Histidine ligand variants of a flavo-diiron protein: effects on structure and activities
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Flavo-diiron proteins (FDPs) contain non-heme diiron and proximal flavin mononucleotide (FMN) active sites and function as terminal components of a nitric oxide reductase (NOR) and/or a four-electron dioxygen reductase (O2R). While most FDPs show similar structural, spectroscopic, and redox properties, O2R and NOR activities vary significantly among FDPs. A potential source of this variability is the iron ligation status of a conserved His residue that provides an iron ligand in all known FDP structures but one, where this His residue is rotated away from iron and replaced by a solvent ligand. In order to test the effect of this His ligation status, we changed this ligating His residue (H90) in Thermotoga maritima (Tm) FDP to either Asn or Ala. The wild-type Tm FDP shows significantly higher O2R than NOR activity. Single crystal X-ray crystallography revealed a remarkably conserved diiron site structure in the H90N and −A variants, differing mainly by either Asn or solvent coordination, respectively, in place of H90. The steady-state activities were minimally affected by the H90 substitutions, remaining significantly higher for O2R versus NOR. The pre-steady-state kinetics of the fully reduced FDP with O2 were also minimally affected by the H90 substitutions. The results indicate that the coordination status of this His ligand does not significantly modulate the O2R or NOR activities, and that FDPs can retain these activities when the individual iron centers are differentiated by His ligand substitution. This differentiation may have implications for the O2R and NOR mechanisms of FDPs.
KeywordsNon-heme iron Nitric oxide Dioxygen X-ray crystallography Enzyme kinetics Site-directed mutagenesis
Nitric oxide reductase
Protein Data Bank
This work was supported by grants from National Institute of Health (RO1 GM040388 to D.M.K. Jr.) and the Robert A. Welch Foundation (AQ-1399 to P. J. H.). Support for the X-ray Crystallography Core Laboratory by the University of Texas Health Sciences Center at San Antonio Executive Research Committee and the Cancer Therapy & Research Center is gratefully acknowledged. We thank Professor Heather Shipley for access to and assistance with the inductively coupled plasma mass spectrometer.