The hemoglobin family

Hemoglobins are the most evolutionarily diverse family of dioxygen carriers. They are found in some plants (e.g., leghemoglobin in the nitrogen-fixing nodules of legumes), many invertebrates (including some insect larvae), crustaceans, 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 archetypical 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 O2 is the tetrameric hemoglobin A. It is readily available from the blood of human donors. * In some invertebrate hemoglobins, especially those of annelids, aggregates may contain as many as 192 binding 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 erythrocruorins (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 hemoglobin.
 

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 O2 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 another carrier with a high affinity for dioxygen, in this case because of a high rate of O2 binding. Since O2 is a poison for the nitrogenase enzyme, yet the nodules also require dioxygen, diffusion of O2 is facilitated, but the concentration of free dioxygen in the vicinity of nitrogen-fixing sites is minimized. 33 Kinetic and thermodynamic data for dioxygen binding and release from a variety of hemoglobins are summarized in Table 4.2. 9,10,3I.34-36 Notice that for the hemoglobin tetramer, which comprises two pairs of slightly dissimilar subunits, the a and {3 chains bind O2 with significantly different affinities and rate constants, especially in the T state. Isolated chains behave like monomeric vertebrate hemoglobins, such as whale myoglobin, which have affinities close to those of R-state hemoglobin. The chlorocruorins have a low affinity compared to other erythrocruorins. Especially for proteins that bind O2 cooperatively, a range of values is specified, since affinities and rates are sensitive to pH, ionic strength, specific anions and cations (allosteric effectors), and laboratory. For example, as we noted above, the O2 affinity of hemoglobin A is sensitive to the concentration of 2,3-DPG and to pH (Bohr effect). Trout hemoglobin I is insensitive to these species, whereas a second component of trout blood, trout hemoglobin IV, is so sensitive to pH (Root effect) that at pH < 7 trout hemoglobin IV is only partially saturated at P(02) = 160 Torr.4 Note that O2 affinities span five orders of magnitude. Since heme catabolism produces carbon monoxide, and since in some environments CO is readily available exogenously, selected data for CO binding are also presented.