7. Which of the following statements regarding structural proteins is true? (A) silk fibroin is made from a repeat of three amino acids where each third amino acid is Gly (B) a-keratin is composed of peptides that contain mostly a-helical structure with the exception of short sequences where the peptide folds back on itself (C) collagen has its own helical structure where three individual peptides are wound around each other in a left-handed helix (D) generally speaking, structural proteins are soluble in water (E) none of the above