Ionization of the heme propionate substituents in pseudomonad cytochromes c‐551

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Abstract
The heme propionate substituents in Pseudomonas cytochrome c‐551 are partially buried by folds of polypeptide in the structure of the protein, and are involved in several hydrogen bonds. The ionization behavior of these groups has been of interest because the oxidation potential of the heme changes with pH in a manner that may parallel ionization of a propionate. The ionization pK a's of these groups have been determined by following the NMR chemical shifts of nearby protons acting as probes of the ionization state of the propionates. In Pseudomonas aeruginosa c‐551 the 13‐propionate (IUB‐IUPAC porphyrin nomenclature) has been assigned a pK a of 3.1, and the 17‐propionate a pK a of 7.2. In the homologous Pseudomonas stutzeri c‐551, the respective propionates both have pK a values of 3.0.