Final answer:
The true statement about pleated sheet foldings within a polypeptide is that they depend on the regular occurrence of CO and NH for hydrogen bonding, and they are part of the protein's secondary structure.
Step-by-step explanation:
The correct statement regarding pleated sheet foldings within a polypeptide is: They depend on the regular occurrence of CO and NH. This refers to the hydrogen bonding that occurs in the secondary structure of proteins. Specifically, the hydrogen bonds form between the oxygen atom in the carbonyl group of one amino acid and the hydrogen of an amino group that is part of the peptide backbone on another amino acid within the polypeptide chain.
In the β-pleated sheet, the R groups of amino acids extend above and below the plane of the sheet and are not involved in the hydrogen bonding that maintains the structure of the sheet. The side chains are not parallel to the plane of the sheet but protrude outwards, alternating in direction.
The β-pleated sheet is not a part of the protein's quaternary structure but instead is a component of its secondary structure. Disulfide bridges also do not primarily hold the loops of the β-pleated sheet in place; it is the hydrogen bonds between the core components of the backbone that maintain the sheet's structure.