This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: nutrition and digestion, Subject name: Biochemistry
Which of the following trace elements is impoant for conversion of procollagen to collagen? 
Select and explain. 
A. Se, B. Cu, C. Zn, D. Mg
B: Cu
Answer: B: Cu 
Explanation: The collagen fibers are strengthened by covalent cross-links between lysine and hydroxylysine residues. The cross-links are formed by lysyl oxidase which conves these amino acids into aldehydes. These are called aldol cross-links. Lysyl oxidase is a copper-containing enzyme, the copper ion being located at its active site. Box 52.1 gives a summary of the steps in collagen formation. In copper deficiency, reduced cross-linking of collagen results (Chapter 35). The aldehyde derivatives of lysine residues can form an aldol condensation. Such aldol cross-links are formed near the amino terminal of the chains.Ref: DM Vasudevan, page no: 582