Hydrogen-bonding network in heme active site regulates the hydrolysis activity of myoglobin

• Mb, L29H Mb and L29E Mb exhibit a distinct pKa value of the heme coordinated water molecule.
• L29E Mb-fluoride complex is more stable by multiple distal hydrogen-bonding interactions.
• L29H Mb undergoes an intermediate with bis-His coordination in pH-jump induced unfolding.
• The heme distal hydrogen-bonding network regulates the hydrolysis activity of Mbs.
• This study provides insights into the structure and hydrolysis activity relationship of heme proteins.

Although heme proteins are extensively studied, the structure and hydrolysis activity relationship of heme proteins is still not well understood. We herein made a comparison of the heme active site of myoglobin (Mb) and its two distal mutants, L29H Mb and L29E Mb, and found that the heme distal hydrogen-bonding network regulates the hydrolysis activity, with L29E Mb being more reactive. Fluoride competitive binding studies revealed that L29E Mb has an axial water molecule readily replaced by fluoride anion, whereas L29H Mb has a much more stable axial water molecule. pH titration studies showed that L29E Mb exhibits a lower pKa and L29H Mb exhibits a higher pKa of the heme coordinated water molecule compared to that of Mb, rationalizing the relative hydrolysis activities. Moreover, pH-jump induced unfolding studies suggested that the heme distal hydrogen-bonding network, especially in L29H Mb, plays a key role in stabilization of the protein. This study thus provides valuable information for understanding the structure and hydrolysis activity relationship of heme proteins, as well as for rational protein design with the knowledge of structural and functional diversity of heme proteins.

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