Sulfur cysteine

Of the twenty amino acids that are normally found in proteins, only two contain sulfur, cysteine and methionine. Cysteine has long been recognized as being easily oxidized and this oxidation is associated with the loss of biological activity of many proteins. In recent years, it has been shown that methionine also shares these characteristics. Methionine was first isolated by Mueller19 and was one of the last amino acids discovered. Its structure was later proven to be y-methylthio-a-aminobutyric acid by Barger and Coyne20 who named the amino acid methionine as a contraction for its chemical name. 

S , bridging sulfurs S , terminal sulfurs (cysteine Sy) N5, coordinating imidazoles av., average distances. 

The building blocks of proteins are the alpha-amino acids, and exclusively those with the L-configuration. There are 20 that occur in na- ture. They too all consist of the four elements C, H, N, and 0 two amino acids additionally contain sulfur (cysteine and methionine). In certain, but vital, enzymes (the peroxidases), sulfur is replaced by selenium. 

In the structure of the [NiFeSe] enzyme from Desulfomicrobium baculatum, one of the four cysteines involved in Ni coordination, one sulfur cysteine is replaced by a selenocysteine (Scheme 77b).409 419... 

An increase in sensitivity is realized by silver-staining, where residues containing sulfur (cysteine, methionine) or basic side chains (arginine, lysine, histidine) reduce Ag+, leading to brown or black colored bands. Here, down to 0.1 ng of protein can be detected. 

Non-heme iron protein Fe Labile Sulfur Cysteine... 

FIGURE 17.2 Active site structures of the three families of mononuclear Mo- and W-enzymes. (a) Coordination around Mo. X and Y represent ligands such as oxygen (oxo, hydroxo, water, serine, aspartic acid), sulfur (cysteine), and selenium atoms (selenocysteine). (b) Structure of the pyranopterin molecule. R=H in eukaryotic enzymes and GMP, AMP, CMP or IMP in bacteria. Adapted from Brondino,... 

Carbon-bonded sulfur (cysteine and methionine) 41 % Non-carbon-bonded sulfur (ester sulfates) 52%... 

Oxidation in the Presence of Sulfur. Cysteine in the presence of finely divided or colloidal sulfur is oxidized to cystine, and, at the same time, sulfur is reduced to hydrogen sulfide ... 

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