Quantification of Intrinsically Disordered Proteins: A Problem Not Fully Appreciated.

Area of Science:

• Biochemistry
• Proteomics
• Molecular Biology

Background:

• Protein quantification is vital for biochemical assays.
• Classical methods like Bradford and UV absorbance inaccurately determine intrinsically disordered protein (IDP) concentrations due to variable amino acid composition.
• Systematic errors in IDP concentration measurement are often overlooked.

Purpose of the Study:

• To evaluate dependable methods for accurate intrinsically disordered protein (IDP) quantification.
• To compare the accuracy of common protein assays against a gold standard for IDPs.
• To highlight the implications of misestimating IDP concentrations on biophysical measurements.

Main Methods:

• Comparison of protein quantification assays against elemental analysis as a gold standard.
• Testing the ninhydrin assay and the commercial Qubit™ Protein Assay on globular proteins and intrinsically disordered proteins (IDPs).
• Demonstrating the impact of concentration errors on circular dichroism spectrum deconvolution and receptor-ligand affinity determination.

Main Results:

• Classical protein quantification methods (Bradford, UV 280 nm) significantly misestimate IDP concentrations.
• The ninhydrin assay and Qubit™ Protein Assay provide reliable IDP quantity data compared to elemental analysis.
• Standardization of even these improved methods is advised for IDPs due to their diverse nature.

Conclusions:

• Accurate quantification of intrinsically disordered proteins (IDPs) requires specialized or validated methods beyond classical assays.
• Misestimation of IDP concentration can lead to significant errors in downstream biophysical analyses and functional descriptions.
• The ninhydrin and Qubit assays are recommended for IDP quantification, contingent upon appropriate standardization.