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

Sara Bari, Rosalía B. Frydman, Claudio Grosman, Benjamfn Frydman

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Biliverdins with extended conformations are reduced by biliverdin reductase (BvR) at higher rates than biliverdins with helical conformations. To find out the molecular basis for this important feature of BvR mechanism, helical and extended biliverdins were titrated for their acid-base equilibria in a protic solvent (methanol). It was found that the basicity of biliverdins increases with the stretching of the conformation. Biliverdin IX γ (all-syn) has a pKa = 3.6; 5,10,15-syn,syn,anti-biliverdin has a pKa = 3.7; 5,10,15-syn,anti,syn-biliverdin has a pKa = 6.1; 5,10,15-syn,anti,anti-biliverdin has a pKa = 6.4; and 5,10,15-all-anti-biliverdin has a pKa = 7.9. The increase in basicity with progressive stretching of conformations closely parallels the increase in the reduction rates by BvR. A biliverdin constrained by a four carbon chain to a helical conformation and which is a very weak base (pKa = 0.4) is not reduced by BvR. Nucleophilic additions of 2-mercaptoethanol at the C10 in biliverdins closely parallel their basicities, as can be expected if the formation of a positive mesomeric species at CIO is linked to the basicity (i.e., the ease of protonation) of the N23 on the pyrrolenine ring.

Original languageEnglish (US)
Pages (from-to)48-56
Number of pages9
JournalBiochemical and Biophysical Research Communications
Issue number1
StatePublished - Oct 15 1992
Externally publishedYes

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Molecular Biology
  • Cell Biology


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