BL/CH 401 Lecture #4 -- Amino Acid Structures

Thiol - Neutral Polar Amino Acid

Polar -- Neutral -- Thiol -- Cys (Cysteine) -- in computers Cys = C

{*Figure 15*}

Cys is essentially thiol-Ala. The thiol (-SH) group of Cys can ionize as shown in graphic.

Thiols ionize at about pH 8 and so usually they are protonated at biological pH.

Hydroxyl groups like in Ser have pK about 15 or so and do not ionize normally (emphasized in Fig. 15).

To make a hydroxyl group in a compound like methanol ionize requires an oxidation/reduction reaction.

A Special Feature of Cys is that it can oxidize (in the presence of oxygen) and react with another Cys to form Cystine or what we can a disulfide bond:

The formation of "Cystine" can take place between 2 polypeptide chains to make a cross-link between them. This is actually an enzyme catalyzed reaction which takes place in the lumen of ER in cells when proteins are being exported from the cell. A very good example is the production of antibodies by cells in the immune response - antibody proteins contain many Cys-Cys or disulfide bonds. Excellular proteins often contain Cys-Cys bonds, while cellular proteins do not usually contain the Cys-Cys since the conditions in the cell are reducing.

In the second part of the graphic above, the general reaction of 2 thiols is shown. In the presence of oxygen or oxidizing conditions, the 2 thiols react to form a disulfide bond between them. Since this is a redox reaction, the hydride ion released by each thiol is usually coupled to an electron acceptor reaction or in simple oxidiation with oxygen, hydrogen peroxide is usually formed with further reduction to water.

©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu

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