The edible gastropod snail helix pomatia uses hemo

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octameric hemoglobins. The edible gastropod (snail) Helix pomatia uses hemo- cyanin, whereas the apparently closely related fresh-water snail Planorbis uses a high-oligomer hemoglobin. Both use a myoglobin as the oxygen-storage pro- tein. The structure of the active site has been extensively probed by EXAFS methods,40,41 and the x-ray crystal structure of a hexameric deoxyhemocyanin is known. 42 Each copper atom is coordinated to three imidazole groups from histidine residues. The pinwheel arrangement of the six subunits, the domain structure of a single subunit, and the domain containing the active site are shown in Figure 4.9. 3. The hemerythrin family The biological occurrence of hemerythrins (Hr in Figure 4.8), the third class of dioxygen carriers, is relatively rare, being restricted to the sipunculid family (nonsegmented worms), a few members of the annelid (segmented worm) fam-
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189 (A) t (B) Domain 2 Cu(B) .,<. . .. Cu(A) His-351 . His-199 His-355 (D) Figure 4.9 Diagram of the structure of deoxyhemocyanin from Panulirus interruptus at 3.2 A resolution: 42c (A) The hexameric arrangement of subunits; (B) The domain structure of one subunit; (C) The tertiary structure of domain 2, which contains the pair of copper atoms: a-helices are repre- sented by cylinders; f3-strands by arrows, and copper atoms by diamonds; (D) The active site and its histidine ligands. Reproduced with permission from B. Linzen, Science 229 (1985), 519-524. ily, a couple of brachiopods (shrimps), and a couple of priapulids. The oxygen- binding site contains, like hemocyanin, a pair of metal atoms, in this case, iron. Upon oxygenation the colorless protein becomes purple-red. Monomeric (myo- hemerythrin), trimeric, and octameric forms of hemerythrin are known; all ap- pear to be based on a similar subunit of about 13.5 kDa. When hemerythrin is extracted from the organism, its oxygen binding is at best only weakly cooper- ative, with Hill coefficients in the range 1.1 to 2.1. 18 In coelomic cells (the tissue between the inner membrane lining the digestive tract and the outer mem- brane of the worm-analogous to flesh in vertebrates), oxygen apparently binds with higher cooperativity (n ~ 2.5).43 Perchlorate ions have been observed to induce cooperativity: since CI0 4 - has no biological role, it appears that in pro- tein purifications the biological allosteric effector is lost. No Bohr effect occurs. Dioxygen binding data are accumulated in Table 4.2. 36 ,44
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190 4 I BIOLOGICAL AND SYNTHETIC DIOXYGEN CARRIERS The structure of hemerythrin in a variety of derivatives (oxy, azido, met, and deoxy) is now well-characterized. With three bridging ligands, a distinctive cofacial bioctahedral stereochemistry is seen (Figure 4.10).45-48 o A (A) Figure 4.10 Structure of hemerythrin: (A) The tertiary structure of octameric hemerythrin 46b with four (X- helices (A, B, C, D) of one of the eight subunits. The filled half-circles denote anion binding sites (e.g., CI0 4 -); the filled circle the Fe2 site; and the cross-hatched oval the N 3 - and SCN- binding sites (FeIIlh and the O 2 binding sites (Fellh. Reproduced with permission from R. E.
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