The Nerve Hemoglobin of the Bivalve Mollusc Spisula solidissima

Abstract

Members of the hemoglobin (Hb) superfamily are present in nerve tissue of several vertebrate and invertebrate species. In vertebrates they display hexacoordinate heme iron atoms and are typically expressed at low levels (M). Their function is still a matter of debate. In invertebrates they have a hexa-or pentacoordinate heme iron, are mostly expressed at high levels (mM), and have been suggested to have a myoglobin-like function. The native Hb of the surf clam, Spisula solidissima, composed of 162 amino acids, does not show specific deviations from the globin templates. UV-visible and resonance Raman spectroscopy demonstrate a hexacoordinate heme iron. Based on the sequence analogy, the histidine E7 is proposed as a sixth ligand. Kinetic and equilibrium measurements show a moderate oxygen affinity (P 50 ϳ0.6 torr) and no cooperativity. The histidine binding affinity is 100-fold lower than in neuroglobin. Phylogenetic analysis demonstrates a clustering of the S. solidissima nerve Hb with mollusc Hbs and myoglobins, but not with the vertebrate neuroglobins. We conclude that invertebrate nerve Hbs expressed at high levels are, despite the hexacoordinate nature of their heme iron, not essentially different from other intracellular Hbs. They most likely fulfill a myoglobin-like function and enhance oxygen supply to the neurons.The recent discovery of globin proteins in the nerve cells of mammals and other vertebrates (1) has created much interest in unraveling the cellular functions and potential biomedical implications of these oxygen binding molecules in the metabolism of the vertebrate nervous system (2). Historically, however, "nerve hemoglobins" (nHbs) 4 were first observed in invertebrate taxa. In 1872, Lankester had already recorded the brilliant red color of the ganglia of the polychaete annelid Aphrodite aculeata (3). Since then, nHbs have also been found in, or associated with, nerve tissues of several other invertebrate taxa, such as molluscs, arthropods, nemerteans, and nematode species (4, 5). Of these, only the nHbs of A. aculeata (6) and the nemertean worm Cerebratulus lacteus (7) have been characterized at the molecular level.With few exceptions (e.g. in air-breathing gastropods), the invertebrate nHbs are expressed at high concentrations (in the mM range) and have therefore been suggested to support nerve function by mediating O 2 storage and/or transport during temporary hypoxia (4). For example, in the gastropod mollusc Aplysia depilans, the oxygenation state of the nHb was correlated with the electrical activity of the neural ganglia, and firing was shown to be proportional to the degree of oxygenation of the globin (8). The nHb in the nerve bundles of the bivalve mollusc Tellina alternata was reported to extend the time of O 2 delivery to the nerves during anoxic periods by as much as 30 min, whereas this effect was not demonstrable in the nerves of a related clam species Tagelius plebeius, which lack the nHb (9). In many cases, the nHbs appear to be specifically expressed in the glial cells surr...

The peculiar heme pocket of the 2/2 hemoglobin of cold-adapted Pseudoalteromonas haloplanktis TAC125

“…Alternatively, the multiple hexacoordinated states may account for multiple functions in the same molecule. Hexacoordinated Hbs are observed in unicellular eukaryotes [17], plants [49], invertebrates [50], and in some tissues of higher vertebrates [48,51], but only a few cases have been examined and reported in the literature for bacterial 2/2 Hbs [52][53][54][55]. The physiological role of these hexacoordinated Hbs is not well understood.…”

Structure and reactivity of hexacoordinate hemoglobins

“…The two other vertebrate hxHbs, “neuroglobins” (Ngb) and “globin X” (GlbX), are more closely related to extracellular oxygen transport Hbs present in animals with closed circulatory systems [19,43,44]. In general, the Ngbs are very strongly coordinated by the distal histidine in both the ferrous and ferric oxidation states, Cgbs are intermediate in this regard, and hxHbs from branch 1 of the tree are most weakly hexacoordinate, on par with the nsHbs from plants [40,41,46–49] (Table 1). …”

“…(4)), several hxHbs have affinity constants appropriate for oxygen transport [4,17,40]. However, rates of oxygen dissociation from hxHbs are generally too slow for oxygen transport, with the exception of the mollusk nHb, DrosHb, and the Class 2 nsHbs [4,24,41,42,48]. Of the hxHbs for which autooxidation has been measured, rates are generally much faster than the pentacoordinate Hbs [46].…”

“…The nerve Hb from the bivalve mollusk, S. solidissima is believed to be an oxygen transport protein [41]. Such glial nerve Hbs are common in species exposed to hypoxia, and enable neuronal function under these conditions [143].…”

Hemoproteins in the cold

“…Hexacoordinated Hbs are also expressed at low structural complexity and observed in bacteria, unicellular eukaryotes (Wittenberg et al, 2002), plants (Watts et al, 2001), invertebrates (Dewilde et al, 2006) and in some tissues of higher vertebrates. In the absence of exogenous ligands, also Ngb and Cygb (de Sanctis et al, 2004) display hexacoordination with distal His E7 coordinating directly with the heme iron, either in ferrous or ferric forms.…”