Electron spin echo envelope modulation studies of lectins: evidence for a conserved Mn(2+)-binding site

Area of Science:

• Biochemistry
• Biophysics
• Structural Biology

Background:

• Lectins are proteins that bind carbohydrates.
• Manganese (Mn2+) ions are often found in the active sites of metalloenzymes and can serve as probes for structural studies.
• Understanding the Mn2+-binding site in lectins is crucial for elucidating their function and interactions.

Purpose of the Study:

• To investigate the Mn2+-binding site in a series of lectins using Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy.
• To determine the number and type of coordinating ligands (nitrogen atoms and water molecules) at the Mn2+ site.
• To assess the effect of saccharide binding on the Mn2+ site structure.

Main Methods:

• Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy was performed on Mn2+-loaded lectins.
• Deuterium oxide exchange experiments were conducted to probe water molecule accessibility.
• Model compound studies were used for data interpretation.
• Nuclear magnetic relaxation dispersion (NMRD) measurements were referenced for comparison.

Main Results:

• ESEEM provided direct evidence for a single nitrogen atom from a conserved residue coordinating to Mn2+ in all studied lectins.
• Two water molecules were found to be coordinated to Mn2+ in all lectins.
• The Mn2+ site in pea and lentil lectins exhibited slow water ligand exchange, contrasting with concanavalin A.
• Saccharide binding had a negligible effect on the Mn2+ binding site structure.

Conclusions:

• The Mn2+-binding site in the investigated lectins is structurally conserved, featuring a nitrogen ligand and two water molecules.
• Differences in water exchange rates suggest variations in the lability of Mn2+-bound water among lectins.
• The Mn2+ site's structural integrity is maintained upon saccharide binding.