Answer:
D. All of the statements are true.
Step-by-step explanation:
Alpha helical secondary structure of proteins is stabilized by hydrogen bonding. The hydrogen bond is formed between a hydrogen atom attached to the electronegative N atom of the peptide bond and oxygen atom of the carboxyl group of the fourth amino acid towards the N terminal of that peptide bond. This internal hydrogen bonding makes the alpha-helices stable structures.
The N atom of protein is a part of a rigid ring and does not permit the free rotation and does not have the H atom to participate in the internal hydrogen bonding. It rather produces a kink in the alpha helix. Glycine has one H atom as its R group and its smaller size makes the alpha helix to assume a coiled structure. Therefore, proline and glycine destabilize the alpha-helices. Alanine, leucine, lysine, etc allow the optimum hydrogen bonding of alpha helices and are frequently found in the alpha-helices. The polypeptide backbone in alpha-helices assumes a helical structure. In beta conformation, the backbone of the polypeptide is extended into the zigzag pattern line a waffle fry.
Beta sheets are stabilized by hydrogen bonds formed between the adjacent parts of the polypeptide chains. The adjacent polypeptide chains may be present in parallel or antiparallel pattern, depending on the same or opposite orientation of amino and carboxyl groups. The antiparallel conformation has opposite amino-to-carboxyl orientations, allowing the formation of linear hydrogen bonds between the adjacent polypeptides which in turn makes these confirmations more stable.