Kinetics and mechanism of hemoglobin denaturation in alkali

pHinfluenced metal ion coordination changes in reconstituted hemoglobin

This paper covers a detailed analysis of the coordination changes taking place at the active sites in both Cu and Ni reconstituted hemoglobin as a function of pH . These experiments provide insight into how proteins are held in their native configuration. The EPR results of CuHb reveal that the species formed in extreme acidic condition were different from those formed at extreme basic condition. At pH 3 we see an isotropic spectrum characteristic of 4-coordinated species, while at pH 12 there is an indication of equilibrium between mixtures of species. Further support for the above coordination changes is obtained from FT-Raman of NiHb at different pH conditions. At pH 3 all the 5-coordination marker bands are lost and there is a shift in the 4-coordination marker band, while at pH 12 both 4- and 5-coordination marker bands are still seen with slight shift in their positions. In addition to this, we could see a new peak at 1633 cm−1. The coordination changes as a function of pH could be seen for both CuHb and NiHb using UV-visible spectroscopic techniques.

Characterization of the structural and functional changes of hemoglobin in dimethyl sulfoxide by spectroscopic techniques

Circular dichroism (CD), fourier transform infrared (FTIR), and fluorescence spectroscopy were used to explore the effect of dimethyl sulfoxide (DMSO) on the structure and function of hemoglobin (Hb). The native tertiary structure was disrupted completely when the concentration of DMSO reached 50% (v/v), which was determined by loss of the characteristic Soret CD spectrum. Loss of the native tertiary structure could be mainly caused by breaking the hydrogen bonds, between the heme propionate groups and nearby surface amino acid residues, and by disorganizing the hydrophobic interior of this protein. Upon exposure of Hb to 52% DMSO for ca. 12 h in a D2O medium no significant change in 1652 cm-1 band of the FTIR spectrum was produced, which demonstrated that alpha-helical structure predominated. When the concentration of DMSO increased to 57%: (1) the band at 1652 cm-1 disappeared with the appearance of two new bands located at 1661 and 1648 cm-1; (2) another new band at 1623 cm-1 was attributed to the formation of intermolecular beta-sheet or aggregation, which was the direct consequence of breaking of the polypeptide chain by the competition of S&z.dbnd6;O groups in DMSO with C&z.dbnd6;O groups in amide bonds. Further increasing the DMSO concentration to 80%, the intensity at 1623 cm-1 increased, and the bands at 1684, 1661 and 1648 cm-1 shifted to 1688, 1664 and 1644 cm-1, respectively. These changes showed that the native secondary structure of Hb was lost and led to further aggregation and increase of the content of 'free' amide C&z.dbnd6;O groups. In pure DMSO solvent, the major band at 1664 cm-1 indicated that almost all of both the intermolecular beta-sheet and any residual secondary structure were completely disrupted. The red shift of the fluorescence emission maxima showed that the tryptophan residues were exposed to a greater hydrophilic environment as the DMSO content increased. CO-binding experiment suggested that the biological function of Hb was disrupted seriously even if the content of DMSO was 20%.

A combined nuclear magnetic resonance and absorbance stopped-flow apparatus for biochemical studies

A combined NMR and absorbance stopped-flow has been developed for monitoring the kinetics of biochemical reactions. We demonstrate its usefulness in following the alkaline denaturation of human hemoglobin. No glassblowing is required in the fabrication of the apparatus. Commercially available valves, syringes, tubing, and tubing connectors are employed whenever possible. Easily fabricated light guides are used to pipe light to and from the optical cell. The stopped-flow uses a 5-mm NMR tube followed by an optical cell with a 0.5-mm optical path length. This allows simultaneous measurements of NMR and absorbance changes. At a terminal flow velocity of 7.5 ml/s, the NMR and optical dead times were 60 and 260 ms, respectively. For the study reported here the oxyhemoglobin was labeled with a 19F probe attached to the beta-93 cysteine. The native protein at pH 7 has an NMR spectrum consisting of a singlet, and the fully denatured hemoglobin sample at pH 12 has a spectrum consisting of three singlets. During the denaturation process another NMR peak appears rapidly and then decays away over the time course of the reaction. The absorbance changes at the high concentration employed for the NMR study (2.08 mM in heme) follow very nearly first-order kinetics. The events monitored by NMR, though in the same time frame as the optical changes, are of much greater complexity, and show the utility of multiple probes for monitoring protein unfolding.