Hill coefficients, dose-response curves and allosteric mechanisms

Area of Science:

• Pharmacology
• Biochemistry
• Computational Chemistry

Background:

• Hill coefficients (nH) are frequently used to characterize ligand-receptor interactions in dose-response studies.
• Interpreting Hill coefficients is complex, particularly in distinguishing between various binding mechanisms and their implications for drug specificity.
• Existing interpretations often oversimplify the information conveyed by Hill coefficients, especially concerning enzymatic dose-inhibition curves.

Purpose of the Study:

• To critically evaluate the information content of Hill coefficients (nH) derived from logistic fits to dose-response data.
• To compare experimental Hill coefficients with theoretical predictions from realistic reaction schemes.
• To clarify the implications of Hill coefficients, particularly those deviating from unity, for understanding ligand binding and enzymatic activity.

Main Methods:

• Analysis of Hill coefficients (nH) obtained from four-parameter logistic fits to dose-response curves.
• Comparison of experimental Hill coefficients with theoretical models of realistic reaction schemes.
• Evaluation of Hill coefficients in the context of enzymatic dose-inhibition curves and allosteric binding models.

Main Results:

• Hill coefficients (nH) can indicate the number of interacting sites but do not differentiate between competitive, non-competitive, or various allosteric binding mechanisms.
• Hill coefficients less than one in enzymatic dose-inhibition curves signify the enzymatic activity of at least one ternary complex, not anticooperative binding.
• Hill coefficients differing from one are definitive evidence of multiple ligand binding, with significant variations often linked to allosteric binding and conformational instability.

Conclusions:

• The interpretation of Hill coefficients requires careful consideration of the underlying reaction mechanisms to avoid oversimplification.
• Observed variations in Hill coefficients may reflect detrimental allosteric effects leading to loss of active conformation, contrasting with desired drug specificity.
• Further discussion is warranted to refine the understanding and application of Hill coefficients in drug discovery and biochemical studies.