Binding affinity and functional potency answer different questions. Occupancy reports how tightly a peptide engages a named molecular target; potency reports how much peptide is needed to move a downstream readout such as enzyme inhibition, cell entry, or microbial growth. Four abstracts first published between 13 July and 1 September 2026 (records captured 30 September 2026) show both aligned shifts and large numerical gaps. They do not support ranking a KD from one paper against an MIC from another.
Occupancy measures engagement; potency measures a response
An mRNA-display campaign identified macrocyclic peptide (MCP) inhibitors that bound the TMPRSS2 active site. The most potent MCPs showed picomolar binding affinities and selectivity across a panel of 22 trypsin-fold serine proteases. Rational optimization yielded the stability-enhanced variant T2-MCP-19, which bound TMPRSS2 with KD = 80 pM yet inhibited uptake of virus-like particles pseudotyped with SARS-CoV-2 spike protein in Calu-3 lung epithelial cells with nanomolar potency. The lead MCP had strong in vitro potency, but proteolytic instability reduced cellular activity. Several MCPs blocked both TMPRSS2 enzymatic activity and its receptor function, shown by inhibition of HKU1 spike-protein binding. That is occupancy versus cellular response, not a spare-receptor analysis (Tombling et al.).
A ClpP-targeting acyldepsipeptide (ADEP) series shows affinity and antibacterial potency moving together inside one program. Among 35 compounds, conformational restriction of the macrocycle and side chain enhanced ClpP1P2 binding and antibacterial potency. ADEP 19 was as much as 12-fold more potent against Mycobacterium tuberculosis (Mtb) and other bacteria than ADEP 4. No numerical KD is reported. Those conclusions rest on biophysical, enzymatic, crystallographic, and microbiological studies in the same paper, not on converting a binding constant into a growth-inhibition titer (Fei et al.).
Computational docking is a weaker occupancy proxy. Engineered MCh-AMP1-A7 reduced the MIC against Candida albicans ATCC 10231 from 16 μg/mL for parent MCh-AMP1 to 8 μg/mL, with 2- to 4-fold MIC reductions against clinical C. glabrata isolates and C. krusei DSM 70079. Docking and molecular-dynamics simulations supported a membrane-associated mechanism (ΔG = -8.8 kcal/mol). That ΔG is modeled, not an experimental KD, and cannot be ranked against the 80 pM TMPRSS2 value or the ADEP 12-fold microbiological shift (Pooshang Bagheri et al.).
The ceftazidime–hexa-arginine conjugate CTZ-R6 shows why affinity at one protein still underdetermines functional potency. Relative to ceftazidime, CTZ-R6 had higher affinity for Bacillus subtilis penicillin-binding protein 2a (PBP2a), lower affinity for PBP1a/b, and a more balanced PBP profile; the abstract gives no KD. It also crossed the Escherichia coli outer membrane by self-promoted uptake, permeabilized that barrier, perturbed cytoplasmic-membrane topology while leaving the electrochemical gradient high, damaged peptidoglycan, and produced β-lactam-like lysis kinetics. The covalent linkage was essential; both moieties contributed. Previously described antibacterial-potency gains of up to 1,000-fold versus ceftazidime against multi-resistant bacteria therefore cannot be read as a simple occupancy increase at a single PBP (Müller et al.).
Receptor reserve and binding kinetics are not reported here
A receptor-reserve claim requires an occupancy–response relationship in the same system: fraction of target bound versus fraction of maximal effect. None of the four abstracts report that analysis. The TMPRSS2 KD-to-cell mismatch is attributed to proteolytic instability, not spare receptors (Tombling et al.). The ADEP work does not state what occupancy is required for ClpP dysregulation (Fei et al.). CTZ-R6 lysis kinetics and a preserved electrochemical gradient are physiological observations, not association or dissociation rate constants (Müller et al.). MCh-AMP1-A7 activity across 10–70°C and pH 2–12 is chemical stability, not binding kinetics (Pooshang Bagheri et al.).
As of 30 September 2026, a remaining question is whether time-resolved occupancy (TMPRSS2 residence time, or ClpP engagement in the bacterial cytoplasm) was measured in the same matrix as the functional readout. These abstracts do not provide those data.
Decision table: which comparisons are supported
Fair comparisons stay inside one chemical series, one target class, and one assay layer. Cross-paper ranking of picomolar KDs against μg/mL MICs, treating docking ΔG as experimental affinity, or inferring that a larger numerical "potency" in one organism implies a better binder in another is unsupported.
| Comparison you want | What these abstracts measured | Supported? |
|---|---|---|
| KD vs cellular activity for the same peptide | T2-MCP-19: 80 pM KD vs nanomolar Calu-3 virus-like-particle uptake potency; lead MCP cellular activity reduced by proteolysis | Yes, as a within-study occupancy–response gap |
| Affinity and antibacterial potency in one series | ADEP 19 vs ADEP 4: conformational restriction enhanced ClpP1P2 binding and up to 12-fold microbiological potency; no numerical KD | Yes, within that ADEP set |
| PBP affinity vs fold-change in antibacterial potency | CTZ-R6: relative PBP affinities plus membrane uptake/permeabilization; covalent linkage required | Only as a multi-mechanism account, not affinity alone |
| Docking ΔG vs experimental KD or MIC | MCh-AMP1-A7: ΔG = -8.8 kcal/mol vs 8 μg/mL C. albicans MIC; no experimental occupancy constant | No: modeled energy vs growth inhibition |
| 80 pM TMPRSS2 KD vs ADEP 12-fold shift or 8 μg/mL antifungal MIC | Different targets, organisms, and endpoints | No |
Checklist when reading a peptide paper:
- Name the molecular target and the functional endpoint separately (binding, enzyme, cell, organism).
- Keep units as published (pM KD, μg/mL MIC, kcal/mol ΔG); do not convert them into a common "potency."
- If cellular activity is weaker than biochemical affinity, use the cause given in that paper (here, proteolytic instability for the TMPRSS2 lead MCP)—do not default to receptor reserve.
- Treat docking ΔG as a modeling result unless an experimental occupancy constant is also reported.
- Do not rank peptides across papers unless target, matrix, and readout match.
Frequently Asked Questions
If a peptide KD is picomolar, should I treat cellular potency as picomolar too?
No. T2-MCP-19 bound TMPRSS2 at KD = 80 pM but inhibited uptake of SARS-CoV-2 spike-pseudotyped virus-like particles in Calu-3 cells with nanomolar potency; the related lead MCP lost cellular activity because of proteolytic instability.
What would be needed to claim receptor reserve from a KD–potency gap?
An occupancy–response relationship in the same system (fraction of target bound versus fraction of maximal effect). These four abstracts do not report that analysis.
Can docking ΔG be ranked against an experimental KD?
Not from these abstracts. MCh-AMP1-A7 reports ΔG = -8.8 kcal/mol from docking and molecular-dynamics simulations, not an experimental occupancy constant.
Explore Further
Browse our research peptide catalog and review third-party lab reports & COAs for published batches.
---
Research use only. The information above is provided for educational and laboratory research purposes only. The compounds discussed are not approved for human or veterinary use, diagnosis, treatment, or the prevention of any disease. Nothing here is medical advice.