Effective Concentrations of Amino Acid Side Chains in an Unfolded Protein

Kamalam Muthukrishnan, Barry T. Nail

Research output: Contribution to journalArticlepeer-review

90 Scopus citations


Preferential interactions between chain segments are studied in unfolded cytochrome c. The method takes advantage of heme ligation in the unfolded protein, a feature unique to proteins with covalently attached heme. The approach allows estimation of the effective concentration of one polypeptide chain segment relative to another, and is successful in detecting differences for peptide chain segments separated by different numbers of residues in the linear sequence. The method uses proton NMR spectroscopy to monitor displacement of the histidine heme ligands by imidazole as guanidine hydrochloride unfolded cytochrome c is titrated with deuterated imidazole. When the imidazole concentration exceeds the effective (local) concentration of histidine ligands, the protein ligands are displaced by deuterated imidazole. On displacement, the histidine ring proton resonances move from the paramagnetic region of the spectrum to the diamagnetic region. Titrations have been carried out for members of the mitochondrial cytochrome c family that contain different numbers of histidine residues. These include cytochromes c from tuna (2), yeast iso-2 (3), and yeast iso-l-MS (4). At high imidazole concentration, the number of proton resonances that appear in the histidine ring C2H region of the NMR spectrum is one less than the number of histidine residues in the protein. So one histidine, probably His-18, remains as a heme ligand. The effective local concentrations of histidines-26, −33, and −39 relative to the heme (position 14–17) are estimated to be (3–16) × 10−3 M. Tentative residue-specific resonance assignments for the displaced histidine C2H ring protons suggest that the effective concentrations of histidine side chains relative to the heme have the order: His-18 ≫ His-26 > His-33 ~ His-39. For cytochromes c with more than two histidine side chains, there are more heme ligands than heme coordination sites. While ligation preferences are discernible, there is sharing of coordination sites among the available histidine ligands. While it is clear that imidazole titrations are capable of detecting structure in unfolded cytochrome c, present results at high denaturant concentrations are largely consistent with those expected for a random-coil polypeptide chain.

Original languageEnglish (US)
Pages (from-to)4706-4710
Number of pages5
Issue number19
StatePublished - May 1 1991
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry


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