The function of hemoglobin in the arcid clam Noetia ponderosa--I. Oxygenation in vitro and in vivo

Hemoglobin wonders: a fascinating gas transporter dive into molluscs

Hemoglobin (Hb) has been identified in at least 14 molluscan taxa so far. Research spanning over 130 years on molluscan Hbs focuses on their genes, protein structures, functions, and evolution. Molluscan Hbs are categorized into single-, two-, and multiple-domain chains, including red blood cell, gill, and extracellular Hbs, based on the number of globin domains and their respective locations. These Hbs exhibit variation in assembly, ranging from monomeric and dimeric to higher-order multimeric forms. Typically, molluscan Hbs display moderately high oxygen affinity, weak cooperativity, and varying pH sensitivity. Hb's potential role in antimicrobial pathways could augment the immune defense of bivalves, which may be a complement to their lack of adaptive immunity. The role of Hb as a respiratory protein in bivalves likely originated from the substitution of hemocyanin. Molluscan Hbs demonstrate adaptive evolution in response to environmental changes via various strategies (e.g. increasing Hb types, multimerization, and amino acid residue substitutions at key sites), enhancing or altering functional properties for habitat adaptation. Concurrently, an increase in Hb assembly diversity, coupled with a downward trend in oxygen affinity, is observed during molluscan differentiation and evolution. Hb in Protobranchia, Heteroconchia, and Pteriomorphia bivalves originated from separate ancestors, with Protobranchia inheriting a relative ancient molluscan Hb gene. In bivalves, extracellular Hbs share a common origin, while gill Hbs likely emerged from convergent evolution. In summary, research on molluscan Hbs offers valuable insights into the origins, biological variations, and adaptive evolution of animal Hbs.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

When are resting water-breathers lacking O2? Arterial PO2 at the anaerobic threshold in crab

The minimum arterial O2 partial pressure (PaO2) at which, in resting conditions, O2 consumption (MO2) can be maintained and below which anaerobic metabolism is initiated was studied in the crabs Eriocheir sinensis and Carcinus maenas at 15 degrees C. Arterial PO2, MO2 (in E. sinensis), blood lactate concentration ([lact]b) and blood copper concentration ([Cu]b, an index of the blood O2 carrying capacity) were determined after 24 h exposure to inspired PO2 (PIO2) ranging from 2.7-2.1 kPa. They were compared to normoxic controls. In normoxia, the most frequently measured PaO2 ranged between 1 and 3 kPa in both species. In hypoxia, the threshold for blood lactate appearance was PaO2 = 2.1 kPa in E. sinensis and 1.3 kPa in C. maenas, but in many individuals anaerobic metabolism was initiated at lower PaO2's. The lowest PaO2 with [lact]b approx. 0 was 0.7 kPa in both species. MO2 was maintained in 4 E. sinensis out of 6 with PaO2 ranging from 0.7-1.2 kPa (PIO2 = 2.1 kPa). The arterial PO2 at which anaerobic metabolism occurred was not related to blood O2 carrying capacity.

How is O2 consumption maintained independent of ambient oxygen in mussel Anodonta cygnea?

The mechanisms of adaptation allowing resting freshwater mussels Anodonta cygnea to maintain their oxygen consumption (MO2) constant when the O2 partial pressure in the inspired water (PIO2) varied were studied at 13 degrees C. Steady-state values of oxygen consumption and/or shell valve activity were determined at prefixed PIO2 for periods ranging from 2 to 15 days. Values of PO2, O2 concentration and acid-base status of arterial blood in the heart were determined after two days. MO2 was maintained constant over PIO2 ranging from 35 to less than 1 kPa. At 0.3 kPa it decreased by 50%. Valves remained open (and MO2 constant) most of the time even during a 4.5 day period at PIO2 approximately 1.5 kPa. Between 35 and 1 kPa, blood PO2 at the heart level can remain low and within a narrow range independent of PIO2. Blood PCO2 increased at high PIO2 and decreased at low PIO2. Data are compared to previous results in crayfish and in wels (sheat-fish). On the basis of data similarity it is proposed that these three animals exhibit the same basic strategy for maintaining resting MO2 when PIO2 varies. This common feature relies mainly on the ability to maintain PO2 in the arterial blood at a value which is low and independent of PIO2. Hence, in terms of O2, homeostasis of the milieu intérieur is accomplished.

A kinetic and equilibrium study of ligand binding to the monomeric and dimeric haem-containing globins of two chitons

The radular muscles of the amphineuran molluscs Amaurochiton glaucus and Sipharochiton pelliserpentis contain both a dimeric and a monomeric form of myoglobin. The dimeric form of the protein is composed of two polypeptide chains covalently linked to each other via one or more disulphide bonds. The dimeric protein shows co-operative O2-binding curves. Kinetic investigations indicate that CO binding is co-operative in the dimeric protein, subsequent to full photolysis, but mono-exponential following 10% photolysis. O2 recombination following part photolysis is mono-exponential in the dimeric form, whereas O2 dissociation kinetics indicates the presence of chain heterogeneity. The monomeric form of the protein exhibits mono-exponential time courses in all the experimental situations explored. Although the rate constants associated with the reactions of individual dimer and monomer molecular species are very different, the two species of chiton investigated show remarkably similar properties when compared with each other. All the reactions studied are pH-independent in the range pH 6-8. Amino acid analysis indicates that the monomeric units that combine to form the dimeric species are not identical with the naturally occurring monomeric form. A comparison is made between the chiton myoglobins and other similar O2-binding proteins.

Anadara ovalis hemoglobins: distinct dissociation and ligand binding characteristics

The red cells of the arcid clam Anadara ovalis contain two electrophoretically distinct hemoglobins: Hb Major (Hb Ma) and Hb Minor (Hb Mi). The major component consists of two electrophoretically indistinguishable tetramers each composed of two heterodimers; the minor hemoblogin is a homodimer whose subunits are different from the tetramer. Functionally, Hb Ma has a higher P50, exhibits a concentration dependent oxygen affinity, has significant ligand cooperativity (n = 2.0), lacks a Bohr effect and is unaffected by ATP. HB Mi has a P50 which is lower and independent of hemoglobin concentration, shows appreciable cooperativity (n = 1.4) and exhibits no heterotropic effects. Both Hb Ma and Mi are resistant to dissociation in the presence of 1.0 M NaI, NaCl and guanidine-HCl but dissociate to monomers when converted to the aquamet but not the cyanmet derivative. The dissociation is completely inhibited by mercaptoethanol. The large number of reactive -SH groups (10-13 per tetramer) suggests that the monomerization is mediated by intra-subunit disulfide bridge formation.

Thermal Vent Clam (Calyptogena magnifica) Hemoglobin

A heterodont bivalve mollusk Calyptogena magnifica, from the East Pacific Rise and the Galápagos Rift hydrothermal vent areas, contains abundant hemoglobin in circulating erythrocytes. No other known heterodont clam contains a circulating intracellular hemoglobin. The hemoglobin is tetrameric and has a relatively high oxygen affinity, which varies only slightly between 2 degrees and 10 degrees C. The presence of hemoglobin in the clam may facilitate the transport of oxygen to be used in chemoautotrophic hydrogen sulfide metabolism.