Article,
Characterization of axial and proximal histidine mutations of the decaheme cytochrome MtrA from Shewanella sp. str. ANA-3: Implications for the electron transport mechanism
journal of bacteriology, (August 2012)
Abstract
Extracellular respiration of solid phase electron acceptors in some microorganisms, require a complex chain of multiheme c-type cytochromes that span the inner and outer membrane. In Shewanella species, MtrA, a ∼35 KDa periplasmic decaheme c-type cytochrome, is an essential component for extracellular respiration of iron(III). The exact mechanism of electron transport has not been resolved yet. But the arrangement of the polypeptide chain may have a strong influence on the capability of the MtrA cytochrome to transport electrons. The iron hemes of MtrA are bound to its polypeptide chain via proximal (CXXCH) and distal histidine residues. In this study we show the effects of mutating histidine residues of MtrA to arginine on protein expression and extracellular respiration using Shewanella sp. str. ANA-3 as a model organism. Individual mutations to six out of nine proximal histidines in CXXCH of mtrA led to decreased protein expression. However, distal histidine mutations resulted in varying degrees of protein expression. In addition, the effects of histidine mutations on extracellular respiration were tested using ferrihydrite and current production in microbial fuel cells. These results show that proximal histidine mutants were unable to reduce ferrihydrite. Mutations to the distal histidine residues resulted in varying degrees of ferrihydrite reduction. These findings indicate that mutations to the proximal histidine residues affect MtrA expression leading to loss of extracellular respiration ability. In contrast mutations to the distal histidine residues are less detrimental to protein expression and extracellular respiration can proceed.
