Abstract
This chapter discusses the sulfhydryl amino acids cysteine and cystine. Cystine is composed of two cysteine groups that have formed an S–S bond either in the same polypeptide chain—an intrachain bond—or between two different chains—an interchain bond. A reactive cysteine group is a part of the biochemical reaction center of an enzyme. A way to use the extreme reactivity of the sulfhydryl group is to modify it so that the protein contains a molecular probe compound that may be an electron paramagnetic resonance (EPR) or nuclear magnetic resonance (NMR) reactive compound. Either of these can be used as a molecular probe. A marker attached to a sulfhydryl group is often easy to locate because of the group's reactivity and the relative scarcity of cysteine in most proteins. Cysteine may have a role as an antioxidant. Keratin is a protein with two major structures: α-helices and sulfhydryl cross-linkages. The numerous S–S links give it some of its strength and some of the functional properties so important to wool or hair.
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