Protein solubility with added cysteine

Figure 7. Protein solubility increases in wheat flour extrudates as a junction oj level of cysteine added as a free radical scavenger during extrusion. From Koh (3), used with permission. Copyright American Association oJ Cereal Chemists, 1996.

Adding up the two reactions, one finds that the aldehyde is oxidized to the corresponding acid, or that the a-kcto-acid is oxidatively decarboxylated to the corresponding acid and CO2, while ATP is assembled, with the thiol playing a catalytic role. This thiol may be a cysteine residue in the enzyme protein or an enzyme-linked phosphopantetheine molecule or coenzyme A, a soluble coenzyme in which phos-phopantetheine is linked to 3 -phospho-AMP or lipoic acid, also known as thioctic acid, a molecule that can serve at the same time as electron carrier and as group carrier, a unique distinction. 

Shaker proteins in which various S4 residues were converted to cysteine were tested for their reactivity with a water-soluble cysteine-modifydng chemical agent that cannot cross the membrane. On the basis of whether the cysteines reacted with the agent added to one side or other of the membrane, the results indicated that in the resting state amino acids near the C-terminus of the S4 helix face the cytosol after the membrane is depolarized, some of these same amino acids become exposed to the exoplasmic surface of the channel. These experiments directly demonstrate movement of the S4 helix across the membrane, as schematically depicted in Figure 7-33 for voltage-gated Na" channels. 

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