Heme orientation affects holo-myoglobin folding and unfolding kinetics.

The research article discusses the impact of heme orientation on the folding and unfolding kinetics of holo-myoglobin, a type of protein. The study shows that the two forms of native myoglobin, differing in heme orientation, exhibit different folding/unfolding kinetics despite having identical absorption spectra and equilibrium-thermodynamic stability.

Understanding the Research

• The study bases its conclusions on observations made on native myoglobin (Mb), a globular protein found in muscle tissue. Myoglobin’s main function is to store oxygen in muscle cells, allowing for the delivery of oxygen when it is needed for energy production.
• The research explains that there are two primary populations of this protein, which are differentiated based on the 180-degree rotation of a component known as heme. Heme is an iron-containing compound that allows the protein to transport oxygen.
• Even though these two versions of myoglobin have identical absorption spectra (the way they absorb light at different wavelengths) and the same equilibrium-thermodynamic stability (meaning they are equally stable under normal conditions), they exhibit different behaviors when it comes to unfolding and refolding.

Folding and Unfolding Kinetics

• The authors state that both fractions of native oxidized myoglobin (derived from horses) unfold and refold in two parallel kinetic reactions. This is, the rates at which the proteins transform from a folded to an unfolded state and vice versa differ significantly, with one form showing a ten-fold difference in the rate constants.
• The term ‘kinetics’ refers to the rate at which these transitions happen. In protein studies, understanding the kinetics of folding and unfolding processes can provide significant insights into how these molecules function and respond to changes in their environment.
• The research also suggests that when the protein is denatured with the introduction of a chemical denaturant (and maintained at pH 7, 20°C), the oxidized heme remains coordinated to unfolded horse myoglobin even in up to 4 M guanidine hydrochloride. Like folding/unfolding kinetics, protein denaturation is another fundamental consequence of changes in the environment of a protein, and understanding it can reveal more about how the protein functions.