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Keiji Shikama

Publications and source records attributed to Keiji Shikama.

8 recordsLinked to original sources
PubMed2005

Nature of the FeO2 bonding in myoglobin and hemoglobin: A new molecular paradigm.

The iron(II)-dioxygen bond in myoglobin and hemoglobin is a subject of wide interest. Studies range from examinations of physical-chemical properties dependent on its electronic structure, to investigations of the stability as a function of oxygen supply. Among these, stability properties are of particular importance in vivo. Like all known dioxygen carriers synthesized so far with transition metals, the oxygenated forms of myoglobin and hemoglobin are known to be oxidized easily to their ferric met-forms, which cannot bind molecular oxygen and are therefore physiologically inactive. The mechanistic details of this autoxidation reaction, which are of clinical, as well as of physical-chemical, interest, have long been investigated by a number of authors, but a full understanding of the heme oxidation has not been reached so far. Recent kinetic and thermodynamic studies of the stability of oxymyoglobin (MbO2) and oxyhemoglobin (HbO2) have revealed new features in the FeO2 bonding. In vivo, the iron center is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the heme pocket from the surrounding solvent and thereby irreversibly displace the bound dioxygen from MbO2 or HbO2 in the form of O2- so that the iron is converted to the ferric met-form. Since the autoxidation reaction of MbO2 or HbO2 proceeds through a nucleophilic displacement following one-electron transfer from iron(II) to the bound O2, this reaction may be viewed as a meeting point of the stabilization and the activation of molecular oxygen performed by hemoproteins. Along with these lines of evidence, we finally discuss the stability property of human HbO2 and provide with the most recent state of hemoglobin research. The HbA molecule contains two types of alphabeta contacts and seems to differentiate them quite properly for its functional properties. The alpha1beta2 or alpha2beta1 contact is associated with the cooperative oxygen binding, whereas the alpha1beta1 or alpha2beta2 contact is used for controlling the stability of the bound O2. We can thus form a unified picture for hemoglobin function by closely integrating the cooperative and the stable binding of molecular oxygen with iron(II) in aqueous solvent. These new views on the nature of FeO2 bonding and the possible role of globin moiety in stabilizing MbO2 and HbO2 are of primary importance, not only for a full understanding of various hemoprotein reactions with O2, but also for planning new molecular designs for synthetic oxygen carriers which may be able to function in aqueous solvent and at physiological temperature.

Binding Sites↗
PubMed2003

1H-NMR study of dynamics and thermodynamics of Cl- binding to ferric hemoglobin of a midge larva (Tokunagayusurika akamusi).

The functional properties of the Arg residue at the E10 helical position in myoglobin and hemoglobin lacking the highly conserved His residue at the E7 position have received considerable interest as to the structure-function relationship of the oxygen-binding hemoproteins, because Arg E10 in such proteins has been shown to play similar roles to those His E7 plays in ordinary proteins. One of the components of hemoglobin from the larval hemolyph of Tokunagayusurika akamusi is also a naturally occurring E7 genetic variant with Ile E7 and Arg E10. This study demonstrated, for the first time, that the positively charged, elongated, and flexible side-chain of Arg E10 in T.akamusi hemoglobin contributes to stabilization of the coordination of biologically relevant Cl(-) to heme iron. Determination of the dynamics of the Cl(-) binding to T. akamusi ferric hemoglobin involving paramagnetic 1H-NMR indicated that the Cl(-) affinity increases with decreasing pH as a result of the fact that the binding rate increases with decreasing pH, whereas the dissociation rate is almost completely independent of pH. The pH-dependent character of the Cl(-) binding rate correlated well with the ionization state of heme peripheral side-chain propionate groups, which was clearly manifested in the pH-dependent shift changes of heme methyl proton signals, suggesting that negative charges of heme propionate groups constitute a kinetic barrier for Cl(-) entry into the heme pocket. These findings provide an insight into the pH-dependent ligand binding properties of T. akamusi hemoglobin.

Animals↗
PubMed2003

The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins.

Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobin VII (Ta-VII Hb) was of particular interest. When its ferric met-form was exposed to pH 5.0 from 7.2, the distal histidine was found to swing away from the E7 position. As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concomitant loss of the axial water ligand. The corresponding spectral changes in the Soret band were therefore followed by stopped-flow and rapid-scan techniques, and the observed first-order rate constants of k(out) = 25 s(-1) and kin = 128 s(-1) were obtained for the outward and inward movements, respectively, of the distal histidine residue in 0.1 m buffer at 25 degrees C. For O2 affinity, Ta-VII Hb showed a value of P50 = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (deltaH+ = -0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxidation rate over a wide range of pH from 4 to 11. The resulting pH-dependence curve was compared with those of another component Ta-V HbO2 and sperm whale MbO2, and described based on a nucleophilic displacement mechanism. In light of the O2 binding affinity, Bohr effect and considerable stability of the bound O2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O2 transport and storage as the major component in the larval hemolymph.

Amino Acid Sequence↗
PubMed2003

Human haemoglobin: a new paradigm for oxygen binding involving two types of alphabeta contacts.

