Question

Collagen synthesized in the presence of radioactively labeled proline (C14 labeling) has radioactive residues incorporated into its sequence. However, collagen synthesized in the presence of radioactively labeled hydroxyproline (C14 labeling) does not become radiolabeled. Why?

a.  
Hydroxyprolin residues lose their labeling upon incorporation into a protein.

   b.  
Hydroxyprolin gets substituted by a non-labeled prolin after protein is synthetized

   c.  
Protein synthesis machinery is only able to incorporate prolin, but not hydroxyprolin, during protein synthesis

   d.  
None of the above answers are correct

Answer 1

C.Protein synthesis machinery is only able to incorporate prolin, but not hydroxyprolin, during protein synthesis. During protein synthesis prolin is incorporated directly into the collagen and following post traslational modification by enzyme prolyl hydroxylase form hydroxyprolin in lumen of endoplamic reticulum. Hydroxyprolin not able to incorporated into collagen protein directly. Hydroxyprolin is important to enhanced the stability of collagen triple helix. Secondary modification (post translational modification) is required to incorpaorate the hydroxyprolin in collagen structure.