Histidine residues underlie Congo red binding to A beta analogs
H Inouye1, J T Nguyen, P E Fraser
1Department of Biology, Boston College, Chestnut Hill, MA 02467-3811, USA.
Summary
Congo red binds to Alzheimer's disease amyloid fibrils, with histidine residues identified as key binding sites. This binding is pH-dependent, with affinity decreasing as pH increases above 5.
Area of Science:
- Biochemistry
- Neuroscience
- Materials Science
Background:
- Alzheimer's disease (AD) is characterized by amyloid fibril formation.
- Congo red (CR) is a dye used to detect amyloid deposits.
- Understanding CR binding to amyloid-beta (A beta) fibrils is crucial for diagnostic and therapeutic strategies.
Purpose of the Study:
- To quantitate the binding affinity and number of binding sites of Congo red to Alzheimer's disease amyloid fibrils.
- To elucidate the role of pH and specific amino acid residues in the binding mechanism.
- To investigate the binding in an 80% ethanol medium, relevant for histological staining.
Main Methods:
- Absorption spectroscopy was used to measure Congo red bound (CR-B) to A beta assemblies.
- Experiments were conducted as a function of CR concentration and pH in 80% ethanol.
- Various A beta analogs, including peptides of different lengths and sequences with specific amino acid substitutions, were studied.
Main Results:
- Congo red binding decreased significantly with increasing pH, becoming negligible above pH 7.
- Histidine residues were identified as the primary binding sites for Congo red.
- Binding affinity (Kd) was found to be 2.8-5.9 microM, with an apparent proton dissociation constant (pK) of 5.0-5.5 for peptides with 1-3 histidines.
Conclusions:
- Histidine residues are critical for Congo red binding to Alzheimer's disease amyloid fibrils in 80% ethanol.
- The binding mechanism is pH-dependent, with optimal binding occurring at lower pH values.
- The findings support the use of Congo red as a diagnostic tool and provide insights into amyloid-dye interactions.
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