A Single Amino Acid Determines Why One Antihistamine Isomer Binds Five Times Stronger
Two molecules can share identical chemical formulas and bond structures while differing in only one property: the geometric arrangement of atoms around a double bond. These geometric isomers - E and Z forms - can behave very differently in biological systems. Doxepin, a drug used both as an antidepressant and an antihistamine, is a case in point. Its Z-isomer binds the histamine H1 receptor roughly five times more tightly than its E-isomer. But the structural reason for that disparity has been unclear - until now.
A team at Tokyo University of Science, led by Professor Mitsunori Shiroishi, combined two precision techniques - isothermal titration calorimetry and molecular dynamics simulations - to dissect the thermodynamic basis of that selectivity. Their findings, published January 26, 2026, in ACS Medicinal Chemistry Letters, trace the difference to a single amino acid residue in the receptor's binding pocket, a threonine at position 112 (Thr1123.37).
Why Thermodynamics Matters in Drug Design
For decades, drug researchers focused primarily on binding affinity - how strongly a molecule sticks to its target protein. More recently, the field has recognized that total binding energy can be broken down into two components with distinct implications: enthalpy (the direct energy of molecular interactions, such as hydrogen bonds and van der Waals contacts) and entropy (a measure of how much molecular disorder increases or decreases upon binding).
This distinction matters because drugs that bind primarily through enthalpy tend to form specific, well-defined contacts with their target - the kind of selectivity that reduces off-target effects. Drugs that rely heavily on entropy for binding often do so by releasing water molecules or by forcing the target into a flexible, less-structured state, which can mean broader and less predictable activity. Enthalpy-driven binding is generally preferred for drug candidates that need to be selective among closely related receptor subtypes.
The histamine H1 receptor (H1R) is a G-protein-coupled receptor - a class that accounts for more than 30% of currently marketed drugs. It mediates allergic reactions, vascular permeability, and airway constriction, as well as wakefulness and some cognitive functions. Antihistamines that block H1R are among the most widely used drugs globally, but current compounds vary considerably in their selectivity and side-effect profiles. Understanding exactly how different ligands interact thermodynamically with H1R could guide the development of better options.
The Experiment: Measuring Energy Directly
The Tokyo team expressed H1R in a yeast-based production system that allows the protein to be purified in sufficient quantities for calorimetric measurement - a technically demanding step, since GPCRs are membrane proteins that are notoriously difficult to work with outside their native cellular environment.
Using isothermal titration calorimetry, they measured the enthalpy and entropy of binding directly for both doxepin isomers, both alone and in competition, against two versions of H1R: the wild-type receptor and a mutant in which threonine at position 112 was replaced by valine (T1123.37V) - a similar but non-polar amino acid incapable of forming hydrogen bonds.
"Doxepin is a compound that has been widely used as an H1R inhibitor. In this study, we successfully measured the thermodynamic signatures of doxepin geometric isomers to the H1R, prepared via a budding yeast expression system, using isothermal titration calorimetry and molecular dynamics simulations," said Professor Shiroishi.
Enthalpy, Entropy, and the Single Threonine
The results showed that total binding energy - how tightly each isomer stuck to the receptor overall - was the same whether the threonine was present or replaced by valine. That is a critical observation: removing the threonine did not change binding strength. What it did change was the internal thermodynamic composition of that binding energy.
When threonine was present (wild-type receptor), both isomers bound primarily through enthalpy, but the Z-isomer showed a larger enthalpic gain coupled with a larger entropic penalty than the E-isomer. The Z-isomer forms tighter, more specific contacts with the receptor - but doing so costs more in terms of molecular flexibility. When threonine was replaced by valine, these differences between the two isomers disappeared entirely. Both isomers bound with similar enthalpy-entropy profiles and similar affinities.
Molecular dynamics simulations explained why. The Z-isomer's higher affinity in the wild-type receptor arises from conformational restriction: the Z geometry places its molecular scaffold in a rigid orientation that makes optimal use of the threonine's hydrogen-bonding capacity. That rigidity translates into strong enthalpy but reduced entropy. The E-isomer, with a different geometric arrangement, cannot adopt the same optimal pose and benefits less from the threonine contact.
What This Means for Future Antihistamines
The findings have direct relevance for rational drug design. By identifying the specific residue that drives isomer selectivity and characterizing its thermodynamic mechanism, the Tokyo team provides a template for designing H1R ligands with improved properties.
"These mechanistic insights into the enthalpy-entropy trade-off in GPCR-ligand interactions highlight the importance of considering conformational constraints and flexibility in designing ligands with optimized thermodynamic properties," said Shiroishi. "This could lead to the development of drugs with improved selectivity, reduced side effects, and longer-lasting therapeutic effects."
The work is still at the basic research stage. Identifying a thermodynamic mechanism at a purified receptor in a calorimetric experiment is several steps removed from developing a new clinical drug. The next challenges include testing whether the same thermodynamic principles apply in cell-based and animal models, and whether designing ligands that maximize enthalpy-driven H1R binding actually translates into improved selectivity and safety profiles in practice.
More broadly, the study demonstrates that the yeast-based expression system can yield enough functional GPCR protein for calorimetric thermodynamic measurements - a methodological advance that could be applied to other receptor-drug pairs across the large and pharmacologically important GPCR family.