All proteins have methionine
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Sulfur-containing amino acids
Cysteine and methionine both contain a sulfur atom. Cysteine is a polar amino acid and is the only amino acid to have a free thiol group, which easily forms a disulfide bridge (S-S bridge) with another cysteine in the vicinity when it is oxidized. This property of cysteine is very important for the structure of proteins: polypeptide chains can be connected to one another or covalent links can be created within a polypeptide chain, which considerably stabilize the conformation of a protein. The term cystine is sometimes used for the two cysteines linked by disulphide bridges. In metalloproteins it is often a cysteine residue that binds zinc, copper or iron atoms. The thiol group of cysteine is the most reactive side group of all, it ionizes under weakly alkaline conditions. This property is used when modifying proteins (e.g. in protein analysis).
Methionine is one of the non-polar amino acids and has a thioether side chain. In contrast to cysteine, this amino acid is relatively unreactive. Ribosomal synthesized proteins always start with this amino acid, which is additionally formylated in bacteria.
- = Molecular weight.
- The hydrophobicity index indicates how much the side chain of an amino acid is displaced by the water. The more negative this number, the more hydrophilic the side chains and the higher the affinity of this amino acid for water.
- Values are listed for the α-amino group (N), the α-carboxy group (C) and, if present, the functional group of the side chain (R).
- Occurrence: This value indicates how often this amino acid occurs in the three main structures of a protein. α = α-helix portion, β = β-sheet portion, t = turn. A value of 1.0 means that this amino acid can be found with a random distribution in this structural motif. Values below 1.0 indicate that this amino acid tends not to appear in a structural motif, values greater than 1.0 mean that the amino acid promotes the formation of the corresponding structure.
Cysteine (Cys, C)
- = 121,15
- S-containing analogue to serine
- weakly charged, forms disulfide bridges
- semi-essential, biosynthesis from methionine
- not hydrophilic (0.17)
- Values: N = 10.78; C = 1.92; SH = 8.33; pI = 5.02
- Occurrence: α = 0.66; β = 1.40; t = 0.54
Methionine (Met, M)
- = 149,21
- Initiator amino acid of protein biosynthesis
- hydrophobic (1.1)
- Values: N = 9.21; C = 2.28; pI = 5.74
- Occurrence: α = 1.30; β = 1.14; t = 0.52
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