Michael C. Marden

Biochemical Characterization and Ligand Binding Properties of Neuroglobin, a Novel Member of the Globin Family

Neuroglobin is a recently discovered member of the globin superfamily that is suggested to enhance the O2 supply of the vertebrate brain. Spectral measurements with human and mouse recombinant neuroglobin provide evidence for a hexacoordinated deoxy ferrous (Fe2+) form, indicating a His-Fe2+-His binding scheme. O2 or CO can displace the endogenous protein ligand, which is identified as the distal histidine by mutagenesis. The ferric (Fe3+) form of neuroglobin is also hexacoordinated with the protein ligand E7-His and does not exhibit pH dependence. Flash photolysis studies show a high recombination rate (k on) and a slow dissociation rate (k off) for both O2 and CO, indicating a high intrinsic affinity for these ligands. However, because the rate-limiting step in ligand combination with the deoxy hexacoordinated form involves the dissociation of the protein ligand, O2 and CO binding is suggested to be slowin vivo. Because of this competition, the observed O2 affinity of recombinant human neuroglobin is average (1 torr at 37 °C). Neuroglobin has a high autoxidation rate, resulting in an oxidation at 37 °C by air within a few minutes. The oxidation/reduction potential of mouse neuroglobin (E′o = −129 mV) lies within the physiological range. Under natural conditions, recombinant mouse neuroglobin occurs as a monomer with disulfide-dependent formation of dimers. The biochemical and kinetic characteristics are discussed in view of the possible functions of neuroglobin in the vertebrate brain.

The redox state of the cell regulates the ligand binding affinity of human neuroglobin and cytoglobin

Neuroglobin and cytoglobin reversibly bind oxygen in competition with the distal histidine, and the observed oxygen affinity therefore depends on the properties of both ligands. In the absence of an external ligand, the iron atom of these globins is hexacoordinated. There are three cysteine residues in human neuroglobin; those at positions CD7 and D5 are sufficiently close to form an internal disulfide bond. Both cysteine residues in cytoglobin, although localized in other positions than in human neuroglobin, may form a disulfide bond as well. The existence and position of these disulfide bonds was demonstrated by mass spectrometry and thiol accessibility studies. Mutation of the cysteines involved, or the use of reducing agents to break the S–S bond, led to a decrease in the observed oxygen affinity of human neuroglobin by an order of magnitude. The critical parameter is the histidine dissociation rate, which changes by about a factor of 10. The same effect is observed with human cytoglobin, although to a much lesser extent (less than a factor of 2). These results suggest a novel mechanism for the regulation of oxygen binding; contact with an appropriate electron donor would provoke the release of oxygen. Hence the oxygen affinity would be directly linked to the redox state of the cell.

Human erythroid cells produced ex vivo at large scale differentiate into red blood cells in vivo.

New sources of red blood cells (RBCs) would improve the transfusion capacity of blood centers. Our objective was to generate cells for transfusion by inducing a massive proliferation of hematopoietic stem and progenitor cells, followed by terminal erythroid differentiation. We describe here a procedure for amplifying hematopoietic stem cells (HSCs) from human cord blood (CB) by the sequential application of specific combinations of growth factors in a serum-free culture medium. The procedure allowed the ex vivo expansion of CD34+ progenitor and stem cells into a pure erythroid precursor population. When injected into nonobese diabetic, severe combined immunodeficient (NOD/SCID) mice, the erythroid cells were capable of proliferation and terminal differentiation into mature enucleated RBCs. The approach may eventually be useful in clinical transfusion applications.

