H-NMR signal of Arenicola marina myoglobin in vivo as an index of tissue oxygenation

Detection of myoglobin desaturation in Mirounga angustirostris during apnea

1H NMR solution-state study of elephant seal (Mirounga angustirostris) myoglobin (Mb) and hemoglobin (Hb) establishes the temperature-dependent chemical shifts of the proximal histidyl N(delta)H signal, which reflects the respective intracellular and vascular PO2 in vivo. Both proteins exist predominantly in one major isoform and do not exhibit any conformational heterogeneity. The Mb and Hb signals are detectable in M. angustirostris tissue in vivo. During eupnea M. angustirostris muscle maintains a well-saturated MbO2. However, during apnea, the deoxymyoglobin proximal histidyl N(delta)H signal becomes visible, reflecting a declining tissue PO2. The study establishes a firm methodological basis for using NMR to investigate the metabolic responses during sleep apnea of the elephant seal and to secure insights into oxygen regulation in diving mammals.

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.

Functional differentiation in trematode hemoglobin isoforms

The Hbs and the major electrophoretic Hb components (isoHbs) were isolated from three species of the trematodes, Explanatum explanatum (Ee), Gastrothylax crumenifer (Gc) and Paramphistomum epiclitum (Pe), that parasitise the common Indian water buffalo Bubalus bubalis. The Hbs are monomeric and resemble the so-called nonfunctional mutant hemoglobins that have Tyr at B10 or E7 positions (replacing Leu and the His residues, respectively). However, they are capable of binding with O2 and CO. O2 equilibrium studies of trematode Hb isoforms reveal extremely high O2 affinities, with half-saturation O2 tension (P50) values up to 800 times lower than those of human hemoglobins. This correlates with Tyr residues at B10 and at the distal position (E7) that decrease the O2 dissociation rate by contributing hydrogen bonds (H-bonds) to the bound O2. These substitutions also increase the O2 association rates either due to orientation of E7-Tyr towards the solvent and/or by sterically hindering the entry of water molecules into the heme pocket. The latter may account for the low rate of autoxidation of trematode Hbs. The Hbs and their isoforms from different species exhibited pronounced variation in O2 affinity, which may relate to subtle differences in the structure of the heme pocket. The O2 affinities of the composite (unfractionated) Hbs were intermediate to those of the individual Hb isoform. The P50 values of Hbs here obtained by direct O2 equilibrium measurements differed from those calculated from kinetic data already published [Kiger, L., Rashid, A. K., Griffon, N., Haque, M., Moens, L.,Gibson, Q. H., Poyart, C., & Marden, M. C. (1998). Biophys. J. 75, 990-998.] Intermediate state(s) due to slow reorientation of E7-Tyr may account for this difference. Some Hb isoforms showed slight (either normal or reverse) Bohr effects. The hyperbolic O2 equilibrium curve, Hill coefficient (n) values near unity accord with a monomeric nature of trematode Hbs. In marked contrast to vertebrate Hbs, CO does not seem to compete effectively with O2 in trematode Hbs, as evident from partition coefficient values (M) below 1.

Primary structures of Arenicola marina isomyoglobins: molecular basis for functional heterogeneity

The primary structures of isomyoglobins MbI and MbII from the body wall musculature of the polychaete Arenicola marina were investigated, aiming to trace the molecular basis for their functional differentiation. Unexpectedly, five chains, MbIa, MbIb, MbIIa, MbIIb and MbIIc, each consisting of 145 amino-acid residues and occurring in a ratio of = 33:17:25:12.5:12.5 were found. All substitutions can be explained by one-point mutations. With the exception of the 41(C6)Asn-->Asp(MbI/MbII) exchange that appears to be the basis for the electrophoretic separation of MbI and MbII, the substitutions do not involve drastic changes in the character of the side-chains. Pairwise comparison of MbIa and MbIIa with other invertebrate globin chains indicate the following sequence of decreasing identities: Aplysia (mollusc) Mb, Chironomus (insect) CTT III hemoglobin, whale Mb and Ascaris (nematode) Mb. The marked difference in O2 affinities between MbI and MbII appears attributable to 62Pro which distorts the E helix around E6 and occurs in all MbII chains, but in only 33% of the MbI chains (MbIb).

Metabolic response in Arenicola marina to limiting oxygen as reflected in the 1H-NMR oxymyoglobin signal

Many intertidal animals can endure prolonged periods of environmental stress and have developed strategies to preserve a functioning energy state in the cell. Recent 1H/31P-NMR techniques have allowed investigators to monitor directly mammalian tissue metabolism in vivo. In particular, the signals of myoglobin (Mb) offer a unique opportunity to explore the intracellular oxygen-partial-pressure [p(O2)] interaction in Arenicola marina, a standard model to study hypoxia tolerance in invertebrates. The present study reveals that the 1H-NMR MbO2 signal at -2.9 ppm is detectable in tissue and reflects directly the oxygenated state. As the p(O2) declines, MbO2 saturation and oxygen consumption decrease. However, phosphotaurocyamine concentration remains unaltered until the MbO2 saturation falls below 33%. The extracellular to intracellular p(O2) gradient appears substantial. The study establishes the 1H-NMR technique as an approach to measure the intracellular p(O2) with an oxygenated state marker and presents the interrelationship between oxygen and the metabolic adaptation during hypoxic stress.