Week 3 HW
A. Compare Cell-Free Reaction Compositions
1. Why do potassium glutamate and magnesium glutamate concentrations differ between compositions? Which is more sensitive?
Different roles → different optimal concentrations
Potassium glutamate is mainly an ionic-environment reagent. K⁺ is the dominant intracellular cation and glutamate the dominant anion in E. coli, so K-glutamate is used to recreate cytoplasmic ionic strength/osmolarity and support general enzyme and ribosome activity. Its response curve is broad: Jewett & Swartz found maximum protein production at 175 mM potassium glutamate, but the optimum sits on a fairly flat plateau, and yields were “nearly insensitive to ammonium glutamate concentrations between 0 and 100 mM.” In other words, you can be off by tens of mM and lose little activity.
Magnesium glutamate is a functional cofactor. Mg²⁺ is required for ribosome structure and translational fidelity, it forms the true substrate complex Mg-NTP, and it is needed by ATP-regenerating enzymes (e.g., pyruvate kinase). Crucially, Mg²⁺ is chelated by phosphorylated compounds — PEP, NTPs/NMPs, and the inorganic phosphate that accumulates during the reaction. So the “right” Mg-glutamate concentration depends on how much phosphorylated material a given formulation loads (a 33 mM PEP system has a very different Mg²⁺ demand than an NMP/ribose system).
Why values differ across formulations
- The energy system sets the Mg²⁺/phosphate balance (PEP-heavy systems need more Mg²⁺ headroom).
- The extract itself carries over salts (~3.3 mM Mg and ~14.4 mM K from S30 buffer in the Swartz-lab systems), so the optimum shifts with every lysate batch.
- Each formulation is empirically re-optimized — Olsen et al. explicitly re-optimized magnesium and phosphate for each new lysate batch and lysate concentration.
Which is more sensitive?
Magnesium. Mg²⁺ has a narrow, bell-shaped optimum — too little stalls ribosomes and ATP regeneration; too much promotes misincorporation, nonspecific enzyme inhibition, and precipitation with phosphate/nucleotides.
This is why Mg-glutamate is the parameter labs re-titrate first when changing extract batches, DNA load, or energy system, whereas K-glutamate tolerates a much wider window (PANOx-SP uses 175 mM, RFopt ends up near ~362 mM — both work).
2. PEP + NTP vs. Ribose + NMP
| Feature | PEP + NTP (e.g., PANOx-SP) | Ribose + NMP (e.g., RFopt) |
|---|---|---|
| Energy regeneration | PEP is a high-energy phosphate donor; pyruvate kinase converts PEP + ADP → pyruvate + ATP in one substrate-level step. Works anaerobically — PEP-driven synthesis is unaffected by oxygen availability and respiration inhibitors. | Ribose is a cheap, non-phosphorylated sugar; the extract’s own metabolism (pentose phosphate pathway → glycolysis → oxidative phosphorylation) regenerates ATP. RFopt starts with zero exogenous ATP yet sustains ~250 µM ATP for the whole reaction. |
| Nucleotide supply | NTPs added pre-made (1.2 mM ATP, 0.85 mM each GTP/UTP/CTP) — immediately available for transcription and translation. | Cheap NMPs are phosphorylated in situ by lysate kinases (NMP kinases, NDP kinase) using regenerated ATP. Salvage even works from bases: GMP can be replaced by guanine + ribose without losing yield. |
| Kinetics | Fast ATP spike within ~30 min, then ATP crashes to near zero by reaction end. | Slower ramp-up (metabolism must boot), but a sustained steady-state — longer-lived protein synthesis. |
| Phosphate burden | PEP hydrolysis liberates ~30+ mM Pi, which sequesters Mg²⁺ and eventually inhibits synthesis; PANOx mitigates this with oxalate. | Phosphate hovers at 10–15 mM, ~3× lower than PANOx-SP, because no phosphate is liberated from PEP. |
| Cost | PEP dominates cost: ~$2,000–3,000 per liter of reaction, >75% of all material costs. | Orders of magnitude cheaper; switching to NMPs + phosphate raised cost-efficiency ~340-fold in the Cytomim study ($0.004 vs $1.64 per mL for energy/nucleotides). |
| Robustness trade-off | Robust to oxygen limitation, vessel geometry, and lysate metabolic state. | Sensitive to oxygen transfer (surface-area-to-volume ratio) and to lysate concentration/quality — RFopt yields are “more sensitive to the amount of lysate added than using the PANOx-SP formulation.” |
In one sentence: PEP+NTP pays upfront for a fast, self-contained, oxygen-independent burst of synthesis, while ribose+NMP outsources ATP and NTP production to the extract’s own metabolism — far cheaper and longer-lived, but only if glycolysis, salvage pathways, and oxidative phosphorylation are active.
