Significance of Tryptophan in Proteins

P-glycoprotein, involved in multidrug resistance of cancer cells, contains five tryptophan residues, one of which is involved in the function of this protein.35 

These data indicate that specific tryptophan residues in various proteins may be involved in binding to (1) guanine or adenine nucleotide cofactors, (2) phospholipids, (3) certain glycosides, (4) cyclosporin A, and possibly other xenobiotics, (5) growth hormone, (6) insulin-like growth factor, and (7) vitamin D. 

Yau et al.42 reported that one of the ubiquitous features of membrane proteins is the preference of tryptophan and tyrosine residues for membrane surfaces that presumably arise from enhanced stability due to distinct interfacial interactions. Indeed, the presence of tryptophan and tyrosine residues for regions corresponding to membrane surfaces is a remarkable feature shared by 10 or so membrane proteins with known three-dimensional structures.43 

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