How does the chemical nature of their side chains link to the potential effect on their protein folding and structure.

i. Cysteine ii. Methionine

 

• Cysteine has chemically reactive side chains made up of disulfide bridges. The disulfide bridges are able to link fragments within the polypeptide chain at long distances which contributes to the formation of the three dimensional conformation in tertiary and quaternary structures of proteins by making it stable. Disulfide bridges also form loops of the backbone by stabilizing them. An example of such loops are the immunoglobins. Basically, disulfide bridges are responsible for any linkage and structure between more than one polypeptide chain which is present in most proteins.
• Methionine has a sulfur-containing side chain which is hydrophobic and as a result methionine is usually buried in proteins. S-aromatic motifs present in the side chain of methionine stabilize protein structure. The methionine side chain is unbranched making it more flexible. The flexible residues are able to bind other peptide chains since it is nonpolar and malleable.

Step-by-step explanation

• An example of a protein linked with disulfide bridges is insulin which has two disulfide bridges between CysA7-CysB7 and between CysA20-CystB19. The disulfide bridges from cysteine also form permanent components of the primary structure which is basic for other structures to form. Sometime, the interchain S-S bridges form the absolute conditions for existence of tertiary structures. The disulfide bridges also play a crucial role in stability of tertiary structure.
• Methionine side chain is hydrophobic and contributes to protein structure by being buried in hydrophobic cores. The side chain also contributes to protein stability through S-aromatic motifs in the side chain. Lack of branching increases flexibility of the amino acid to bind with other peptide chains which contributes to the secondary and tertiary structure of proteins.