Thiols, also named as mercaptan, are a class of organic compounds containing a sulfhydryl group (SH). They are similar to the alcohols and phenols but with the oxygen atom replaced by a sulfur atom. Actually, thiols are responsible for high volatility and unpleasant odor resembling that of garlic or rotten eggs. Thiols also contribute to the telltale odor skunks sprays in self-defense and the scent of freshly chopped onions. A concentration of 10 ppb can be recognized by human nose. Because of their disagreeable odours, it is easy to say whether thiols exist in petroleum and natural gas. They are also the same compounds responsible for the smell associated with bad breath and flatulence in humans. However, not the odors properties of all thiols are offensive. The thiols in many botanicals lead to their aromatic properties. For example, thioterpineol, a thiol in grapefruit, contributes to the distinctive yet not unpleasant fragrance of grapefruit. Thiols also are involved in heady aroma familiar to those brew beer or wine at home.
Different types of proteins like antibodies, receptors, hormones, and enzymes often include one or more Cys residues, existing as free thiols or as intra- and inter-molecular disulfide bond(s). The free Cys residue of some enzymes is part of the catalytic activity site, while a disulfide bond in peptides and proteins enhance the conformational rigidity on a protein. The formation of non-native intramolecular disulfide bond(s) during the folding process may cause protein misfolding, which in turn may lead to precipitation and aggregation. Native disulfide bonds in proteins are formed during the oxidative folding process in the ER lumen in eukaryotes and in the periplasm in prokaryotes. This oxidative folding process in eukaryotes is assisted by a series of enzymes like Erv2, Ero1 (endoplasmic reticulum oxireductin-1) and PDI (protein disulfide isomerases). Through a series of thiol-disulfide exchange reactions, PDI works together with Ero1 or Erv2 to oxidize thiol groups in a protein directly. The Ero1 enzyme inside the ER, which is associated with the ER membrane, can oxidize PDI and form a disulfide between Ero1 and PDI. PDI will catalyze the reduction of incorrectly formed disulfide bonds and, then will re-oxidize the protein substrate to form the correct disulfide bond. The protein disulfide oxidoreductase (PDO), which contributing to oxidization of proteins from the cysteines to disulfides, mainly exists in cytoplasm. Therefore, generally speaking, proteins in cytoplasm contain free thiols, while proteins found in other compartments like the ER usually including disulfide bonds.
HPLC-UV is widely used for the quantitative determination of thiols in proteins, because thiols can react with different reagents and show maximum absorption at different wavelength. For example, in the reaction with DTNB, the earliest reagent widely used for thiol group quantification, thiol group(s) produces an equivalent amount of 5-thio-2-nitrobenzoic acids, with maximum absorbance at 412 nm.
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