3. Literature comparison: PANOx-SP (PEP-based) vs. RFopt (ribose-based)
PANOx-SP — Jewett, M.C. & Swartz, J.R. (2004) Biotechnol. Bioeng. 86:19–26.
RFopt — Olsen, M.L. et al. (2026) Nature Communications (doi:10.1038/s41467-026-69605-8; screened >1,200 formulations via definitive screening designs).
| Component | PANOx-SP (2004) | RFopt (2026) |
|---|---|---|
| Energy source | 33 mM PEP | 50 mM ribose + 10 mM glucose |
| Nucleotides | 1.2 mM ATP + 0.85 mM each GTP/UTP/CTP | NMPs (AMP/CMP/GMP/UMP, 5 ≤ mM; GMP replaceable by guanine + ribose) |
| K-glutamate | 175 mM | ~362 mM |
| Mg-glutamate | 20 mM (incl. ~3.3 mM extract carryover) | 8 mM (re-titrated per lysate) |
| NH₄-glutamate | 10 mM | omitted |
| Buffer | none (HEPES deliberately removed) | 75 mM HEPES, pH 7.5 |
| Phosphate | not added (accumulates from PEP); 2.7 mM sodium oxalate to sequester Pi/Mg | 15 mM K-phosphate deliberately added (feeds salvage/energy metabolism) |
| Polyamines | 1.5 mM spermidine + 1 mM putrescine (replacing PEG) | screened out — not required |
| Cofactors / supplements | 0.33 mM NAD, 0.26 mM CoA, cAMP, 34 µg/mL folinic acid, 170 µg/mL E. coli tRNA, 100 µg/mL T7 RNAP | tRNA/CoA/NAD/folinic acid omitted (retained in non-dialyzed lysate); 4 mM nicotinamide as an NAD precursor |
| Yield / cost | ~1.3 g/L sfGFP; ~$4,672 per L of reaction | 2.4 ± 0.3 g/L sfGFP (87% higher than PANOx-SP); $133/L (~$55/g protein) — an 84–99% cost reduction; 3.7 ± 0.2 g/L with O₂ supplementation at 4-mL scale |
Contrasts
The design philosophies are 22 years apart.
- PANOx-SP was hand-crafted to mimic the cytoplasm (swap acetate → glutamate, PEG → polyamines, single-variable titrations).
- RFopt was found by design-of-experiments screening — 524 formulations tested across a 2²¹ (~2.1 million) combination design space — which stripped out everything non-essential (no NTPs, tRNA, CoA, folinic acid, polyamines) and deliberately added phosphate and buffer.
- The practical trade-off mirrors Q2: PANOx-SP is oxygen-insensitive and tolerant of lysate variability; RFopt is ~35× cheaper and higher-yielding but demands good oxygen transfer and per-batch Mg/phosphate tuning.
B. Design One Reaction Composition for Each of the Three Proteins
1. sfGFP Reaction Composition
JSON
Rationale
- Energy: ribose 40 mM + glucose 6.94 mM + NMPs (AMP 0.63, CMP 0.38, GMP 0.5, UMP 0.38 mM): This composition is shared by every ≥3.0 g/L model; avoids PEP/NTP costs entirely.
- Substrates: AA 3.75 mM + Cys 4.0 mM (cysteine enrichment is in all top-4 models) + Tyr 1.25 mM.
- pH control: HEPES 45 mM + phosphate 5.6/5.6 mM dual buffer — #77’s exact recipe.
- Spermidine: 1.25 mM kept as cheap insurance: not strictly required (#77 won without it), but it stabilizes ribosomes/mRNA and sits in 3 of 5 top models at ~1.2 mM — minimal cost, plausible upside, no shown downside for a one-shot bet.
- Everything else omitted: catalase, spermidine, pyruvate, adenosine, guanine, DTT/GSSG/GSH, PEG, etc.: the data shows they add cost, not yield, for sfGFP.
2. PETase
3. Reteplase
Rationale for PETase and Reteplase
- Ribose + NMPs replaces PEP + NTPs + NAD + CoA + folinic acid: Because ribose is cheaper.
- Acidic phosphate (mono:di, 10 mM): mirrors the sfGFP-Ribose reference; counters pH drop from ribose/glucose catabolism.
- Total Mg ~10 mM, HEPES 60 mM, AA 3 mM: squarely inside the range of the best-performing GPT-5 conditions.
- Reteplase: gets a strongly oxidizing redox pair (4:1 GSSG:GSH) and no added cysteine.
- PETase: gets a milder redox pulse suited to its fewer disulfides.