week-09-hw-cell-free-systems

Part A: General & Lecturer-Specific Questions

General Homework 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.

   Rapid Iteration and Throughput

• Direct Use of Linear DNA Templates
  Traditional methods require time-consuming cloning of DNA into circular plasmids before they can be inserted into a host cell.
  In CFPS, you can use raw PCR products directly as the instruction manual, allowing you to move from a genetic design to a functional protein in just a few hours.

• High-Throughput Screening Compatibility
  Because the reaction occurs in a simple liquid phase without the need for incubator space or shaking flasks, it can be easily scaled down into 96-well or 384-well plates.
  This allows robotic systems to simultaneously test hundreds of different protein variants or reaction conditions under identical parameters.

• Elimination of Cell Recovery and Lysis
  In living systems, you must wait for the culture to reach a specific density and then physically break the cells open to harvest the protein.
  CFPS skips these steps entirely because the protein is synthesized

2. Name at least two cases where cell-free expression is more beneficial than cell production.

• Production of Cytotoxic Proteins
  In traditional in vivo production, the target protein often interferes with the host cell’s survival. For example, if you are trying to produce antimicrobial peptides (AMPs) or pore-forming toxins, the protein will kill the E. coli or yeast “factory” as soon as it is expressed, leading to zero yield.
  CFPS Benefit: Since there is no living cell to keep alive, the system is indifferent to the toxicity of the product. This allows for the high-titer production of potent toxins, lytic enzymes, and other proteins that are normally “undruggable” or unproduceable in living hosts.

• Incorporation of Non-Canonical Amino Acids (ncAAs)
  If you want to create a “designer” protein with chemical properties not found in nature—such as adding a fluorescent tag, a “click-chemistry” handle, or a post-translational modification—you must use ncAAs. In a living cell, this requires complex metabolic engineering to ensure the cell doesn’t accidentally incorporate the synthetic amino acid into its own essential proteins, which would be lethal.
  CFPS Benefit: You can directly manipulate the translation machinery by adding pre-charged tRNAs and orthogonal synthetases without worrying about cross-reactivity with the host’s proteome. This provides a high degree of “chemical site-specificity,” allowing for the production of sophisticated protein-drug conjugates and advanced biomaterials.

3. Describe the main components of a cell-free expression system and explain the role of each component.
   Main Components of a Cell-Free Expression System

• The Crude Extract (The Machinery)
  The extract is the heart of the system, typically derived from cells like E. coli, wheat germ, or rabbit reticulocytes that have been physically lysed.
  Role: Provides the essential molecular “hardware” required for translation, including ribosomes, tRNAs, aminoacyl-tRNA synthetases, and translation factors. It also contains endogenous enzymes needed for energy regeneration.

• The DNA Template (The Instructions)
  This is the genetic blueprint for the protein you want to synthesize. Unlike in vivo methods, this can be a circular plasmid or a simple linear PCR product.
  Role: Contains promoter and terminator sequences that tell the machinery where to start and stop. Serves as the instruction manual for mRNA production (transcription) and subsequent protein synthesis (translation).

• Energy Regeneration System
  Protein synthesis is energetically expensive. Since the system is no longer part of a living cell, it cannot “eat” or perform cellular respiration to stay powered.
  Role: Typically consists of high-energy phosphate compounds (like phosphoenolpyruvate or creatine phosphate) and corresponding kinases. Acts as a “battery pack” to continuously regenerate ATP and GTP, which are consumed during amino acid chain assembly.

• Substrates and Cofactors (The Building Blocks)
  These are the raw materials added to the reaction buffer to facilitate biochemical reactions.

  • Amino Acids: The 20 standard building blocks or even non-canonical ones, used to assemble the protein chain.
  • Nucleotides (NTPs): Used for transcribing DNA into mRNA and as energy carriers.
  • Salts and Buffers: Magnesium ($Mg^{2+}$) and potassium ($K^{+}$) ions are strictly required for ribosome stability and enzymatic activity, while buffers maintain a stable pH.

