- START HERE — how to read acetoacetyl-CoA + CoA → 2 acetyl-CoA A thiolase cartoon on the human ACAT1 / T2 frame that actually exists: 2IBY (1.85 Å X-ray) chain A with crystal free CoA + K+ + Cl−. Occupancy is free CoA, not acetoacetyl-CoA, not acetyl-CoA — never read CoA as acetoacetyl-CoA. Cys126 on this crystal is CSO (S-hydroxycysteine), not acetyl-Cys. Teaching Cys126 omits the extra OD. Cys126 ★ is the nucleophilic cysteine (UniProt ACT_SITE). His385 is named, not starred (UniProt Site 385). Cys413 is named — UniProt P24752 ACT_SITE is Cys413, not His413. Covalent E~acetyl is compiled and disclosed — not a pure A-page. K+ stays. Blue is nucleophile, not hydride. V34A is the construct mutation; residue 34 is not on this mesh. Extrahepatic after OXCT1. Ketogenesis ACAT1 is the other page (condensation). ACAT2 and ACAA2 are other genes. Measured Å are deposited 2IBY free-CoA pairs only.
- Crystal / X-ray structures PDB 2IBY — human mitochondrial acetoacetyl-CoA thiolase + CoA + K + Cl, 1.85 Å X-ray (Haapalainen 2007). Coordinates are public. Occupancy is free CoA. Cys126 is CSO. V34A is engineered. Human ACAT1 UniProt P24752.
- Mechanism ΓÇö Cys126 nucleophile; thiolysis direction UniProt P24752 EC 2.3.1.9: acetoacetyl-CoA + CoA = 2 acetyl-CoA. This page teaches thiolysis (extrahepatic ketolysis after OXCT1). Condensation of two acetyl-CoA is the ketogenesis ACAT1 page, not this page. On this mesh the nucleophilic cysteine is Cys126. His385 is named, not starred. Covalent acetyl-Cys126 is compiled, not this occupancy. ACAT2 is another gene. ACAA2 is another gene. Source: UniProt P24752 ┬╖ Haapalainen 2007
- 0 — Setup: empty thiolase pocket 2IBY occupancy is free CoA + K + Cl, not acetoacetyl-CoA, not acetyl-CoA. Cys126 is CSO, not acetyl-Cys. The pocket is shown empty so binding can start. K+ stays. Opaque, run:null (no autoplay). Cys126 ★ is the nucleophile. V34A is the construct mutation; residue 34 is not on this mesh (Cα 37–427). Extrahepatic after OXCT1. Ketogenesis acat1 is the other page. ACAT2 and ACAA2 are other genes.Source: PDB 2IBY (1.85 Å X-ray)
- ① — Acetoacetyl-CoA binds [schematic] Acetoacetyl-CoA docks on the 2IBY CoA pose — not a deposited acetoacetyl-CoA. CoA on 2IBY is free CoA, not acetoacetyl-CoA. Overlay Å are not geomClaims. K+ stays.Source: 2IBY CoA pose + 1F0Y CAA thiol-end overlay
- ② — Cys126 attacks C3 → acetyl-enzyme + acetyl-CoA (hypothetical, lit.-based) Blue nucleophile rides from Cys126 SG onto schematic acetoacetyl C3. Acetoacetyl-CoA cross-fades to schematic first acetyl-CoA and a compiled acetyl-Cys126 stub. Covalent E~acetyl is disclosed — not a pure A-page. This crystal has CSO, not acetyl-Cys. Slow-motion on purpose. Blue = nucleophile, not hydride.Source: compiled thiolase ping-pong · UniProt P24752 Cys126
- Γæó ΓÇö First acetyl-CoA leaves The first acetyl-CoA (original CoA, C1ΓÇôC2 of acetoacetyl) departs. Compiled acetyl-Cys126 stays. K+ stays. Cys126 is CSO on 2IBY, not acetyl-Cys. Free CoA is not acetoacetyl-CoA.Source: thiolase ping-pong bookkeeping
