Week 9 HW: Cell-Free Systems
Part A: Conceptual Questions
1. Explain the main advantages of cell-free protein synthesis over traditional in vivo methods, specifically in terms of flexibility and control over experimental variables. Name at least two cases where cell free expression is more beneficial than cell production.
Cell-free protein synthesis (CFPS) has transitioned from a niche laboratory technique to a powerful platform for synthetic biology and biomanufacturing. By removing the constraints of maintaining cell viability, CFPS offers a level of engineering precision that traditional in vivo methods cannot match.
Advantages of CFPS: Flexibility and Control
In traditional in vivo expression, the cell’s “homeostasis” is the primary obstacle. The cell prioritizes its own survival over your recombinant protein. CFPS bypasses this by using only the necessary molecular machinery.
- Direct Access: Since there is no cell membrane, you can directly manipulate the reaction environment. You can add non-natural amino acids, chaperones, or specific detergents without worrying about cellular toxicity.
- Variable Control: You can precisely calibrate the concentrations of DNA templates, T7 polymerase, and energy substrates. In cells, these levels are dictated by the organism’s metabolic state.
- Elimination of Toxicity: Many proteins (like antimicrobial peptides or certain enzymes) kill the host cell upon expression. In a cell-free system, the “host” is already dead, allowing for the synthesis of highly toxic molecules.
Two cases where CFPS is more beneficial
Rapid Prototyping (Design-Build-Test): Screening 100 variants of a protein takes days with CFPS (just add PCR products to the mix) compared to weeks for microbial transformation and cloning.
Incorporation of Non-Canonical Amino Acids (ncAAs): CFPS allows for the easy expansion of the genetic code to create “bio-orthogonal” proteins with unique chemical properties that might otherwise interfere with a living cell’s metabolism.
2. Describe the main components of a cell-free expression system and explain the role of each component.
A standard cell-free system consists of three functional groups:
- Cell Extract (S30/S12): Provides the “hardware”: ribosomes, aminoacyl-tRNA synthetases, and initiation/elongation factors.
- Energy Buffer: Contains ATP, GTP, and an energy regeneration substrate (e.g., phosphoenolpyruvate) to fuel the high-energy cost of translation.
- Reaction Mix: Includes the DNA/mRNA template, salts (Mg2+, K+), amino acids, and often a T7 RNA polymerase for coupled transcription-translation.
3. Why is energy provision regeneration critical in cell-free systems? Describe a method you could use to ensure continuous ATP supply in your cell-free experiment.
Protein synthesis is energetically expensive; every peptide bond consumes multiple high-energy phosphates. In a batch CFPS reaction, endogenous ATP is depleted within minutes due to “side reactions” and phosphatase activity.
To maintain a steady-state concentration of ATP, we use secondary energy sources. A common method is the Creatine Phosphate/Creatine Kinase (CP/CK) system.
- Method: We add Creatine Phosphate and the enzyme Creatine Kinase to the mix. The enzyme transfers a phosphate group from CP to ADP, constantly “recharging” the ATP pool as it is consumed. This prevents the accumulation of inorganic phosphate, which can eventually inhibit the reaction.
4. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.
| Feature | Prokaryotic (e.g., E. coli) | Eukaryotic (e.g., Wheat Germ/CHO) |
|---|---|---|
| Speed/Yield | Very high; fast turnaround. | Slower; lower yields. |
| Folding | Limited post-translational modifications. | Complex folding and glycosylation. |
- Prokaryotic Choice: T7 Polymerase. It is a simple, robust protein that doesn’t require complex folding or glycosylation, making E. coli extracts the most cost-effective choice.
- Eukaryotic Choice: Human Erythropoietin (EPO). This requires specific glycosylation patterns to be biologically active. A CHO (Chinese Hamster Ovary) cell-free system would be used because it contains the microsomes/ER vesicles necessary for these modifications.
5. How would you design a cell-free experiment to optimize the expression of a membrane protein? Discuss the challenges and how you would address them in your setup.
Membrane proteins are notoriously difficult because they are hydrophobic and aggregate in aqueous buffers.
The Design: To optimize expression, we must provide a “hydrophobic mimic” during synthesis. I would use Nanodiscs—small, discoidal bilayers held together by membrane scaffold proteins.
- Challenge: Insoluble aggregation and misfolding.
- Strategy: Perform a “co-translational” setup where Nanodiscs are added directly to the CFPS reaction. As the ribosome produces the hydrophobic transmembrane helices, they spontaneously insert into the Nanodisc bilayer, maintaining their native conformation and solubility.
6. Imagine you observe a low yield of your target protein in a cell-free system. Describe three possible reasons for this and suggest a troubleshooting strategy for each.
If I observe a low yield, I would consider these three potential points of failure:
- Magnesium (Mg2+) Concentration:
- Reason: Ribosome stability and polymerase activity are extremely sensitive to magnesium levels.
- Strategy: Perform a magnesium titration. Run several small-scale reactions with Mg2+ concentrations ranging from 5mM to 20mM to find the “sweet spot.”
- Template Degradation:
- Reason: Endogenous nucleases in the cell extract may be chewing up your DNA or mRNA template.
- Strategy: Use circular plasmid DNA instead of linear PCR products, or add RNase inhibitors to the reaction mix.
- Codon Bias:
- Reason: The tRNA pool in the extract (e.g., E. coli) might not match the codon usage of our target gene (e.g., a human gene).
- Strategy: Supplement the reaction with extra tRNAs for rare codons or use a commercially available “Extra” extract enriched with rare tRNA species.