Oxygen equilibrium studies of the radular muscle myoglobins of the gastropod molluscs, buccinum undatum L. and Bustcon canaliculatum L

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.

Ligand binding in the ferric and ferrous states of Paramecium hemoglobin

The unicellular protozoan Paramecium caudatum contains a monomeric hemoglobin (Hb) that has only 116 amino acid residues. This Hb shares the simultaneous presence of a distal E7 glutamine and a B10 tyrosine with several invertebrate Hbs. In the study presented here, we have used ligand binding kinetics and resonance Raman spectroscopy to characterize the effect of the distal pocket residues of Paramecium Hb in stabilizing the heme-bound ligands. In the ferric state, the high-spin to low-spin (aquo-hydroxy) transition takes place with a pK(a) of approximately 9.0. The oxygen affinity (P(50) = 0.45 Torr) is similar to that of myoglobin. The oxygen on- and off-rates are also similar to those of sperm whale myoglobin. Resonance Raman data suggest hydrogen bonding stabilization of bound oxygen, evidenced by a relatively low frequency of Fe-OO stretching (563 cm(-1)). We propose that the oxy complex is an equilibrium mixture of a hydrogen-bonded closed structure and an open structure. Oxygen will dissociate preferentially from the open structure, and therefore, the fraction of open structure population controls the rate of oxygen dissociation. In the CO complex, the Fe-CO stretching frequency at 493 cm(-1) suggests an open heme pocket, which is consistent with the higher on- and off-rates for CO relative to those in myoglobin. A high rate of ligand binding is also consistent with the observation of an Fe-histidine stretching frequency at 220 cm(-1), indicating the absence of significant proximal strain. We postulate that the function of Paramecium Hb is to supply oxygen for cellular oxidative processes.

A myoglobin evolved from indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme

The distribution, isolation, spectral and oxygen-binding properties, stability of ferrous state (autoxidation), amino acid sequence and gene structure of indoleamine 2,3-dioxygenase (IDO)-like myoglobins are summarized, and their evolution is discussed. Although it has long been thought that all hemoglobins and myoglobins have evolved from a common ancestral gene encoding a 14-16 kDa polypeptide, the discovery of IDO-like myoglobin from several gastropod molluscs clearly indicates that there was an alternative pathway for myoglobin evolution.

Comparative studies of the indoleamine dioxygenase-like myoglobin from the abalone Sulculus diversicolor

The abalone Sulculus diversicolor contains abundant myoglobin in its buccal mass. The myoglobin is homodimeric and the molecular mass of the constituent polypeptide chain is 41,000 Da. The amino acid sequence and gene structure are highly homologous with those of a vertebrate tryptophan-degrading enzyme, indoleamine dioxygenase (IDO). Thus Sulculus myoglobin evolved from an IDO gene, and represents a typical case of functional convergence. The oxygen equilibrium properties of Sulculus myoglobin were examined and compared with those of myoglobins from other sources. It binds oxygen reversibly, and the P50 was determined to be 3.8 mmHg at 20 degrees C and pH 7.4, showing that the oxygen affinity of Sulculus myoglobin is significantly lower than those of usual 16 kDa myoglobins. It also displays no cooperativity (nmax: 1.02-1.06) and no alkaline Bohr effect between pH 7.0 and 7.9. The cDNA-derived amino acid sequences of vertebrate IDOs, molluscan IDO-like myoglobins and a homolog in the yeast Saccharomyces were aligned, and several amino acid residues were proposed as candidates for key residues to control the function of IDO or myoglobin.

Amino-acid sequence of a cooperative, dimeric myoglobin from the gastropod mollusc, Buccinum undatum L

The complete amino-acid sequence of a dimeric myoglobin from the radular mussel of the gastropod mollusc, Buccinum undatum L. has been determined. The globin, which shows cooperative binding of oxygen, contains 146 amino acids, is N-terminal aminoacetylated, and has histidine residues at position 65 and 97, corresponding to the heme-binding histidines seen in mammalian myoglobins. It shows about 75% and 50% homology, respectively, with the dimeric molluscan myoglobins from Busycon canaliculatum and Cerithidea rhizophorarum, the former of which also shows weak cooperatively, but much less similarity to other species of myoglobin and hemoglobin.

Ligand binding equilibrium and kinetic measurements on the dimeric myoglobin of Busycon canaliculatum and the comparative ligand binding of diverse non-cooperative heme proteins

The rate constants and delta H degrees for the non-cooperative dimeric Busycon myoglobin are: oxygen, k' = 4.75 X 10(7) M-1 sec-1, k = 71 sec-1, and CO, l'= 3.46 X 10(5) M-1 sec-1, l = 0.0052 sec-1 at 20 degrees C, pH 7, delta H degrees = -3 kcal/mol for O2 and CO.2. Log-log plots of k vs K for oxygen and of l' vs L for CO binding for numerous non-cooperative hemoglobins and myoglobins point to a large steric influence of the protein on heme ligation reactions. Many of the proteins behave as "R" state for one ligand, but "T" for the other.

Dimeric hemoglobin of the bivalve mollusc Anadara broughtonii: complete amino acid sequence of the globin chain

The complete amino acid sequence of a dimeric hemoglobin (HbI) from the marine bivalve mollusc Anadara broughtonii was determined by sequencing of the intact chain and peptide fragments produced by cleavage at two asparaginylglycine bonds and at methionyl, arginyl, and tryptophanyl residues. The clam hemoglobin consists of two identical polypeptide chains. The globin chain has 146 amino acid residues with a proline at the NH2 terminus and a leucine at the COOH terminus. The calculated molecular mass of the native hemoglobin was 32945 daltons. The clam hemoglobin contains only two histidine residues, which correspond to the distal and proximal heme-linked positions. Compared with human beta chain, an additional segment of seven residues is present in the NH2-terminal region and also five less residues in the COOH-terminal region. Although such an amino-terminal elongation has been known to be characteristic of hemoglobins from the most primitive living vertebrates Cyclostomata, a very similar structure was found to occur in the hemoglobin from the primitive invertebrate arcid clam.

Oxygen equilibrium and subunit aggregation of a holothurian hemoglobin

The hemoglobin of the sea cucumber Cucumaria miniata Brandt has a mol. wt of about 36000 in the oxy- form with a s20,w equal to 2.9 and a subunit molecular weight of 18000 by sodium dodecylsulfate gel electrophoresis. This pigment aggregates when deoxygenated to an oligomer with a s20,w equal to 4.7, an aggregation which is reversible upon subsequent oxygenation. The hemoglobin shows a sigmoid binding equilibrium with "n" equal to 1.8 and a decrease in oxygen affinity with an increase in pigment concentration. This hemoglobin is compared with other hemoglobins showing oxygenation-linked subunit aggregation.