which is a scavenger for electrophilic compounds and is reduced to GS SG in defense of reactive oxygen species [28]. 

When the urea and thiol are removed by dialysis (see p. 78), secondary and tertiary structures develop again spontaneously. The cysteine residues thus return to a suf ciently close spatial vicinity that disulfide bonds can once again form under the oxidative effect of atmospheric oxygen. The active center also reestablishes itself In comparison with the denatured protein, the native form is astonishingly compact, at 4.5 2.5 nm. In this state, the apolar side chains (yellow) predominate in the interior of the protein, while the polar residues are mainly found on the surface. This distribution is due to the hydrophobic effect (see p. 28), and it makes a vital contribution to the stability of the native conformation (B). 

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