Bohr effect of human hemoglobin A: Magnitude of negative contributions determined by the equilibrium between two tertiary structures

Hemoglobin structure at higher levels of hemoglobin A1C in type 2 diabetes and associated complications

BACKGROUND

Fourier transform infrared (FTIR) spectroscopy technique has not been used as a diagnostic tool for diabetes in clinical practice. This study was linked to structural changes in hemoglobin (Hb) in type 2 diabetes patients at higher levels of HbA1C using FTIR spectroscopy.

METHODS

Fifty-three diabetic patients from the Bahawal Victoria Hospital, Bahawalpur, Pakistan were categorized as group A (6% < HbA1C < 7%; n = 25) and group B (HbA1C ≥9%; n = 28). Another group (group N) of twenty blood samples was taken from healthy people from the Islamia University Bahawalpur, Pakistan. Data from all groups were collected from January 1, 2018 to March 31, 2019. The structure of Hb was studied by FTIR spectroscopy and impact of glucose on the fine structure of HbA1C was estimated.

RESULTS

Hb secondary structure erythrocyte parameters were altered by changing glucose concentrations. From FTIR spectra of all three groups it was found that Hb structure was slightly altered in group A, but significantly changed in group B (P < 0.05). There was an increase in β-sheet structure and a reduction in α-helix structure at elevated levels of HbA1C (group B) in type 2 diabetes.

CONCLUSION

We suggest that higher level of glycation reflected by increased HbA1C might be a contributing factor to structural changes in Hb in type 2 diabetic patients. FTIR spectroscopy can be a novel technique to find pathogenesis in type 2 diabetes.

R and R2 quaternary structures of carbonmonoxyhemoglobins: Differential effect of inositol hexakisphosphate on their affinity for Ellman's reagent

The reaction of 5,5'-dithiobis(2-nitrobenzoate), DTNB, with hemoglobin sulfhydryl groups is linked to three negatively contributing Bohr effect groups: His2β is present in all avian hemoglobins but absent in some mammalian hemoglobins; His77β and His143β are absent in avian but present in nearly all mammalian hemoglobins. To probe the consequences of these differences, we determined the influence of inositol hexakisphosphate (inositol-P₆) on the DTNB affinities of avian and mammalian carbonmonoxyhemoglobins. Inositol-P₆decreases by two orders of magnitude the DTNB affinity of guinea pig hemoglobin, which has His2β and the R2 quaternary structure. It decreases, or has no effect on, the DTNB affinities of hemoglobins that have His2β and whose structures lie along the R2 ⇌ R quaternary equilibrium. Finally, inositol-P₆increases by one to two orders of magnitude the DTNB affinities of hemoglobins that lack His2β. Thus His2β, DTNB and inositol-P₆, in combination, distinguish the R2 from the R quaternary structure.

Bohr effect and oxygen affinity of carp, eel and human hemoglobin: Quantitative analyses provide rationale for the Root effect

The functional properties of most fish hemoglobins are more complex than those of human hemoglobin. This complexity arises in the form of the Root effect, in which the oxygen affinity of such fish hemoglobins decreases rapidly with pH relative to that of human hemoglobin. Cooperative ligand binding is also diminished below pH ≈ 6.5. The Bohr effect, determined by acid-base titration, has been reported for the Root effect carp and anodic eel hemoglobins. Unlike for mammalian hemoglobins, the Wyman equation for the Bohr effect fails to account quantitatively for these Bohr data. We present a successful quantitative accounting for these data based on evidence for multiple T states in various fish hemoglobins and on their lack of sixhistidine Bohr groups, with pK^oxy > pK^deoxy. On the same bases we also provide a rationale for the higher pH sensitivity of the oxygen affinity of carp compared to human hemoglobin.

Bohr effect of human hemoglobin: Separation of tertiary and quaternary contributions based on the Wyman equation

As a prelude to separating tertiary from quaternary structure contributions to the Bohr effect, we employed the Wyman equation to analyze Bohr data for human hemoglobin to which 2,3-bisphosphoglycerate, 2,3-BPG, is bound. Changes in the pK_as of the histidine Bohr groups result in a net reduction of their contributions to the Bohr effect at pH 7.4 compared to their contributions in stripped hemoglobin. The non-histidine 2,3-BPG binding groups - the β-chain terminal amino group and Lys82β - make negative and positive contributions, respectively, to the Bohr effect. The final result is that the Bohr effect at physiological pH is higher for 2,3-BPG bound compared to stripped hemoglobin. Contributions linked to His2β, His77β and His143β enable us to separate tertiary from quaternary Bohr contributions in stripped and in 2,3-BPG bound hemoglobin. Both contributions serve to make the Bohr effect for 2,3-BPG bound hemoglobin higher than for stripped hemoglobin at physiological pH.

The impact of the HbA1c level of type 2 diabetics on the structure of haemoglobin

This study explores the impact of HbA₁c levels on the structure of haemoglobin (Hb) in patients with type 2 diabetes. Seventy-four diabetic patients were classified into the following two groups based on their level of HbA₁c: group A, patients with good glycaemic control (HbA₁c < 7.0%, n = 36); group B, patients with persistent hyperglycaemia (HbA₁c ≥ 9.0%, n = 38). Thirty-four healthy people served as controls (group H). Hb structure was examined by Fourier transform infrared spectroscopy (FTIR), and diabetic erythrocytes were modelled to estimate the impact of glucose on these cells and Hb. Increasing glucose concentrations altered both erythrocyte parameters and the Hb secondary structure. Group B differed significantly from group H (p < 0.05): in the former, the ordered Hb secondary structure had a strong tendency to transform into a disordered secondary structure, decreasing structural stability. We presumed here that high HbA₁c levels might be a factor contributing to Hb structural modifications in diabetic patients. FTIR spectral analysis can provide a novel way to investigate the pathogenesis of type 2 diabetes mellitus.