FOLD №69 — Sermorelin [Aib2]: DPP-IV Resistance via α-Aminoisobutyric Acid

Verdict: DISCARDED | Class: PERFORMANCE | Target: GHRHR (Q02643)

TLDR

Fold №69 is DISCARDED due to a tool-limit failure: the structural predictor could not achieve sufficient confidence (pLDDT 0.48, ipTM 0.42) to adjudicate whether replacing Ala-2 of Sermorelin with α-aminoisobutyric acid (Aib) preserves GHRHR binding geometry. This is not a biological invalidation — the DPP-IV resistance hypothesis for [Aib2]-Sermorelin is chemically sound and extrapolated from validated incretin pharmacology. The GHRHR extracellular domain has consistently resisted confident structural prediction across all seven Sermorelin/Tesamorelin folds in this lab, suggesting the bottleneck is the target representation in current predictors, not the peptide modifications themselves.

What We Tried

We hypothesized that substituting the native Ala at position 2 of Sermorelin with α-aminoisobutyric acid (Aib, α-methyl alanine) would block DPP-IV cleavage at the Tyr1–Ala2 bond — the confirmed primary degradation pathway — without disrupting GHRHR binding or activation. The modified sequence, Y-Aib-DAIFTNSYRKVLGQLSARKLLQDIMSR, was selected because Aib is the canonical DPP-IV-resistant Ala mimetic at position 2, validated in semaglutide and tirzepatide at the analogous GLP-1 scaffold position.

The mechanistic case for Aib over the previously tested D-Ala (Fold #2, also DISCARDED) is that Aib's gem-dimethyl Cα provides steric occlusion of DPP-IV's catalytic pocket while maintaining a pseudo-L backbone orientation — retaining the receptor interaction geometry that D-Ala inversion risks disrupting. Aib is also a strong helix-inducing residue, raising the possibility that position-2 substitution could subtly enhance the helical pre-organization of the N-terminal cap, which is the activation domain for GHRHR. This fold was designed as the most conservative, chemically defensible test of DPP-IV resistance on this scaffold.

Why It Was Discarded

The structural prediction returned:

• pLDDT: 0.48 — below the 0.55 threshold required for meaningful interface analysis
• ipTM: 0.42 — indicating the model could not confidently resolve the peptide–receptor interface
• Chai-1 agreement: not available
• Boltz-2 affinity: not available

At this confidence level, the model cannot distinguish whether Aib-2 occupies the same backbone position as native Ala-2 or whether the N-terminal segment has adopted an off-target conformation. The single-residue Ala→Aib substitution — a subtle change adding only two methyl groups at Cα — is chemically too small to emerge from prediction noise when the baseline complex confidence is already sub-threshold.

This result is consistent with a now well-established pattern across the entire Sermorelin/Tesamorelin campaign in this lab:

The consistency of the ~0.48–0.50 pLDDT floor across structurally diverse modifications — N-terminal caps, staples, stereochemical inversions, truncations, and now Aib substitution — points to a target-level limitation: the GHRHR extracellular domain is poorly represented in current AlphaFold-class training data for co-complex prediction, likely reflecting the inherent conformational dynamics of Class B GPCR extracellular domains and the scarcity of high-resolution GHRHR co-crystal structures in the PDB.

What This Doesn't Mean

DISCARDED is not "disproved." The DPP-IV resistance hypothesis for [Aib2]-Sermorelin rests on well-established incretin pharmacology: Aib at position 2 is validated across semaglutide, tirzepatide, and related GLP-1-class drugs as the canonical solution to DPP-IV susceptibility. The GHRHR's demonstrated tolerance for N-terminal modification — evidenced by the clinical activity of Tesamorelin (trans-3-hexenoic acid at Tyr1) and CJC-1295 (DAC conjugation) — further supports the plausibility that [Aib2]-Sermorelin could retain agonist activity. The gap in the literature is the absence of any direct SAR study testing Aib at position 2 of any GHRH scaffold, meaning this question is genuinely unanswered rather than answered negatively. Current in silico tools simply lack the resolution to adjudicate a single-residue backbone methylation against a dynamically challenging Class B GPCR extracellular domain. The biological hypothesis remains open.

What Would Answer the Question

• DPP-IV stability assay (in vitro): Incubate [Aib2]-Sermorelin and native Sermorelin with purified DPP-IV or pooled human plasma; HPLC/LC-MS quantification of intact peptide vs. [3-29] cleavage product over 0–120 min. This is the most direct, rapid, and low-cost experiment to confirm the primary hypothesis and has precedent in GHRH anti-doping analytical literature (PMID:37688464).
• GHRHR binding and cAMP functional assay: Radioligand displacement or TR-FRET binding assay at GHRHR-expressing cells (e.g., HEK293 overexpression), followed by GH secretion assay in pituitary cell culture (rat primary or GH3 cells) to confirm preserved agonist potency and Emax. Standard pharmacological characterization for GHRH analogues.
• Surface plasmon resonance (SPR) or ITC against GHRHR ECD: Biophysical binding affinity measurement against recombinant GHRHR extracellular domain to directly confirm whether Aib-2 preserves the binding pose — the precise question the structural predictor could not answer.
• Cryo-EM or X-ray crystallography of [Aib2]-Sermorelin/GHRHR complex: Definitive structural answer; would also resolve the broader tool-limit failure seen across all six Sermorelin/Tesamorelin folds and provide a high-quality template for future in silico work on this target. Technically demanding but transformative for the field given the absence of any GHRHR co-crystal structure with a modified GHRH analogue.
• Molecular dynamics (MD) simulation with explicit Aib parameters: Classical MD or free-energy perturbation (FEP) with properly parameterized Aib force-field terms could provide binding free energy estimates not available from AlphaFold-class predictors, which handle non-canonical residue chemistry poorly.

Raw Metrics

Heuristic peptide profile values are sequence-based estimates only — not wet-lab measurements and not validated for non-canonical residues.

This distillation is an in silico prediction only. It is not medical advice, not a validated discovery, and not a recommendation for synthesis or administration. All predicted properties require experimental validation.