title: Heme-bound SiaA from Streptococcus pyogenes: Effects of mutations and oxidation state on protein stability.
abstract: The protein SiaA (HtsA) is part of a heme uptake pathway in Streptococcus pyogenes. In this report, we present the heme binding of the alanine mutants of the axial histidine (H229A) and methionine (M79A) ligands, as well as a lysine (K61A) and cysteine (C58A) located near the heme propionates (based on homology modeling) and a control mutant (C47A). pH titrations gave pKa values ranging from 9.0 to 9.5, close to the value of 9.7 for WT SiaA. Resonance Raman spectra of the mutants suggested that the ferric heme environment may be distinct from the wild-type; spectra of the ferrous states were similar. The midpoint reduction potential of the K61A mutant was determined by spectroelectrochemical titration to be 61±3mV vs. SHE, similar to the wild-type protein (68±3mV). The addition of guanidine hydrochloride showed two processes for protein denaturation, consistent with heme loss from protein forms differing by the orientation of the heme in the binding pocket (the half-life for the slower process ranged from less than half a day to two days). The ease of protein unfolding was related to the strength of interaction of the residues with the heme. We hypothesize that kinetically facile but only partial unfolding, followed by a very slow approach to the completely unfolded state, may be a fundamental attribute of heme trafficking proteins. Small motions to release/transfer the heme accompanied by resistance to extensive unfolding may preserve the three dimensional form of the protein for further uptake and release.
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA.
authors.forename: Neval
authors.initials: N
authors.lastname: Akbas
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA.
authors.forename: Elizabeth B
authors.initials: EB
authors.lastname: Draganova
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58108-6050, USA.
authors.forename: Darci R
authors.initials: DR
authors.lastname: Block
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA.
authors.forename: Brian R
authors.initials: BR
authors.lastname: Sook
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA.
authors.forename: Yau Fong
authors.initials: YF
authors.lastname: Chan
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA.
authors.forename: Joy
authors.initials: J
authors.lastname: Zhuo
authors.affiliation: Department of Biology, Georgia State University, Atlanta, GA 30303, USA.
authors.forename: Zehava
authors.initials: Z
authors.lastname: Eichenbaum
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58108-6050, USA.
authors.forename: Kenton R
authors.initials: KR
authors.lastname: Rodgers
authors.affiliation: Department of Chemistry, Georgia State University, Atlanta, GA 30302-3965, USA. Electronic address: ddixon@gsu.edu.
authors.forename: Dabney W
authors.initials: DW
authors.lastname: Dixon
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D017510:Protein Folding; D055550:Protein Stability; D013816:Thermodynamics
chemical_list: D001425:Bacterial Outer Membrane Proteins; D005290:Ferric Compounds; D005296:Ferrous Compounds; C523920:SiaA protein, Streptococcus pyogenes; D006418:Heme
keywords: Denaturation; Guanidine; Heme; Kinetics; Resonance Raman; Streptococcus pyogenes; Uptake; unfolding