- ④ — CoA binds Crystal CoA docks — deposited 2IBY occupancy, the second substrate, not acetoacetyl-CoA. Cys126 SG ⋯ CoA S1P = 4.26 Å is the deposited free-CoA contact, not an acetyl-enzyme geometry.Source: PDB 2IBY CoA
- ⑤ — CoA-S attacks acetyl-Cys → acetyl-CoA (hypothetical, lit.-based) Blue nucleophile rides from CoA S onto compiled acetyl-Cys C. CoA + acetyl stub cross-fade to schematic second acetyl-CoA. His385 is named, not starred; Cys413 is named. Cys126 SG ⋯ His385 NE2 = 3.45 Å is deposited 2IBY. Slow-motion on purpose. Blue = nucleophile, not hydride.Source: compiled thiolase thiolysis · 2IBY Cys126 / His385
- ⑥ — Second acetyl-CoA leaves — ketolysis complete The second acetyl-CoA departs. K+ stays. Extrahepatic ketolysis yields two acetyl-CoA from circulating acetoacetate (OXCT1 then this page). This page does not condense (that is ketogenesis ACAT1) and does not run ACAA2 or ACAT2. Cys126 is CSO on 2IBY, not acetyl-Cys. Overlay Å are not geomClaims. K+ ⋯ Tyr219 OH = 2.74 Å is deposited 2IBY.Source: ketolysis bookkeeping · EC 2.3.1.9
- How to read this page — a first-pass primer First pass: Setup (empty pocket, Cys126 ★, CSO not acetyl, K stays, V34A, extrahepatic) → ① schematic acetoacetyl-CoA docks on CoA pose → ② blue Cys126-S → C3, fade to first acetyl-CoA + acetyl-Cys → ③ first acetyl-CoA leaves → ④ crystal CoA docks → ⑤ CoA-S → acetyl-Cys, fade to second acetyl-CoA → ⑥ leave; ketolysis complete. Second pass: 4.26 / 3.45 / 2.74 Å are crystal facts on 2IBY free-CoA + K complex; acetoacetyl-CoA and acetyl-Cys are [schematic]; CoA is free CoA, not substrate until ④. Third pass: open “Disagreements” for CSO vs Cys, free CoA vs acetoacetyl, UniProt P24752 Cys413, not His413, ketogenesis ACAT1 vs this page, and ACAT2 / ACAA2 vs this gene.
- Chemistry primer ΓÇö ping-pong thiolase (thiolysis) ACAT1 Cys126 attacks acetoacetyl-CoA C3, releasing the first acetyl-CoA and leaving an acetyl-enzyme. Incoming CoA then thiolyzes that acetyl-Cys to the second acetyl-CoA. Blue is nucleophile, not hydride. This is not ACAA2 (EC 2.3.1.16) and not ACAT2 (cytosolic). Ketogenesis ACAT1 condensation is the other page.
- Words used on screen ΓÇö free CoA ┬╖ CSO ┬╖ Cys126 Γÿà ┬╖ compiled acetyl-Cys ┬╖ K stays Free CoA ΓÇö 2IBY occupancy, not acetoacetyl-CoA, not acetyl-CoA. CSO ΓÇö Cys126 is S-hydroxycysteine on this crystal; teaching omits OD. Cys126 Γÿà ΓÇö nucleophile on this PDB = UniProt. Compiled acetyl-Cys ΓÇö covalent intermediate, not this occupancy. K stays ΓÇö deposited potassium, activator, not a reactant.
Legend & Fidelity
- Mechanism fidelity — measured vs. inferred The measured layer: 2IBY (1.85 Å X-ray) human ACAT1 + free CoA + K + Cl; Cys126 SG ⋯ CoA S1P = 4.26 Å; Cys126 SG ⋯ His385 NE2 = 3.45 Å; K+ ⋯ Tyr219 OH = 2.74 Å. The inferred layer ([schematic]): acetoacetyl-CoA on the CoA pose, transferring nucleophile spheres, acetyl-Cys stub, two acetyl-CoA products. Binding order is compiled staging, not a kinetic fit. The protein mesh is static. Free-CoA Å are not acetoacetyl-CoA geomClaims. Cys126 is CSO, not acetyl-Cys.
