Molecular oxygen migration through the xenon docking sites of human hemoglobin in the R-state.

A nanosecond laser flash-photolysis technique was used to study bimolecular and geminate molecular oxygen (O2) rebinding to tetrameric human hemoglobin and its isolated α and ß chains in buffer solutions equilibrated with 1atm of air and up to 25atm of xenon. Xenon binding to the isolated α chains and to the α subunits within tetrameric hemoglobin was found to cause a decrease in the efficiency of O2 escape by a factor of ~1.30 and 3.3, respectively. A kinetic model for O2 dissociation, rebinding, and migration through two alternative pathways in the hemoglobin subunits was introduced and discussed. It was shown that, in the isolated α chains and α subunits within tetrameric hemoglobin, nearly one- and two-third escaping molecules of O2 leave the protein via xenon docking sites, respectively. The present experimental data support the idea that O2 molecule escapes from the ß subunits mainly through the His(E7) gate, and show unambiguously that, in the α subunits, in addition to the direct E7 channel, there is at least one alternative escape route leading to the exterior via the xenon docking sites.

Gastrointestinal bioavailability of 2.0 nm diameter gold nanoparticles.

The use of gold nanoparticles as imaging agents and therapeutic delivery systems is growing rapidly. However, a significant limitation of gold nanoparticles currently is their low absorption efficiencies in the gastrointestinal (GI) tract following oral administration. In an attempt to identify ligands that facilitate gold nanoparticle absorption in the GI tract, we have studied the oral bioavailability of 2.0 nm diameter gold nanoparticles modified with the small molecules p-mercaptobenzoic acid and glutathione, and polyethylene glycols (PEG) of different lengths and charge (neutral and anionic). We show that GI absorption of gold nanoparticles modified with the small molecules tested was undetectable. However, the absorption of PEGs depended upon PEG length, with the shortest PEG studied yielding gold nanoparticle absorptions that are orders-of-magnitude larger than observed previously. As the oral route is the most convenient one for administering drugs and diagnostic reagents, these results suggest that short-chain PEGs may be useful in the design of gold nanoparticles for the diagnosis and treatment of disease.

Effects of amino acid substitutions at beta 131 on the structure and properties of hemoglobin: evidence for communication between alpha 1 beta 1- and alpha 1 beta 2-subunit interfaces.

Substitutions of Asn, Glu, and Leu for Gln at the beta131 position of the hemoglobin molecule result in recombinant hemoglobins (rHbs) with moderately lowered oxygen affinity and high cooperativity compared to human normal adult hemoglobin (Hb A). The mutation site affects the hydrogen bonds present at the alpha(1)beta(1)-subunit interface between alpha103His and beta131Gln as well as that between alpha122His and beta35Tyr. NMR spectroscopy shows that the hydrogen bonds are indeed perturbed; in the case of rHb (beta131Gln --> Asn) and rHb (beta131Gln --> Leu), the perturbations are propagated to the other alpha(1)beta(1)-interface H-bond involving alpha122His and beta35Tyr. Proton exchange measurements also detect faster exchange rates for both alpha(1)beta(1)-interface histidine side chains of the mutant rHbs in 0.1 M sodium phosphate buffer at pH 7.0 than for those of Hb A under the same conditions. In addition, the same measurements in 0.1 M Tris buffer at pH 7.0 show a much slower exchange rate for mutant rHbs and Hb A. One of the mutants, rHb (beta131Gln --> Asn), shows the conformational exchange of its interface histidines, and exchange rate measurements have been attempted. We have also conducted studies on the reactivity of the SH group of beta93Cys (a residue located in the region of the alpha(1)beta(2)-subunit interface) toward p-mercuribenzoate, and our results show that low-oxygen-affinity rHbs have a more reactive beta93Cys than Hb A in the CO form. Our results indicate that there is communication between the alpha(1)beta(1)- and alpha(1)beta(2)-subunit interfaces, and a possible communication pathway for the cooperative oxygenation of Hb A that allows the alpha(1)beta(1)-subunit interface to modulate the functional properties in conjunction with the alpha(1)beta(2) interface is proposed.

Site-directed mutagenesis in hemoglobin: test of functional homology of the F9 amino acid residues of hemoglobin alpha and beta chains.

The cysteine residue at F9(93) of the human hemoglobin (Hb A) beta chain, conserved in mammalian and avian hemoglobins, is located near the functionally important alpha1-beta2 interface and C-terminal region of the beta chain and is reactive to sulfhydryl reagents. The functional roles of this residue are still unclear, although regulation of local blood flow through allosteric S-nitrosylation of this residue is proposed. To clarify the role of this residue and its functional homology to F9(88) of the alpha chain, we measured oxygen equilibrium curves, UV-region derivative spectra, Soret-band absorption spectra, the number of titratable -SH groups with p-mercuribenzoate and the rate of reaction of these groups with 4, 4'-dipyridine disulfide for three recombinant mutant Hbs with single amino acid substitutions: Ala-->Cys at 88alpha (rHb A88alphaC), Cys-->Ala at 93beta (rHb C93betaA) and Cys-->Thr at 93beta (rHb C93betaT). These Hbs showed increased oxygen affinities and impaired allosteric effects. The spectral data indicated that the R to T transition upon deoxygenation was partially restricted in these Hbs. The number of titratable -SH groups of liganded form was 3.2-3.5 for rHb A88alphaC compared with 2.2 for Hb A, whereas those for rHb C93betaA and rHb C93betaT were negligibly small. The reduction of rate of reaction with 4,4'-dipyridine disulfide upon deoxygenation in rHb A88alphaC was smaller than that in Hb A. Our experimental data have shown that the residues at 88alpha and 93beta have definite roles but they have no functional homology. Structure-function relationships in our mutant Hbs are discussed.