The 2-methyl substituent on 2-methylimidazole is not sterically active in the five-coordinate structures Fe(PF)(2-MeIm) and Fe(TPP)(2-MeIm), since the iron atom is displaced from the plane of the porphyrin by the expected amount and the Fe-N1m bond is unstretched and similar to that in deoxyhemoglobin (low O2 affinity) and deoxyMb (higher O2 affinity). Moreover, resonance Raman measurements also indicate little strain t in this bond. 200 In other words, there is no "tension at the heme," a key concept in early discussions of cooperativity before structures on model systems and high-resolution, refined protein structures became available. IIa On moving into the plane of the porphyrin upon oxygenation, the 2-methyl substituent prevents the Fe-imidazole group from achieving its optimum geometry with the iron at the center of the porphyrin hole, as seen in the structure of Fe(PF)(l-MeIm)(02)' Thus, the sterically active 2-methyl substituent leads to lowered O2 (and CO) affinity relative to the I-methyl analogue. In metrical terms the lowered affinity is reflected in an increase in the sum of the axial bond lengths from 1.75 + 2.07 = 3.82 A to 1.90 + 2.11 = 4.01 A.
In the crystal structure of Fe(C2Cap)(l-MeIm)(CO) the cap is about 5.6 A from the porphryin plane. I2Id Hence, in the crystal structures of the free base H2(C2Cap)201a and FeCI(C2Cap)20Ib species, in which the cap is screwed down to approximately 4.0 A from the porphyrin plane, considerable conformational rearrangement of the cap and the four chains attaching it to the porphyrin is needed to provide room for a small ligand such as CO. This is even more pronounced in a Co(C3Cap) complex where the cap is only 3.49 A from the mean porphyrin plane. 202 Thus not only is affinity for CO lowered, but some additional discrimination against it is induced, since a linear, perpendicular coordination creates considerable strain energy elsewhere in the molecule. For the pocket porphyrin (Figure 4.23), structural data are available on the carbonyl adduct. 12lb The CO ligand is unable to achieve the linear perpendicular geometry seen in the high-affinity picket-fence porphyrin derivative, Fe(PF)(lMeIm)( CO), IIO and distortion of the porphyrin core is greater. In the pocketporphyrin system, O2 affinity is unaffected, but CO affinity is lowered.
The crystal structure of partially oxygenated hemoglobin, [a-Fe02h[t3-Feh, 191a reveals that the quaternary structure, except in the immediate vicinity of the a hemes, which have O2 coordinated, resembles that of T-state deoxyhemoglobin rather than R-state liganded hemoglobin. In accord with the low affinity of Tstate hemoglobin, the Fe--Nlm bonds for the six-coordinate a-hemes at 2.37 A are significantly longer than those in fully oxygenated R-state oxyhemoglobin, [a-Fe02h[l3-Fe02h in the notation above, (1.94 (a-hemes) and 2.06 A(13 hemes» and that found in oxymyoglobin (2.07 A). In contrast to the R-state structure and oxyMb, the a-hemes are folded as seen in the deoxy parent, leaving the Fe still substantially displaced (0.2 A) from the plane of the four pyrrole nitrogen atoms. The deoxyhemoglobin T-state quaternary structure also has been observed in two other partially liganded hybrid hemoglobins, [a-FeCOh [f3-Mn(II)h203 and [a-Nih[f3-FeCOh- 191d Again, structural changes upon coordination do not propagate beyond the immediate vicinity of the liganded heme to the critical alf32 interfaces.
Note that although the crystal structure of hemoglobin A reveals that access to the binding site for the 13 chains is blocked by groups at the entrance to the cavity above the iron center, this does not prevent facile access to the binding site; the rate of O2 binding is slowed by a factor of only five. A similar situation occurs also for vertebrate myoglobins.
The large structural differences that exist between deoxy (n and oxy (R) hemoglobin and the much smaller differences between deoxy (n and partially liganded (n hybrid hemoglobin are shown in Figure 4.32. 203 Because of the steric hindrance afforded by the distal histidine, all biological systems have low affinity for CO relative to the picket-fence porphyrins, with the exception of mutants where the distal histidine has been replaced by glycine. Thus low affinity to CO is associated primarily with the inability of the Fe-CO group to achieve its preferred linear geometry perpendicular to the porphyrin.
Low-affinity O2binding in the hemoglobins appears to be associated with the inability of the Fe-proximal histidine unit to move into the plane of the porphyrin and less so to distal effects, such as a cavity too small to accommodate the coordinated ligand. The blocked access to the site affects the kinetics but not necessarily the thennodynamics of ligand binding, as evidenced by the structure of T-state [a-Nih[f3-FeCOh- 19Id Some similarities between the structures and properties of partially oxygenated (T-state) [a-Fe02h[f3-Feh hemoglobin and Fe(PF)(2-MeIm)(02) are provided in Table 4.10. In the synthetic systems low O2 affinity can be induced by 2-methyl substituents-a restraint on the movement of the Fe-imidazole moiety analogous to that provided by the protein chain. A second means is by distal effects, such as caps and straps.
Tidak ada komentar:
Posting Komentar