title: Near-UV circular dichroism and UV resonance Raman spectra of individual tryptophan residues in human hemoglobin and their changes upon the quaternary structure transition.
abstract: The aromatic residues such as tryptophan (Trp) and tyrosine (Tyr) in human adult hemoglobin (Hb A) are known to contribute to near-UV circular dichroism (CD) and UV resonance Raman (RR) spectral changes upon the R → T quaternary structure transition. In Hb A, there are three Trp residues per αβ dimer: at α14, β15, and β37. To evaluate their individual contributions to the R → T spectral changes, we produced three mutant hemoglobins in E. coli; rHb (α14Trp→Leu), rHb (β15Trp→Leu), and rHb (β37Trp→His). Near-UV CD and UVRR spectra of these mutant Hbs were compared with those of Hb A under solvent conditions where mutant rHbs exhibited significant cooperativity in oxygen binding. Near-UV CD and UVRR spectra for individual Trp residues were extracted by the difference calculations between Hb A and the mutants. α14 and β15Trp exhibited negative CD bands in both oxy- and deoxy-Hb A, whereas β37Trp showed positive CD bands in oxy-Hb A but decreased intensity in deoxy-form. These differences in CD spectra among the three Trp residues in Hb A were ascribed to surrounding hydrophobicity by examining the spectral changes of a model compound of Trp, N-acetyl-l-Trp ethyl ester, in various solvents. Intensity enhancement of Trp UVRR bands upon the R → T transition was ascribed mostly to the hydrogen-bond formation of β37Trp in deoxy-Hb A because similar UVRR spectral changes were detected with N-acetyl-l-Trp ethyl ester upon addition of a hydrogen-bond acceptor.
authors: Research Center for Micro-Nano Technology, Hosei University, Koganei, Tokyo 184-0003, Japan. masako.nagai.gv@hosei.ac.jp
authors: Masako
authors: M
authors: Nagai
authors: Shigenori
authors: S
authors: Nagatomo
authors: Yukifumi
authors: Y
authors: Nagai
authors: Kenichi
authors: K
authors: Ohkubo
authors: Kiyohiro
authors: K
authors: Imai
authors: Teizo
authors: T
authors: Kitagawa
mesh_terms: D002942:Circular Dichroism / Q000379:methods*; D018360:Crystallography, X-Ray / Q000379:methods; D006441:Hemoglobin A / Q000737:chemistry* / Q000235:genetics; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D009154:Mutation; D020836:Protein Structure, Quaternary; D013059:Spectrum Analysis, Raman / Q000379:methods*; D014364:Tryptophan / Q000737:chemistry* / Q000235:genetics
chemical_list: D014364:Tryptophan; D006441:Hemoglobin A