Kinetic study of CO and O2 binding to horse heart myoglobin reconstituted with synthetic hemes lacking methyl and vinyl side chains.

This research takes a look at how the binding rates and affinities of carbon monoxide (CO) and oxygen (O2) vary for horse heart myoglobins reconstituted with synthetic hemes that lack peripheral methyl and vinyl groups. It finds that there is a correlation between heme size and ligand specificity, but the correlation fails with the most dealkylated heme due to protein conformational changes.

Details of Research

• The study conducts experiments on horse heart myoglobins, proteins typically associated with muscle tissue in vertebrates, including horses. These proteins carry oxygen from the lungs to the muscle tissues. They are reconstituted with synthetic hemes, a part of hemoglobin and myoglobin, lacking peripheral methyl and vinyl groups. The exclusion of these groups from the synthetic heme molecules is central to the study.
• The researchers measure the binding rates and affinities of carbon monoxide and oxygen with these differently constructed myoglobins. Binding rates refer to the speed at which these gases form a complex with the myoglobin, while affinity refers to the strength of the binding.
• The results show an apparent correlation between the heme size and ligand specificity. In this context, ligand specificity refers to the selectivity of the heme protein toward binding with either CO or O2. Hemes with a larger m value, indicating a greater ratio of CO vs O2 association rates, tend to be smaller in size.

Breaking Down of Correlation

• The study notes that this correlation between heme size and ligand specificity breaks down when it comes to the most dealkylated heme, i.e., the heme lacking the most peripheral methyl and vinyl groups. The researchers interpret this as the result of conformational changes in the protein that occur due to the altered heme structure.
• These conformational changes, cause an alteration in the steric crowdedness at the O2-binding site. Steric crowdedness refers to the spatial arrangement of atoms or groups within the myoglobin protein. Changes in this arrangement could affect the accessibility of the site to inbound oxygen atoms, hence altering binding rates and affinities.

Supporting Observations

• The research also points to additional evidence supporting their observations. The spectral properties of the heme (the absorption, emission or scattering of light by the molecule), and the rates of autoxidation (a reaction of oxygen with organic compounds occurring in the absence of external influences) back the researchers’ view of the impact of heme size and protein conformational changes on the binding of CO and O2.