Myriapoda 8 i hr i ch i hb i er i mb i arthropoda

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myriapoda [8£] I Hr I Ch I Hb I Er I Mb I... "" / arthropoda gastropoda Hc Hb & cephalopoda Hc mollusca ~mphineura Hc Mb bivalvia Hb siPunculids~r priapulids Hr brachiopods Hr legume root nodules / PLANTS ~"moci"m ~ Y'''t ffilliI chordata I Hb I Mb I echinodermata [8li] 0 nemerteans ~ffilliI "'''\~~ t"m,,:~:t:V' I E' I M:~,:e~& o~ \ 0 ~ Q.<J:'/ ~ primitive acoel flatworms I ANIMALS Figure 4.8 Phylogenetic distribution of oxygen-carrier proteins: Hb, hemoglobin; Mb, myoglobin; Er, ery- throcruorin; Ch, chlorocruorin; He, hemocyanin; Hr, hemerythrin. 15a Reproduced with permis- sion from K. E. van Holde and K. I. Miller, Quart. Rev. Biophys. 15 (1982), 1-129. suffice to give some hints about how respiratory proteins evolved, a subject that is outside the scope of this book. 1. The hemoglobin family Hemoglobins are the most evolutionarily diverse family of dioxygen car- riers. They are found in some plants (e.g., leghemoglobin in the nitrogen-fixing nodules of legumes), many invertebrates (including some insect larvae), crusta- ceans, molluscs (especially bivalves and snails), almost all annelid worms, and in all vertebrates with one possible exception, the Antarctic fish Cyclostomata. With few exceptions the monomeric and oligomeric hemoglobins all share a basically similar building block: a single heme group is embedded in a folded polypeptide with a molecular weight of about 16 kDa (see Figure 4.2), and is anchored to the protein by coordination of the iron center to an imidazole ligand from a histidine residue. Mammalian myoglobin is often taken as the archetyp- ical myoglobin (see Table 4.1). Sperm whale, bovine, or equine myoglobin are specific examples; the muscle tissue from which they may be extracted is more available than that from Homo sapiens. The archetypical oligomeric hemoglobin that shows cooperative binding of O 2 is the tetrameric hemoglobin A. It is read-
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I. INTRODUCTION: BIOLOGICAL DIOXYGEN TRANSPORT SYSTEMS 185 ily available from the blood of human donors. * In some invertebrate hemoglo- bins, especially those of annelids, aggregates may contain as many as 192 bind- ing sites, to give a molecular weight of about 3 x 10 6 Dalton. These and other high-molecular-weight hemoglobins of arthropods are often referred to as ery- throcruorins (Er). In a few annelid worms, the otherwise ubiquitous heme b or protoheme is replaced by chloroheme (see Figure 4.2) to give chlorocruorins (Ch), which tum green upon oxygenation (chloros, Greek for green). Some organisms, for example the clam Scapharca equivalvis, feature a dimeric hemo- globin. The only known anomalous hemoglobin is Hb Ascaris, which comes from a parasitic nematode found in the guts of pigs. It has a molecular weight of about 39 kDa per heme; this value is not a multiple of the myoglobin building block. 31 Moreover, presumably in response to the low availability of O 2 in pigs' guts, Hb Ascaris has an extraordinarily high affinity for dioxygen, in large part owing to an extremely slow rate of dioxygen release. 32 Leghemoglobin is an- other carrier with a high affinity for dioxygen, in this case because of a high rate of O 2 binding. Since O 2 is
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