Fibrous proteins, which serve mainly structural roles, have simple repeating elements of secondary structure.Given below are some statements related to fibrous protein –

1) Fibrous proteins are soluble in water, a property conferred by a high concentration of hydrophilic amino acid residues both in the interior of the protein and on its surface.

2) Two strands of -keratin, oriented in antiparallel, are wrapped about each other to form a super twisted coiled coil. The super twisting amplifies the strength of the overall structure, just as strands are twisted to make a strong rope.

3) The collagen helix is a unique secondary structure quite distinct from the - helix. It is left-handed and has three amino acid residues per turn.

4) Glycine cannot be replaced by another amino acid residue without substantial deleterious effects on collagen structure.

5) Fibroin is rich in Ala and Gly residues, permitting a close packing of -sheets and an interlocking arrangement of R groups.

6) Elastin synthesis starts from the precursor molecule known as desmosine , the structural unit of elastin.

Which one of the following combinations of statements is incorrect?