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The apolar channel in Cerebratulus lacteus hemoglobin is the route for O2 entry and exit
Salter, M.; Nienhaus, K.; Nienhaus, G.; Dewilde, S.; Moens, L.; Pesce, A.; Nardini, M.; Bolognesi, M.; Olson, J. (2008). The apolar channel in Cerebratulus lacteus hemoglobin is the route for O2 entry and exit. J. Biol. Chem. 283(51): 35689-35702. https://dx.doi.org/10.1074/jbc.M805727200
In: Journal of Biological Chemistry. American Society for Biochemistry and Molecular Biology: Baltimore, etc.. ISSN 0021-9258; e-ISSN 1083-351X
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  • Salter, M.
  • Nienhaus, K.
  • Nienhaus, G.
  • Dewilde, S.
  • Moens, L.
  • Pesce, A.
  • Nardini, M.
  • Bolognesi, M.
  • Olson, J.

Abstract
    The major pathway for O2 binding to mammalian myoglobins (Mb) and hemoglobins (Hb) involves transient upward movement of the distal histidine (His-64(E7)), allowing ligand capture in the distal pocket. The mini-globin from Cerebratulus lacteus (CerHb) appears to have an alternative pathway between the E and H helices that is made accessible by loss of the N-terminal A helix. To test this pathway, we examined the effects of changing the size of the E7 gate and closing the end of the apolar channel in CerHb by site-directed mutagenesis. Increasing the size of Gln-44(E7) from Ala to Trp causes variation of association (k'O2) and dissociation (kO2) rate coefficients, but the changes are not systematic. More significantly, the fractions (Fgem ˜ 0.05–0.19) and rates (kgem ˜ 50–100 µs-1) of geminate CO recombination in the Gln-44(E7) mutants are all similar. In contrast, blocking the entrance to the apolar channel by increasing the size of Ala-55(E18) to Phe and Trp causes the following: 1) both k'O2 and kO2 to decrease roughly 4-fold; 2) Fgem for CO to increase from ~0.05 to 0.45; and 3) kgem to decrease from ~80 to ~9 µs-1, as ligands become trapped in the channel. Crystal structures and low temperature Fourier-transform infrared spectra of Phe-55 and Trp-55 CerHb confirm that the aromatic side chains block the channel entrance, with little effect on the distal pocket. These results provide unambiguous experimental proof that diatomic ligands can enter and exit a globin through an interior channel in preference to the more direct E7 pathway.

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