Cysteine readily forms disulfide linkages because of the ease with which it dissociates to form a thiolate anion. This accounts for the use of sulfur in methionine replacement of the sulfur with oxygen would result in a much less hydrophobic amino acid. Indeed, oxygen is the second most electronegative element in the periodic table.
Because both sulfur and oxygen belong to the same group (Group 6) of the Periodic Table and, therefore, are capable of making similar covalent linkages, the question may be restated: why would methionine and cysteine analogs, in which the sulfur atom is replaced by oxygen, not serve the same functions? One of the critical differences between oxygen and sulfur is sulfur's lower electronegativity. Why does nature employ sulfur in her repertoire of amino acids? The other canonical amino acids are comprised only of carbon, hydrogen, oxygen, and nitrogen atoms. However, homocysteine and taurine also play important physiological roles ( Fig. Methionine and cysteine may be considered to be the principal sulfur-containing amino acids because they are 2 of the canonical 20 amino acids that are incorporated into proteins. Methionine, cysteine, taurine, homocysteine, S-adenosylmethionine Taurine is present in many tissues at higher concentrations than any of the other amino acids. Cysteine may be converted to such important products as glutathione and taurine. This pathway, which is significant only at high methionine concentrations, produces a number of toxic endproducts. Methionine may also be metabolized by a transamination pathway. Homocysteine may be remethylated to methionine or converted to cysteine by the transsulfuration pathway. S-Adenosylhomocysteine, which is a product of these methyltransferases, gives rise to homocysteine. In animals, the great bulk of S-adenosylmethionine is used in methylation reactions. This is a cofactor of extraordinary versatility, playing roles in methyl group transfer, 5′-deoxyadenosyl group transfer, polyamine synthesis, ethylene synthesis in plants, and many others.
Methionine metabolism begins with its activation to S-adenosylmethionine. Cysteine, by virtue of its ability to form disulfide bonds, plays a crucial role in protein structure and in protein-folding pathways. Within proteins, many of the methionine residues are buried in the hydrophobic core, but some, which are exposed, are susceptible to oxidative damage.
Methionine is the initiating amino acid in the synthesis of virtually all eukaryotic proteins N-formylmethionine serves the same function in prokaryotes. This difference accounts for some of the distinctive properties of the sulfur-containing amino acids. Sulfur belongs to the same group in the periodic table as oxygen but is much less electronegative. Methionine, cysteine, homocysteine, and taurine are the 4 common sulfur-containing amino acids, but only the first 2 are incorporated into proteins.
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