β-Lactoglobulin binds retinol and protoporphyrin IX at two different binding sites

Measurement of tryptophan fluorescence quenching and the excitation energy transfer from tryptophanyl residues to the bound ligand indicates that β‐lactoglobulin binds tightly to hemin and protoporphyrin IX in a ligand‐to‐protein stoichiometric ratio. The apparent dissociation constants of hemin‐β‐lactoglobulin and protoporphyrin IX‐β‐lactoglobulin complexes are 2.5 × 10−7 M and 4 × 10−7 M, respectively. The addition of β‐lactoglobulin (final concentration = 10 μM, phosphate buffer 50 mM, pH 7.1) to the solution containing retinol and protoporphyrin IX triggers an energy transfer between β‐lactoglobulin tryptophan and protoporphyrin IX as well as between retinol and protoporphyrin IX. The efficiency of energy transfer depends on the distance between the donor (retinol) and the acceptor (protoporphyrin IX). Using the Förster theory, a retinol‐protoporphyrin IX distance of 25 Å was calculated. These results indicate that retinol and protoporphyrin IX are bound to the β‐lactoglobulin monomer at two different sites.

Effectors of hemoglobin. Separation of allosteric and affinity factors

The relative contributions of the allosteric and affinity factors toward the change in p50 have been calculated for a series of effectors of hemoglobin (Hb). Shifts in the ligand affinity of deoxy Hb and the values for 50% ligand saturation (p50) were obtained from oxygen equilibrium data. Because the high-affinity parameters (liganded conformation) are poorly determined from the equilibrium curves, they were determined from kinetic measurements of the association and dissociation rates with CO as ligand. The CO on-rates were obtained by flash photolysis measurements. The off-rates were determined from the rate of oxidation of HbCO by ferricyanide, or by replacement of CO with NO. The partition function of fully liganded hemoglobin for oxygen and CO is only slightly changed by the effectors. Measurements were made in the presence of the effectors 2,3-diphosphoglycerate (DPG), inositol hexakisphosphate (IHP), bezafibrate (Bzf), and two recently synthesized derivatives of Bzf (LR16 and L35). Values of p50 change by over a factor of 60; the on-rates decrease by nearly a factor of 8, with little change in the off-rates for the liganded conformation. The data indicate that both allosteric and affinity parameters are changed by the effectors; the changes in ligand affinity represent the larger contribution toward shifts in p50.

Trematode Hemoglobins Show Exceptionally High Oxygen Affinity

Ligand binding studies were made with hemoglobin (Hb) isolated from trematode species Gastrothylax crumenifer (Gc), Paramphistomum epiclitum (Pe), Explanatum explanatum (Ee), parasitic worms of water buffalo Bubalus bubalis, and Isoparorchis hypselobagri (Ih) parasitic in the catfish Wallago attu. The kinetics of oxygen and carbon monoxide binding show very fast association rates. Whereas oxygen can be displaced on a millisecond time scale from human Hb at 25 degrees C, the dissociation of oxygen from trematode Hb may require a few seconds to over 20 s (for Hb Pe). Carbon monoxide dissociation is faster, however, than for other monomeric hemoglobins or myoglobins. Trematode hemoglobins also show a reduced rate of autoxidation; the oxy form is not readily oxidized by potassium ferricyanide, indicating that only the deoxy form reacts rapidly with this oxidizing agent. Unlike most vertebrate Hbs, the trematodes have a tyrosine residue at position E7 instead of the usual distal histidine. As for Hb Ascaris, which also displays a high oxygen affinity, the trematodes have a tyrosine in position B10; two H-bonds to the oxygen molecule are thought to be responsible for the very high oxygen affinity. The trematode hemoglobins display a combination of high association rates and very low dissociation rates, resulting in some of the highest oxygen affinities ever observed.