This review summarizes the most recent state of haemoglobin (Hb) research based on the literature and our own results. In particular, an attempt is made to form a unified picture for haemoglobin function by reconciling the cooperative oxygen binding with the stabilization of the bound dioxygen in aqueous solvent. The HbA molecule contains two types of alphabeta contacts. One type is the alpha1beta2 or alpha2beta1 contacts, called sliding contacts, and these are strongly associated with the cooperative binding of O2 to the alpha2beta2 tetramer. The other type is the alpha1beta1 or alpha2beta2 contacts, called packing contacts, but whose role in Hb function was not clear until quite recently. However, detailed pH-dependence studies of the autoxidation rate of HbO2 have revealed that the alpha1beta1 and alpha2beta2 interfaces are used for controlling the stability of the bound O2. When the alpha1beta1 or alpha2beta2 contact is formed, the beta chain is subjected to a conformational constraint which causes the distal (E7) histidine to be tilted slightly away from the bound dioxygen, preventing the proton-catalysed nucleophilic displacement of O2- from the FeO2 by an entering water molecule. This is one of the most characteristic features of HbO2 stability. Finally we discuss the role of the alpha1beta1 or alpha2beta2 contacts by providing some examples of unstable haemoglobin mutants. These pathological mutations are found mostly on the beta chain, especially in the alpha1beta1 contact regions. In this way, HbA seems to differentiate two types of alphabeta contacts for its functional properties.

Hemoglobins↗
PubMed2002

Yeast flavohemoglobin from Candida norvegensis. Its structural, spectral, and stability properties.

Flavohemoglobin was isolated directly from the yeast Candida norvegensis and studied on its structural, spectral, and stability properties. In Candida flavohemoglobin, the 155 N-terminal residues make a heme-containing domain, while the remaining 234 C-terminal residues serve as a FAD-containing reductase domain. A pair of His-95 and Gln-63 was assigned to the proximal and distal residues, respectively. In purification procedure FAD was partially dissociated on a Butyl-Toyopearl column, so that FAD-lacking flavohemoglobin was also obtainable. In this ferric species, the Soret and charge-transfer bands were all characteristic of a penta-coordinate form. Compared with the recombinant heme domain expressed in Escherichia coli, we have measured the autoxidation rate over a wide pH range. The resulting pH dependence curves were then analyzed in terms of a nucleophilic displacement mechanism. As a result, the heme domain was found to be extremely susceptible to autoxidation, its rate being more than 100 times higher than that of sperm whale MbO2. However, this inherently high oxidation rate was dramatically suppressed in Candida flavohemoglobin to an extent almost comparable to the stability of mammalian myoglobins. These new findings lead us to conclude that Candida flavohemoglobin, differently from bacterial flavohemoglobins, can serve as an oxygen storage protein in aerobic conditions.

Amino Acid Sequence↗
PubMed2002

The alpha 1 beta 1 contact of human hemoglobin plays a key role in stabilizing the bound dioxygen.

When the alpha and beta chains were separated from human oxyhemoglobin (HbO(2)), each individual chain was oxidized easily to the ferric form, their rates being almost the same with a very strong acid-catalysis. In the HbO(2) tetramer, on the other hand, both chains become considerably resistant to autoxidation over a wide range of pH values (pH 5-11). Moreover, HbA showed a biphasic autoxidation curve containing the two rate constants, i.e. k(f) for the fast oxidation due to the alpha chains, and k(s) for the slow oxidation to the beta chains. The k(f)/k(s) ratio increased from 3.2 at pH 7.5-7.3 at pH 5.8, but became 1 : 1 at pH values higher than 8.5. In the present work, we used the valency hybrid tetramers such as (alpha(3+))2(beta O(2))(2) and (alpha O(2)(2)(beta(3+))(2), and demonstrated that the autoxidation rate of either the alpha or beta chains (when O2- ligated) is independent of the valency state of the corresponding counterpart chains. From these results, we have concluded that the formation of the alpha 1 beta 1 or alpha 2 beta 2 contact suppresses remarkably the autoxidation rate of the beta chain and thus plays a key role in stabilizing the HbO(2) tetramer. Its mechanistic details were also given in terms of a nucleophilic displacement of O(2)(-) from the FeO(2) center, and the emphasis was placed on the proton-catalyzed process performed by the distal histidine residue.

Dimerization↗
PubMedDate not supplied

Structure-function relationships in unusual nonvertebrate globins.

Based on the literature and our own results, this review summarizes the most recent state of nonvertebrate myoglobin (Mb) and hemoglobin (Hb) research, not as a general survey of the subject but as a case study. For this purpose, we have selected here four typical globins to discuss their unique structures and properties in detail. These include Aplysia myoglobin, which served as a prototype for the unusual globins lacking the distal histidine residue; midge larval hemoglobin showing a high degree of polymorphism; Tetrahymena hemoglobin evolved with a truncated structure; and yeast flavohemoglobin carrying an enigmatic two-domain structure. These proteins are not grouped by any common features other than the fact they have globin domains and heme groups. As a matter of course, various biochemical functions other than the conventional oxygen transport or storage have been proposed so far to these primitive or ancient hemoglobins or myoglobins, but the precise in vivo activity is still unclear. In this review, special emphasis is placed on the stability properties of the heme-bound O2. Whatever the possible roles of nonvertebrate myoglobins and hemoglobins may be (or might have been), the binding of molecular oxygen to iron(II) must be the primary event to manifest their physiological functions in vivo. However, the reversible and stable binding of O2 to iron(II) is not a simple process, since the oxygenated form of Mb or Hb is oxidized easily to its ferric met-form with the generation of superoxide anion. The metmyoglobin or methemoglobin thus produced cannot bind molecular oxygen and is therefore physiologically inactive. In this respect, protozoan ciliate myoglobin and yeast flavohemoglobin are of particular interest in their very unique structures. Indeed, both proteins have been found to have completely different strategies for overcoming many difficulties in the reversible and stable binding of molecular oxygen, as opposed to the irreversible oxidation of heme iron(II). Such comparative studies of the stability of MbO2 or HbO2 are of primary importance, not only for a full understanding of the globin evolution, but also for planning new molecular designs for synthetic oxygen carriers that may be able to function in aqueous solution and at physiological temperature.

Amino Acid Sequence↗