Metrical details are given in table 48 llc 192 table

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metrical details are given in Table 4.8. llc ,118,121.172.183,187,188,190-192
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Table 4.8 Metrical details of selected liganded hemoglobins and their models a Resol. Fe:N p Fe-pprph DOf!ling Fe-L, Fe-L 2 Fe-XY 0, 02 Tilt Compound A A A A A A deg. deg. deg. deg. Dioxygen adducts Fe(PF)(1- - 1.98(1) -0.03 0.02 2.07(2) 1. 75(2) 131(2) 20 45 0 Melm)(02) Fe(PF)(2- - 1. 996(4) 0.09 0.02 2.107(4) 1.898(7) 129(1 ) 22 45 7 Melm)(02) Mb0 2 1.6 1.95(6) 0.18(3) 0.01 2.07(6) 1.83(6) 115(4) 1 ~O 4 EXAFS 2.02(2) 2.06(2) 1.80(2) 123(4) 148(8? Er0 2 1.4 2.04 0.38 -0.08 2.1 1.8 150 7 3 Hb0 2 a 2.1 1. 99(5) 0.12(8) 0.04 1.94(9) 1.66(8) 153(7) 11 0 3 f3 1. 96(6) -0.11(8) 0.11 2.07(9) 1.87(3) 159(12) 27 45 5 EXAFS 1.99(2) 2.05(2) 1.82(2) 122(4) 143(8)b [a-Fe02Mf3-Feh a 2.1 2.04(4) 0.19(5) 0.17(5) 2.24(10) 1.82(4) 153(4) 6 11 f3 0.3 2.2 Carbonmonoxy adducts MbCO 1.5 1.97(3) 0.00 0.03 2.2 1.9 140 30 EXAFS 2.01 (2) 2.20(2) 1.93(2) 127(4) 145(8) b EreO 1.4 2.01 -0.11 -0.10 2.1 2.2 161 (9) 7 HbCO a 2.2 2.02 -0.10 1.95 1.83 175(15) f3 2.03 0.10 2.20 1.70 171(15) [a-Nil [f3-FeCO] a 2.6 f3 0.15 0.12 2.23(5) Fe(TPP)(Py)(CO) 2.02(3) -0.02 -0.02 2.10(2) 1.77(2) 179(2) 45 ~O EXAFS 2.02(2) - 2.09(2) 1.81(2) 138(6) 180(II)b Fe(poc)(1,2-Melm)(CO) 1.973(8) 0.001 2.079(5) 1.768(7) 172.5(6) Fe(C 2 Cap )( 1- Melm)(CO) 1.990(7) 0.01 2.043(6) 1. 742(7) 172.9(6) 1. 988( 13) 0.02 2.041(5) 1.748(7) 175.9(6) a See Figure 4.25 for definition of symbols. b Alternative interpretation of EXAFS data.
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III. STRUCTURAL BASIS OF LIGAND AFFINITIES OF OXYGEN CARRIERS 235 b. Interactions of Coordinated Ligands with Distal Groups Without ex- ception to date (but see footnote 1 in Reference 168), in structurally character- ized oxyhemoglobins, the coordinated dioxygen ligand is hydrogen-bonded to the distal histidine or to a water molecule--even though theoretical calculations show that hydrogen bonding would destabilize M-0 2 moieties. 192 This univer- sal observation of hydrogen bonding in these biological systems is consistent with notions that electron density accumulates on the dioxygen molecule upon coordination. Given the errors associated with atomic positions (at best, ± 0.20 A) the x-ray crystallographic evidence could be equivocal, since hydrogen atoms on the distal imidazole are not observed. There are at least three lines of evi- dence that support the existence of a specific O 2 ... HN interaction. First, the EPR spectrum of cobalt oxyhemoglobin indicates that the coordinated dioxygen is hydrogen-bonded to something. 177 - 179 Second, and more directly, in the neu- tron-diffraction structure of oxymyoglobin,189 where hydrogen and especially deuterium nuclides scatter strongly, the imino hydrogen or deuteron was located on the nitrogen atom closest to the coordinated dioxygen, as illustrated in Figure 4.31A. In contrast, in the neutron-diffraction structure of carbonmonoxymy- oglobin, the alternative imidazole tautomer was observed (Figure 4.3IB). 125,189 The absence of hydrogen bonding of the distal imidazole residue with the co- ordinated CO molecule is consistent with other lines of evidence that there is little accumulation of electron density on the carbonyl ligand. Third, but less directly, genetically engineered mutants have been produced in which the distal histidine has been replaced by glycine-sperm whale Mb E7His- .... ? Gly, and HbA Q'E7His- .... ?Gly and HbA /3E7His- .... ?Gly.35b,192 For the myoglobin mutant, the O 2 binding rate constant at room temperature increases by an order of magnitude, but the dissociation rate constant increases by two orders of magnitude, leading
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