Page - (000048) - in Biomedical Chemistry: Current Trends and Developments

1.1.4.8.1 Disulfide Bridges ‒ Oxidized Thiols Disulfide bridges are essential in a protein to allow stable structural scaffolds. They are S‒S bonds between cysteine residues (Scheme 1.1.34). Note that the sulfur loses a bond to a hydrogen and gains one to another sulfur atom, so the formation of this linkage is a redox reaction with thiols being oxidized. The oxidant (to be reduced) is a glutathione dimer (GSSG), which consists in two glutathione molecules connected by a disulfide bridge (Scheme 1.1.35). Scheme 1.1.34: Disulfide bridge formation. Scheme 1.1.35: Mechanism of disulphide linkage formation by a glutathione dimer, releasing two reduced glutathione molecules. 1.1.5 The Organic Mechanisms of Biological Transformations Previously, we highlighted that common organic reaction mechanisms can be used to understand the biosynthetic pathways. Herein some illustrative examples are presented. 1.1.5.1Cis/trans-Isomers Interconversion in the Vision Pathway It is common knowledge that vitamin A, retinol (Scheme 1.1.36), plays an important role in our vision. Although the sequence of reactions and detailed transformations are not in the scope of this chapter, it is remarkable that a simple oxidation and change in configuration is ultimately responsible for a complex process such as vision. Cis/trans