Neuroglobin Ligand Binding Kinetics

Neuroglobin, cytoglobin, and hemoglobins from Drosophila melanogaster and Arabidopsis thaliana were studied for their ligand binding properties versus temperature. These globins have a common feature of being hexacoordinated (via the distal histidine) under deoxy conditions, displaying an enhanced amplitude for the alpha absorption band at 560 nm. External ligands can bind, but the transition from the hexacoordinated form to the ligand (L) bound species is slow, as expected for a replacement reaction Fe‐His ↔ Fe ↔ Fe‐L. Histidine binding is on the order of 1 ms; dissociation times are variable, and may be as long as 1 s for the highest histidine affinities. Oxygen binds rapidly but dissociates slowly, requiring as much as 1 s. These rates would correspond to a very high affinity for the pentacoordinated form; however, competition with the distal histidine leads decreases the affinity for the external ligand. The observed oxygen affinity remains in the range of 1 to 10 mm Hg. The low oxygen dissociation indicates a stabilization via H‐bonds as for certain globins from parasites (Ascaris, the trematodes). Other ligands such as CO, or CN for the ferric form, show a decreased affinity, since only the competition with the E7 histidine, but not the stabilizing H‐bond, plays a role. In addition, the competitive internal ligand leads to a weaker observed temperature dependence of the ligand affinity, since the difference in equilibrium energy for the two ligands is much lower than that of ligand binding to pentacoordinated hemoglobin. This effect could be of biological relevance for certain organisms, since it would lead to an oxygen affinity that is nearly independent of temperature. IUBMB Life, 56: 709‐719, 2004

Cytoglobin conformations and disulfide bond formation

The oligomeric state and kinetics of ligand binding were measured for wild‐type cytoglobin. Cytoglobin has the classical globin fold, with an extension at each extremity of about 20 residues. The extended length of cytoglobin leads to an ambiguous interpretation of its oligomeric state. Although the hydrodynamic diameter corresponds to that of a dimer, it displays a mass of a single subunit, indicating a monomeric form. Thus, rather than displaying a compact globular form, cytoglobin behaves hydrodynamically like a tightly packed globin with a greater flexibility of the N‐ and C‐terminal regions. Cytoglobin displays biphasic kinetics after the photolysis of CO, as a result of competition with an internal protein ligand, the E7 distal histidine. An internal disulfide bond may form which modifies the rate of dissociation of the distal histidine and apparently leads to different cytoglobin conformations, which may affect the observed oxygen affinity by an order of magnitude.

Structure and function evolution in the superfamily of globins

The superfamily of globins has emerged some 4000 Myr from a common ancestor, which was among the basic protein components required for life. Globins are present in the three kingdoms of life. From a structure point of view, these molecules are defined by the presence of a characteristic protein fold, rich in alpha-helix, surrounding a heme group. Depending on the species or organs, they may be physiologically active as monomers, tetramers or large size polymers. Their function varies from the classical reversible binding of oxygen for transport and storage to cytoprotection against reactive oxygen species, NO scavenging, signaling in oxygen dependent metabolic pathways, or possibly other specific properties involving ligand or electron transfer. All these aspects are discussed in this review.

Hyperthermal stability of neuroglobin and cytoglobin.

Neuroglobin (Ngb) and cytoglobin (Cygb), recent additions to the globin family, display a hexa‐coordinated (bis‐histidyl) heme in the absence of external ligands. Although these proteins have the classical globin fold they reveal a very high thermal stability with a melting temperature (Tm) of 100 °C for Ngb and 95 °C for Cygb. Moreover, flash photolysis experiments at high temperatures reveal that Ngb remains functional at 90 °C. Human Ngb may have a disulfide bond in the CD loop region; reduction of the disulfide bond increases the affinity of the iron atom for the distal (E7) histidine, and leads to a 3 °C increase in the Tm for ferrous Ngb. A similar Tm is found for a mutant of human Ngb without cysteines. Apparently, the disulfide bond is not involved directly in protein stability, but may influence the stability indirectly because it modifies the affinity of the distal histidine. Mutation of the distal histidine leads to lower thermal stability, similar to that for other globins. Only globins with a high affinity of the distal histidine show the very high thermal stability, indicating that stable hexa‐coordination is necessary for the enhanced thermal stability; the CD loop which contains the cysteines appears as a critical region in the neuroglobin thermal stability, because it may influence the affinity of the distal histidine.