Answer:
Secondary
Explanation:
Collagen has a special secondary structure made of triple helix. It has three polypeptide chains with each chain having a repetition of amino acid sequence glycine-X-Y where X and Y are usually proline or hydroxyproline.
Hydrogen bond is formed between the NH group of glycine and CO group of other residues which stabilizes the triple helix structure. Since glycine is the smallest amino acid it leads to the tight packing of the three chains. When glycine gets replaced by phenylalanine due to a mutation, this close packing is disturbed because phenylalanine is much bigger in size. Hence the triple helix is not able to form disrupting the secondary structure of collagen.
