Final answer:
Proline is the amino acid that introduces a 'kink' in polypeptide chains due to its ring-like side chain, which disrupts normal hydrogen bonding in secondary structures and affects the protein's tertiary structure. Therefore, correct option is a.
Step-by-step explanation:
The student is asking about an amino acid that introduces a 'kink' into polypeptide chains due to an unusual ring in its structure. Among the options given (Proline, Glycine, Histidine, Lysine), the correct answer is Proline.
Proline's side chain is linked to its amino group, forming a unique ring-like structure that disrupts the regular helical structure of proteins, causing a bend or kink. This distinct feature forces nearby groups away from the ring, influencing the folding and structure of the protein.
Secondary structures like the alpha-helix and beta-pleated sheets are stabilized by intrachain hydrogen bonds, with R groups projecting outward from the backbone. Proline is an exception as it can disrupt these structures due to the covalent cyclization of its side chain with the backbone, preventing standard hydrogen bonding.
The tertiary structure of proteins, involving the arrangement of secondary structures in three-dimensional space, is significantly affected by proline residues. The presence of proline can lead to turns and loops within the protein, which can be important for the function and overall shape of the protein.