Sulfur, in proteins

Schneider JF, Westley J. 1969. Metabolic interrelations of sulfur in proteins, thiosulfate, and cystine. J Biol Chem 244 5735-5744. 

Y. M. Torchinsky, Sulfur in Proteins, Pergamon Press, Oxford, 1977, p. 133. 

Figure 9.21 Depth profiling of phosphorus and sulfur in protein spots of a 2D gel measured by LA-ICP-MS. Transient signals show decreasing ion intensity with increasing replication number (proportional to measurement time).

Table 9.29 Identification and quantification of sulfur in proteins using MALDI-FTICR-MS.

Cystine chemistry must obviously be a matter of some importance in keratins which, as proteins, are characterized by their high-sulfur content. An early review of keratin sulfur chemistry is given by Alexander and Hudson (1954), while the more recent review by Cecil and McPhee (1959) includes extensive reference to work on keratins. Some treatment of the topic is also to be found in articles in Sulfur in Proteins (Benesch et al., 1959). 

Various algae samples from an area of industrial contamination (Trondheimsfjord) and from an area free of industrial contamination (Reine in Lofoten) contained 0.05-0.24 ppm selenium. There was more variation between species of algae than between sources of material. The low selenium in these plants as compared with the selenium content in fish is attributed by Lunde (46) to the different organic forms of sulfur in the plants and in fish—selenium being enriched relative to sulfur in proteins. 

An analysis by Chakrabarti [21] of protein structures in the PDB showed that metal ions approach the sulfur of methionine at about 38(5)° from the perpendicular to the C-S-C group. This is similar to values in the range foimd, as just described, for smaU-molecule crystal structures in which the metal ion is presumed to interact with a sulfur lone-pair orbital [18]. It was also found that metal ions approach cysteine residues such that the M- -S-C-C torsion angle is 90° or 180°, and that the conformation of the cysteine side chain is generally affected by the metal ion. The metal ions that readily bind to sulfur in proteins are copper, iron, mercury, and zinc. The geometry of binding of metal ions to methionine or cysteine did not appear to depend on the identity of the individual metal. 

W. Kauzmann, Relative probabilities of isomers in cystine-containing randomly coiled polypeptides. In Sulfur in Proteins, Proc. of Symposium, Falmouth, Mass. 1958, p. 93-108 (1959)... 

The heavy metals lead, cadmium and mercury are toxic because they combine with sulfur in proteins. They can occupy sites where no metal is normally present, a serious disturbance, or they can push vital metals such as iron out of their positions. The liver prepares a remarkable protein, metallothionein, especially rich in cysteine groups, which can act as a scavenging substance, taking care of toxic elements up to a certain level. Even essential elements, copper for instance, may be removed if present in too high, toxic concentrations. 

The important role of sulfur in protein quantification via hyphenated ICP-IDMS techniques is exemplified in Figure 8.22, which illustrates the key role of sulfur in protein quantification by ICP-MS [108]. Cysteine and methionine are sulfur-containing amino adds that are present in most proteins. The enzymatic digestion... 

Sulfur exists in cysteine and methionine and is one of the most abundant elements in natural proteins. The cumulative abundance of sulfur in proteins is about 5%. The relative amount of proteins can be obtained by the isotopelabeling method with the sulfhydryl group and molecular mass spectrometry measurement, such as the isotope-coded affinity tags (ICAT) method. On the other hand, when the amino acid sequence of a protein has been identified, the absolute amount of a protein can be obtained via the determination of sulfur with ICP-MS. In comparison with other naturally existing elements in proteins, sulfur is preferable as an internal standard for protein quantification due to its high abundance. ... 

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