- Disagreements on record (a) Occupancy vs substrate. 2IBY holds free CoA, not acetoacetyl-CoA, not acetyl-CoA. (b) CSO vs Cys. Cys126 is S-hydroxycysteine on this crystal ΓÇö not acetyl-Cys and not unmodified Cys. Teaching omits OD. (c) Covalent acetyl-Cys126 is compiled identity, not this occupancy. (d) His385 is named, not starred. (e) UniProt P24752 ACT_SITE is Cys413, not His413 ΓÇö labels use PDB ids. (f) Concerted vs split nucleophile/thiolysis ΓÇö this page splits Γæí then Γæñ for 1-event/1-step teaching. (g) ACAT2 is cytosolic, another gene. (h) ACAA2 is T1 thiolysis (EC 2.3.1.16), another gene. (i) V34A is engineered; residue 34 is not on this mesh (C╬▒ 37ΓÇô427). (j) MES / GOL / Cl are named, not substrate. (k) Ketogenesis ACAT1 condensation is the other page ΓÇö same gene, opposite flux. (l) Extrahepatic after OXCT1; liver ketogenesis does not run this page.
- Regulation ΓÇö K+ ┬╖ isoforms ┬╖ extrahepatic K+ activates (not Na+) ΓÇö Haapalainen 2007. This cartoon does not simulate ion-binding kinetics. This thiolysis is monofunctional ACAT1 / T2 only, extrahepatic after OXCT1. Cytosolic ACAT2 is another gene ΓÇö not this mesh. ACAA2 / T1 is ╬▓-ox thiolysis, another gene. Homotetramer assembly is not this cartoon. Matrix localization is assumed, not a catalytic event on this page. Ketogenesis condensation is the other ACAT1 page.Source: UniProt P24752 ┬╖ PDB 2IBY ┬╖ Haapalainen 2007
- Kinetics scope (no measured timecourse on this page) One acetoacetyl-CoA + CoA → 2 acetyl-CoA event is drawn, not a steady-state run. Compiled BRENDA values for EC 2.3.1.9 are not re-asserted here — no measured kcat/Km on this page [schematic]. Pair with a kinetics chapter.Source: BRENDA EC 2.3.1.9 — no kcat/Km re-asserted
- Sequence Human ACAT1 UniProt P24752 ┬╖ NCBI Gene 38. This screen uses 2IBY numbering (Cys126 = UniProt Cys126). Chain A. C╬▒ 37ΓÇô427. Transit 1ΓÇô33 and V34A are not this mesh.
- Simulation map — what you see is what is written Setup = 2IBY pocket empty, K stays (opaque, no autoplay). ① schematic acetoacetyl-CoA moves in. ② slow-mo: blue Cys126-S → C3; fade to first acetyl-CoA + acetyl-Cys. ③ first acetyl-CoA leaves. ④ crystal CoA moves in. ⑤ slow-mo: CoA-S → acetyl-Cys; fade to second acetyl-CoA. ⑥ second acetyl-CoA leaves. Only these events move.
- Sources — bibliography of this page S1 — PDB 2IBY: human ACAT1 + CoA + K + Cl, 1.85 Å X-ray — public coordinates; measured Cys126 SG⋯CoA S1P = 4.26 Å · Cys126 SG⋯His385 NE2 = 3.45 Å · K+⋯Tyr219 OH = 2.74 Å S2 — Haapalainen et al., 2007, PMID:17371050 — human mitochondrial acetoacetyl-CoA thiolase; K+ and Cl− S3 — Sequence: UniProt P24752 · NCBI Gene 38 S4 — Kinetics: BRENDA EC 2.3.1.9 — no kcat/Km re-asserted on this page
› Backbones and CoA poses are taken directly from the listed PDB entries, superposed into one common frame. Where no crystalline state exists for a step (e.g., acetoacetyl-CoA on 2IBY, acetyl-Cys126, nucleophile flight, product leave), the pose is drawn schematically and labeled as such in the narration. Occupancy is free CoA, not acetoacetyl-CoA. Cys126 is CSO, not acetyl-Cys. Free-CoA Å are not acetoacetyl-CoA geomClaims.