The interplay between basicity, conformation, and enzymatic reduction in biliverdins

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• Biliverdin reductase (BvR) catalyzes the reduction of biliverdin, a key step in heme catabolism.
• The conformation of biliverdin influences its interaction with BvR and its reduction rate.
• Understanding the molecular basis of this interaction is crucial for elucidating BvR's catalytic mechanism.

Purpose of the Study:

• To investigate the molecular basis for the differential reduction rates of biliverdin conformations by BvR.
• To correlate the acid-base properties (basicity) of biliverdins with their conformational flexibility and BvR-mediated reduction.

Main Methods:

• Acid-base equilibria titrations of various biliverdin conformations in methanol.
• Measurement of biliverdin reduction rates by BvR.
• Analysis of the relationship between biliverdin basicity, conformation, and BvR activity.

Main Results:

• Biliverdins with more extended conformations exhibit higher basicity (pKa values ranging from 3.6 to 7.9).
• Increased basicity of biliverdins directly correlates with accelerated reduction rates by BvR.
• A conformationally constrained, weakly basic biliverdin (pKa = 0.4) was not reduced by BvR.
• Nucleophilic addition at C10 by 2-mercaptoethanol parallels biliverdin basicity, suggesting a link to protonation at N23.

Conclusions:

• The basicity of biliverdin, which increases with conformational extension, is a critical factor determining its reduction rate by BvR.
• The findings elucidate a key aspect of the BvR catalytic mechanism, linking substrate conformation to enzyme activity via acid-base properties.
• This study provides molecular insights into how BvR distinguishes between different biliverdin conformations.