Sulfur, amino acid structure

In addition to their importance as essential amino acids for humans, the quantitative determination of cysteine and methionine seems to be growing in importance in the animal feed industry. The dietary requirements for the sulfur amino acids tend to be very high in many animals. This is presumably due to the magnitude of hair/feather growth and the fact that the structural proteins that comprise hair/feathers often have high cyst(e)ine content. 

Sulfur was recognised as a common element in proteinaceous material in the early eighteenth century, and the disulfide cystine (1) was probably the first sulfur amino acid to be discovered (Morner, 1899) its structure was elucidated by Friedmann(1903). 

Cysteine is similar in structure to serine, but has an SH group on the b-carbon, instead of an OH group as in serine. This amino acid is extremely important in protein structure with regard to forming disulfide bonds and potential chelation. Cysteine is not considered an essential amino acid in people because it can be formed from serine and methionine. However, since methionine is an essential amino acid, in some cases improved growth can be obtained by adding cysteine to the diet, because it will spare the amount of methionine required to form the cysteine. Thus, on low-methionine diets, cysteine can be beneficial toward growth. A number of proteins are low in sulfur amino acids and, therefore, this methionine-cysteine relationship may become important. 

Proteins are macromolecules built up of amino acids. Sulfur amino acids, which are present in all plant and animal cells, structure of two amino acids is shown in Figure 48.2. ... 

Waxy maize zein is rich in sulfur amino acids(Rishi et al, 2001). The sulfur amino acids were connected via relatively strong disulfide bonds and hydrophobic bonds (Pomes et al, 1971), which may contribute to the perfect film-forming characteristic of waxy maize zein. The waxy maize zein solution was deposited on mica, the concentration of the zein molecules would increase as the solvent vaporized. When the concentration of the zein molecules reached a certain value, the monomer or polymer molecules would aggregate to form a film structure via hydrogen bonds, disulfide bonds and hydrophobic bonds. By AFM, it was... 

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

Molybdenum hexafluoride. 3,1412 Molybdenum-iron-sulfur complexes, 4,241 Molybdenum oxide amino acid formation prebiotic systems, 6, 872 Molybdenum storage protein microorganisms, 6, 681 Molybdenum telluride, 3, 1431 Molybdenum tetraalkoxides physical properties, 2, 347 Molybdenum tribromide, 3,1330 Molybdenum trichloride, 3,1330 Molybdenum trifluoride, 3, 1330 Molybdenum trihalides, 3, 1330 bond lengths, 3, 1330 magnetic moments, 3,1330 preparation, 3,1330 properties, 3, 1330 structure, 3,1330 Molybdenum triiodide, 3,1330 Molybdenum trioxide complexes, 3, 1379 Molybdenum triselenide, 3, 143)... 

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. 

There are hundreds of iron-containing enzymes. In general, the iron can exist as (a) a mononuclear site, in which it is coordinated by a tetrapyrrole structure (hemes) or strictly by amino acid residues that donate oxo, nitrogen, or sulfur ligands (b) a dinuclear site in which the irons are bridged by oxo, nitrogen, or sulfur coordination (c) a trinuclear site as in the 3Fe-4S clusters or (d) a tetranuclear site as in the [4Fe-4S] clusters. 

---