Transferrin Is a Protein That Binds Iron

Transferrin is a protein that binds iron.It enters cells through transferrin receptors located on the cell membrane.A research group made a mutant transferrin receptor protein by replacing three asparagine residues with glycine.Results of their study are summarized in the table below. Which statement is consistent with these results?

A) Iron binds to transferrin by recognizing oligosaccharides on the transferrin protein surface.
B) Iron binds to cells by recognizing oligosaccharides on the transferrin receptor surface.
C) Transferrin binds to the transferrin receptor protein on the cell surface by recognizing oligosaccharides attached to the receptor.
D) Asparagine residues provide the critical recognition sites for iron to bind to and enter cells.
E) Transferrin recognizes asparagine residues on the transferrin receptor protein to bind and enter cells.