The magnitude and sign of these effects are shown in

Info icon This preview shows pages 250–254. Sign up to view the full content.

View Full Document Right Arrow Icon
. The magnitude and sign of these effects are shown in Figure 4.27. For the coordination of alkylisocyanide molecules to * For Glycera CoMb0 2 no change in EPR parameters occurs on substituting D 2 0 for H 2 0.!6S No hydrogen bond between O 2 and a distal group comparable in strength to that in whale CoMb0 2 was inferred,
Image of page 250

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
free energy (G) Compress Fe - N'm M + 02(CO) deoxy Fe-L, nonoptimal Fe-L 2 nonoptimal Stretch Fe -N 1m M0 2 (MCO) oxy (carbonyl) Hydrogen bonding (oxyonly) t Destabilize M0 2 (MCO) Stabilize M0 2 (MCO) t 229 reaction coordinate Figure 4.27 Proximal and distal effects on the ligand affinities. hemoglobin, the steric effects of different alkyl groups have been quantified. 35 Lowered affinity occurs with increasing alkyl chain length, with the exception of methyl isocyanide. C. Detailed Structures of Hemoglobins and Model Systems With thermodynamic background and general structural features relevant to li- gand affinity enumerated, attention may now be turned to the detailed structural aspects of the active site and its surroundings. As was shown crudely in Figure 4.3, the ligand affinity of an iron porphyrin may be perturbed either by modu- lating the structure of the deoxy material or by modulating the structure and surroundings of the liganded material or both. The model systems provide the reference points against which the protein structures may be compared. 1. Structures relevant to deoxy hemoglobins The structure of the picket-fence porphyrin compound, Fe(PF)(2-MeIm), is shown in Figure 4.28. 172 Minus the pickets, it is essentially a magnified view of the active site of deoxymyoglobin, shown in Figure 4.29. 181 Some metrical details of these structures, of a very similar unsubstituted tetraphenylpor- phyrin, I 10 and of several other deoxyhemoglobins I Ie, 182-185 are listed in Table 4.7. In general they are all similar, but important differences exist. In all structures, except deoxyerythrocruorin,183 the iron atom is displaced about 0.4 to 0.5 A from the plane of the porphyrin toward the axial base. For deoxyerythrocruorin the displacement is less than half this, perhaps because the water molecule is weakly coordinated to the iron center.
Image of page 251
230 (6) Figure 4.28 (A) ,,,,,,OO'ag,,'''' of th< ,w etu ", of P<lPFll2-M,\n»'" (B) Structure of Fe(PF)(I_Melm)(Oz)' \81
Image of page 252

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
231 Figure 4.29 Structure of metmyoglobin and deoxymyoglobin at 2.0 A resolution near the heme. \8\a Solid bonds are for metmyoglobin; open bonds for deoxymyoglobin. Note the water molecule coordi- nated to the iron center and hydrogen bonded to the distal imidazole group in metmyoglobin. Reproduced with permission from T. Takano, J. Mol. Biol. 110 (1977), 569-584. An imidazole group from a histidine residue-the distal histidine E7 in po- sition 7 on helix labeled E-hovers over the binding site for most vertebrate hemoglobins, except for genetically engineered mutants of human hemoglobin (j3E7His -?
Image of page 253
Image of page 254
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern