Protein degradation via the ubiquitin-proteasome system is a three-enzyme relay followed by ATP-driven unfolding:
E1 + Ub + ATP → E1~Ub (activated, AMP + PPi released)
E1~Ub + E2 → E2~Ub
E2~Ub + E3 + substrate-Lys → substrate-Ub(n) + E2
repeat → K48-linked poly-Ub chain grows on substrate
Chain length L follows a birth-death balance between ligation and deubiquitinase (DUB) removal:
dL/dt = k_lig · [E3 active] − k_dub · L
capture when L ≥ threshold (this sim: 2–8, canonical K48 minimum ≈ 4)
Once a substrate crosses the threshold, the 19S regulatory particle's ring of six AAA+ ATPases grips an exposed unstructured tail and threads the chain through its central pore, unfolding the protein processively — one ~2 residue "step" per ATP hydrolysed — into the 20S core's proteolytic chamber, where it is cleaved into short peptides. Ubiquitin itself is recycled by proteasome-associated DUBs before the substrate enters the pore, so the ubiquitin pool never leaves the system.
- E3 ligase rate — how fast poly-Ub chains grow on each tagged substrate.
- DUB rate — competing chain-trimming; too high and substrates never reach threshold (a real proofreading checkpoint against premature degradation).
- K48 threshold — minimum chain length the proteasome's Rpn10/Rpn13 ubiquitin receptors need to commit a substrate to degradation.
- ATP / unfoldase speed — throughput of the two proteasomes shown; each processes one substrate at a time.
This top-down 2D view shows the same reaction network as the 3D version, laid flat: a cytosol arena bounded on all sides, two docking proteasomes at its left and right edges, and substrates that drift, get tagged, get captured, and get threaded through the pore in real time. Drag to pan the arena and scroll/pinch to zoom in on any substrate's growing ubiquitin chain.
Real-world relevance: this pathway degrades >80% of cellular proteins, drives cell-cycle checkpoints (cyclin destruction), clears misfolded proteins, and is the direct target of proteasome-inhibitor cancer drugs (bortezomib) and PROTAC-style targeted-degradation therapeutics.