Cloning, expression and physicochemical characterization of a di-heme cytochrome c(4) from the psychrophilic bacterium Pseudoalteromonas haloplanktis TAC 125

The 20-kDa di-heme cytochrome c(4) from the psycrophilic bacterium Pseudoalteromonas haloplanktis TAC 125 was cloned and expressed in Escherichia coli and investigated through UV-vis and H-1 NMR spectroscopies and protein voltammetry. The model structure was computed using the X-ray structure of Pseudomonas stutzeri cytochrome c(4) as a template. The protein shows unprecedented properties within the cytochrome c(4) family, including (1) an almost nonpolar surface charge distribution, (2) the absence of high-spin heme Fe(III) states, indicative of a thermodynamically stable and kinetically inert axial heme His,Met coordination, and (3) identical E degrees' values for the two heme centers (+0.322 V vs the standard hydrogen elecrode). At pH extremes, both heme groups undergo the 'acid' and 'alkaline' conformational transitions typical of class I cytochromes c, involving ligand-exchange equilibria, whereas at intermediate pH values their electronic properties are sensitive to several residue ionizations.