Electronic and steric factors affecting ligand binding: horse hemoglobins containing 2,4-dimethyldeuteroheme and 2,4-dibromodeuteroheme.

This research paper examines how electronic and steric factors affect ligand binding in horse hemoglobin recombined with 2,4-dimethyldeuteroheme and 2,4-dibromodeuteroheme, assessing the oxygen affinity and the kinetic rate constants for oxygen dissociation in these hemoglobins.

Research Procedure

• Horse globin was recombined with 2,4-dimethyldeuteroheme and 2,4-dibromodeuteroheme to create the corresponding reconstituted hemoglobins.
• The ligand binding reactions of these reconstituted hemoglobins were analyzed in detail.

Examination of Oxygen Affinity

• The researchers note that both reconstituted hemoglobins have high n values, a commonly used measure in chemical equilibrium studies.
• However, the 2,4-dimethyldeuterohemoglobin exhibits a consistently higher oxygen affinity compared to native (natural) horse hemoglobin. This suggests that it can bond with oxygen more easily.
• Conversely, 2,4-dibromodeuterohemoglobin shows consistently lower oxygen affinity than the native horse hemoglobin, indicating a lesser readiness to bond with oxygen.

Kinetic Rate Constants for Ligand Binding and Dissociation

• The rate constants (l’ and 1’4) for the binding of the first and fourth molecules of carbon monoxide (CO) respectively for both reconstituted hemoglobins do not deviate significantly from those of the native hemoglobin.
• However, the kinetic rate constants for oxygen dissociation from these reconstituted hemoglobins are noticeably altered.
• The rate constants for oxygen dissociation in the presence of dithionite for 2,4-dimethyl- and 2,4-dibromodeuterohemoglobin are 8s-1 and 72s-1 respectively, compared to 33s-1 for native hemoglobin.

Acid-Alkaline Transition in Reconstituted Hemoglobins

• Furthermore, the study found that the pK of the acid-alkaline transition in the Met forms of the two reconstituted hemoglobins (pK’) also varied substantially between 2,4-dimethyl- and 2,4-dibromodeuterohemoglobin.
• pK is a measure of acid strength. The “Met forms” refers to the methemoglobin forms (oxidized form) of the reconstituted hemoglobins.