Final answer:
Collagen fibrils are reinforced by crosslinks between lysine and hydroxylysine residues, vital for the structural stability of the collagen fiber in connective tissues.
Step-by-step explanation:
Collagen fibrils are strengthened by covalent crosslinks between lysine and hydroxylysine residues on adjacent collagen molecules. These crosslinks are critical for the stability and structural integrity of the collagen fiber, which is essential in providing tensile strength to connective tissues. The collagen triple helix structure is formed by three polypeptide chains winding around each other, with a repeating sequence of Gly-X-Y, where X and Y are often proline and hydroxyproline, respectively.
Hydroxylysine residues can be further modified, contributing to crosslink formation. The synthesis and structure of collagen involve important posttranslational modifications catalyzed by various enzymes, with vitamin C (ascorbic acid) serving as a crucial cofactor for these enzymatic reactions.