Fibrillar superstructure formation of hemoglobin A and its conductive, photodynamic and photovoltaic effects.

The fabrication of biomaterials which serve as functional scaffolds exhibiting diversified effects has been valued. We report here a unique strategy to fibrillate hemoglobin A (HbA), which exhibits multiple photoelectrochemical properties, and a subsequent specific defibrillation procedure. A subtle structural rearrangement of the α/ß-subunits within the quaternary structure of HbA is responsible for the HbA fibril formation in the presence of 0.5% CHCl3. The narrow pH dependence of the suprastructure formation around pH 7.4 illustrates the highly sensitive nature of the structural alteration. The CHCl3-induced fibrils become disintegrated by ascorbic acid, indicating that the oxidation-reduction process of the iron within the heme moiety could be involved in stabilization of the fibrillar structures. The electron-transferring property of the iron allows the fibrils to exhibit not only their conductive behavior but also a photodynamic effect generating hydroxyl radicals in the presence of H(2)O(2) with light illumination. A photovoltaic effect is also demonstrated with the HbA fibrils, which generate an electric current on the fibril-coated microelectrode upon irradiation at 405nm. Taken together, the multiple effects of HbA fibrils and the selective fibrillation/defibrillation procedures could qualify the fibrils to be employed for various future applications in biotechnology, including bio-machine interfaces.

Molecular dynamics simulations of hemoglobin A in different states and bound to DPG: effector-linked perturbation of tertiary conformations and HbA concerted dynamics.

Recent functional studies reported on human adult hemoglobin (HbA) show that heterotropic effector-linked tertiary structural changes are primarily responsible for modulating the oxygen affinity of hemoglobin. We present the results of 6-ns molecular dynamics simulations performed to gain insights into the dynamical and structural details of these effector-linked tertiary changes. All-atom simulations were carried out on a series of models generated for T- and R-state HbA, and for 2,3-diphosphoglycerate-bound models. Cross-correlation analyses identify both intra- and intersubunit correlated motions that are perturbed by the presence of the effector. Principal components analysis was used to decompose the covariance matrix extracted from the simulations and reconstruct the trajectories along the principal coordinates representative of functionally important collective motions. It is found that HbA in both quaternary states exists as ensembles of tertiary conformations that introduce dynamic heterogeneity in the protein. 2,3-Diphosphoglycerate induces significant perturbations in the fluctuations of both HbA states that translate into the protein visiting different tertiary conformations within each quaternary state. The analysis reveals that the presence of the effector affects the most important components of HbA motions and that heterotropic effectors modify the overall dynamics of the quaternary equilibrium via tertiary changes occurring in regions where conserved functionally significant residues are located, namely in the loop regions between helices C and E, E and F, and F and G, and in concerted helix motions. The changes are not apparent when comparing the available x-ray crystal structures in the presence and absence of effector, but are striking when comparing the respective dynamic tertiary conformations of the R and T tetramers.

Critical residues responsible for self-association differences of hemoglobin alpha and beta chains: analysis by molecular modeling.

Although the alpha and beta chains of adult human hemoglobin (Hb A) are very similar, when isolated the individual chains display marked differences in the propensities to form homotetramers: alpha chains alone associate weakly into dimers, while beta chains form relatively stable tetramers. We have examined the origin of this difference using computer-based model building and energy minimization. For oxyhemoglobin (R state) structures, interfaces have been compared for energy minimized alpha 2 beta 2, beta 4, and hypothetical alpha 4 tetramers. For the alpha 1-beta 1 interface (also designated as the X-interface) 19 alpha chain and 19 beta chain residues were identified that each contribute at least 1% to the energy of the contact in Hb A. This interface has a high degree of pseudo-symmetry, with identical residues at 6 of these positions for both chains. The geometry of the X-interface is similar for the homotetramers, with all 6 of these residues retained at the interface in beta 4 and 4 of the 6 found at the interface in alpha 4, although the alpha-alpha interface involves fewer contacts and less buried surface area. For the alpha 1-beta 2 interface (also designated as the Z-interface) 10 alpha chain and 10 beta chain residues are identified as contributing at least 1% to the energy of the contact in Hb A; about half of the contact residues are identical for corresponding positions of alpha and beta chains and most of these residues are retained at the interfaces in the two types of homotetramers, but with fewer alpha-alpha contacts.(ABSTRACT TRUNCATED AT 250 WORDS)