The major pathway for O 2 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 (k O2 ) rate coefficients, but the changes are not systematic. More significantly, the fractions (F gem ≈ 0.05-0.19) and rates (k gem ≈ 50 -100 s ؊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 k O2 to decrease roughly 4-fold; 2) F gem for CO to increase from ϳ0.05 to 0.45; and 3) k gem to decrease from ϳ80 to ϳ9 s ؊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.The structural mechanisms for O 2 binding to myoglobins (Mb) 3 and hemoglobins (Hb) have been the topic of much research since Kendrew (1) and Perutz et al. (2) first reported the three-dimensional structures of Mb and Hb over 45 years ago. In the case of mammalian myoglobins, ligand binding is well understood (Refs. 3-10 and references therein) and consists of four major steps. Weakly bound water is displaced to create a vacant distal pocket above the heme iron atom. Ligands migrate into the protein through a short channel that is created when the distal histidine (His-64 at the E7 helical position) 4 transiently rotates upward and then are captured in the interior of the distal pocket. This noncovalent intermediate is often called the B state because it can also be generated by photolysis of the equilibrium bound or A state. Covalent bond formation between the internal ligand and the iron atom of the heme group then competes with ligand escape back out through the His(E7) gate. The bound ligand can be further stabilized by electrostatic interactions with surrounding polar amino acid side chains. This E7 gate pathway appears to occur in most if not all animal hemoglobins, with the classic 3-on-3 ␣-helical globin fold and a distal histidine, although experimental verification has only been done rigorously for vertebrate Mbs using time-resolved crystallography (4 -8), site-directed mutagenesis (3, 9, 11), and time-resolved absorbance and FTIR spectroscopy (12-18).In con...

Background: O 2 pathways in animal hemoglobins and myoglobins are controversial. Results: Ligands enter and exit sperm whale Mb and Cerebratulus lacteus Hb by completely different pathways. Conclusion: Rational mutagenesis mapping can identify ligand migration pathways and provides experimental benchmarks for testing molecular dynamics simulations. Significance: Globins can use either a polar gate or an apolar tunnel for ligand entry.

The large apolar tunnel traversing the mini-hemoglobin from Cerebratulus lacteus (CerHb) has been examined by xray crystallography, ligand binding kinetics, and molecular dynamic simulations. The addition of 10 atm of xenon causes loss of diffraction in wild-type (wt) CerHbO 2 crystals, but Leu-86(G12)Ala CerHbO 2 , which has an increased tunnel volume, stably accommodates two discrete xenon atoms: one adjacent to Leu-86(G12) and another near Ala-55(E18). Molecular dynamics simulations of ligand migration in wt CerHb show a low energy pathway through the apolar tunnel when Leu or Ala, but not Phe or Trp, is present at the 86(G12) position. The addition of 10 -15 atm of xenon to solutions of wt CerHbCO and L86A CerHbCO causes 2-3-fold increases in the fraction of geminate ligand recombination, indicating that the bound xenon blocks CO escape. This idea was confirmed by L86F and L86W mutations, which cause even larger increases in the fraction of geminate CO rebinding, 2-5-fold decreases in the bimolecular rate constants for ligand entry, and large increases in the computed energy barriers for ligand movement through the apolar tunnel. Both the addition of xenon to the L86A mutant and oxidation of wt CerHb heme iron cause the appearance of an out Gln-44(E7) conformer, in which the amide side chain points out toward the solvent and appears to lower the barrier for ligand escape through the E7 gate. However, the observed kinetics suggest little entry and escape (<25%) through the E7 pathway, presumably because the in Gln-44(E7) conformer is thermodynamically favored. Although molecular dynamics (MD)7 simulations suggest multiple interior pathways for O 2 entry into and exit from globins, most experiments with mammalian myoglobins (Mbs) and hemoglobins (Hbs) suggest a well defined single pathway involving a short channel between the heme propionates and the heme iron atom that is gated by the distal E7 histidine (1). To search for and define an interior ligand migration trajectory, we chose to use the neuronal mini-hemoglobin from Cerebratulus lacteus as a model globin system to examine its long apolar tunnel that leads from the distal portion of the heme pocket to an exit point between the C-terminal regions of the E and H helices of the tertiary fold, a pathway that is roughly 180°opposite the E7 channel and appears to be a major route for ligand entry (2). This model globin provides a well defined system to examine both experimentally and theoretically the effects of xenon binding, mutagenesis, and conformational heterogeneity on the competition between movement through the E7 gate versus an internal apolar pathway.Nerve tissue Hbs occur in both vertebrates and invertebrates (3). Among these, the nerve Hb from the nemertean worm C. lacteus (CerHb) is the smallest functional globin known, being composed of 109 amino acids instead of the ϳ140 -160 residues typical of most monomeric globins (4, 5). Analysis of the three-dimensional structure of CerHb has shown a markedly edited 3-over-3-globin fold with deletion of

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