The active site of hemerythrin as determined by X-ray absorption fine structure

Area of Science:

• Biophysical Chemistry
• Structural Biology
• Inorganic Chemistry

Background:

• Hemerythrin is an oxygen-binding protein found in some marine invertebrates.
• The active site of hemerythrin contains two iron atoms bridged by an oxygen atom.
• Understanding the structural dynamics of hemerythrin is crucial for elucidating its oxygen transport mechanism.

Purpose of the Study:

• To investigate the structural changes in the active site of hemerythrin during oxygen binding and release.
• To determine the precise iron-iron and iron-oxygen distances in oxy- and deoxyhemerythrin.
• To assess and minimize errors in X-ray absorption fine structure (XAFS) analysis for metalloproteins.

Main Methods:

• Extensive X-ray absorption fine structure (XAFS) measurements were performed on azidomet-, methemerythrin, oxyhemerythrin, and deoxyhemerythrin.
• Analysis of XAFS data was conducted using synthesized models mimicking the hemerythrin active site.
• A novel error analysis formulation was developed to improve the accuracy of structural parameter determination.

Main Results:

• The study identified a mu-oxo bridge between iron atoms in oxyhemerythrin (Fe-O distance: 1.8 Å).
• Upon oxygen release, this converts to a mu-hydroxo bridge in deoxyhemerythrin (Fe-O distance: 2.0 Å).
• The iron-iron distance expands from 3.24 Å in oxyhemerythrin to 3.57 Å in deoxyhemerythrin, maintaining a constant Fe-O-Fe angle.

Conclusions:

• The experimental findings provide strong support for proposed structures of oxy- and deoxyhemerythrin.
• The conversion between mu-oxo and mu-hydroxo bridges is a key structural event in hemerythrin's function.
• Accurate XAFS analysis, with careful error assessment, is vital for determining metalloprotein active site structures.