Microperoxidase 11: A model system for porphyrin networks and heme-protein interactions

Daniel Verbaro, Andrew Hagarman, Ajay Kohli, Reinhard Schweitzer-Stenner

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


We measured the circular dichroism (CD) and absorption spectra of the B-band region of microperoxidase 11 (MP11) as a function of temperature and peptide concentration. At micromolar concentrations, small MP11 dimers or trimers lead to excitonic coupling between low-spin and high-spin heme groups, to which the NH2 group of the MP11 N-terminal and H2O are bound as a sixth ligand, respectively. These aggregates convert into monomers with hexacoordinated high-spin heme groups with increasing temperature. This transition can be described by a two-state model. Aggregation becomes more extended at 50 μM concentration and causes some B-band hyperchromism, which reflects a J-type arrangement of heme groups linked together in the aggregates formed. At near-millimolar concentration, the CD and absorption spectra of the B-band region suggest the existence of even more extended and thermally stable aggregates, which might involve μ-oxo dimers of the heme groups. The degree of aggregation at 50 and 500 μM concentration increases substantially if the sample is freed from most of its oxygen in a N2 atmosphere. The CD spectrum of the monomeric high-spin species is reminiscent of that observed for the unfolded alkaline conformation of the intact protein. Finally, we investigated the binding of acetylmethionine (AcM) ligands to the heme at aggregation-supporting conditions (500 μM concentration). The data suggest that the ligand prevents any substantial aggregation. As a surprising result, our data reveal that AcM-MP11 complexes exhibit a high-spin/low-spin mixture, with the high-spin configuration being stabilized at high temperatures.

Original languageEnglish (US)
Pages (from-to)1289-1300
Number of pages12
JournalJournal of Biological Inorganic Chemistry
Issue number8
StatePublished - Nov 2009
Externally publishedYes


  • Cytochrome
  • Electronic structure
  • Heme
  • Peptide
  • Porphyrin

ASJC Scopus subject areas

  • Biochemistry
  • Inorganic Chemistry


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