4. 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.
   • Energy Demand of Translation
     Each amino acid added to a growing peptide chain consumes two ATP (for tRNA charging) and two GTP (for ribosome movement), making protein synthesis one of the most energy-intensive processes in biochemistry.

• Risk of Rapid Depletion
  In a closed system without recycling, the initial ATP/GTP pool would be exhausted within minutes, stalling protein production. Accumulated phosphate byproducts can also bind magnesium ions, destabilizing the reaction.

• Enzymatic Regeneration Pathways
  We can add high-energy donor molecules (e.g., phosphoenolpyruvate or creatine phosphate) with kinases like pyruvate kinase. These enzymes recycle ADP back into ATP, acting as a biological “battery chagrer.”

• Dialysis-Based Continuous Supply
  Advanced setups use semi-permeable membranes to allow fresh nutrients and ATP to diffuse in while removing inhibitory byproducts. This maintains chemical equilibrium and enables sustained protein synthesis for days.

Source:

5. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.
   Comparison of Prokaryotic vs. Eukaryotic Cell-Free Systems

   Example 1: Prokaryotic System (E. coli)

• Protein to Produce: Green Fluorescent Protein (GFP)
• Reasoning: GFP is small, robust, and does not require post-translational modifications to fluoresce.
• Efficiency: E. coli extracts have the highest translation rates, enabling vast quantities of GFP production within hours.
• Monitoring: Fluorescence can be tracked in real-time. GFP serves as an ideal reporter protein for testing new cell-free reaction conditions or energy regeneration strategies, since its folding is simple enough for bacterial machinery.

Example 2: Eukaryotic System (CHO Cells)

• Protein to Produce: Tissue Plasminogen Activator (tPA)
• Reasoning: tPA is a complex human enzyme used to dissolve blood clots and is difficult to produce in bacteria.
• Disulfide Bonding: Contains 17 disulfide bonds. Bacterial cytoplasm is highly reducing and fails to form these correctly. Eukaryotic extracts with microsomal membranes provide the oxidative environment and chaperones (e.g., Protein Disulfide Isomerase) for proper folding.
• Glycosylation: Requires specific sugar chains for stability and activity in the human body. Eukaryotic cell-free systems can be supplemented with microsomes (ER-derived vesicles) to perform these modifications, which are impossible in E. coli systems.

I have tried to sum up the advantages and disadvantages comparision of both expression systems here:

6. 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.

The key factors required to design a cell-free experiment for membrane proteins are:

• 1. Artificial Lipid Environments
  Membrane proteins are usually hydrophobic and require a lipid bilayer to fold correctly. In cell-free systems, researchers introduce artificial lipid structures such as liposomes, nanodiscs, or microsomes to mimic the natural membrane. These environments stabilize the protein during synthesis and facilitate proper insertion and folding. The MEMPLEX platform, for example, generates thousands of lipid-protein combinations to identify optimal conditions for each membrane protein.

• 2. Controlled Chemical Interactions
  Since CFPS allows precise control over the chemical environmentm, we can independently vary lipid composition, ionic strength, redox potential, and chaperone concentrations. This enables the fine-tuning of protein-protein and protein-lipid interactions, which are critical for membrane protein stability and functionality. MEMPLEX uses machine learning to predict and optimize these combinations, accelerating the design of functional synthetic environments.

The following problems may be encountered:

• Challenge A: Protein Aggregation and Misfolding
  Hydrophobic transmembrane helices tend to clump together or stick to the tube walls without a lipid bilayer.
  Solution: Implement nanodiscs — small, uniform discoidal bilayers wrapped in membrane scaffold proteins (MSPs). Unlike liposomes, nanodiscs keep membrane proteins soluble and monomeric, making them ideal for structural studies such as Cryo-EM.

• Challenge B: Low Yields due to Resource Depletion
  Membrane protein synthesis is slower and consumes more energy than soluble protein synthesis, leading to rapid depletion of ATP and accumulation of inhibitory byproducts.
  Solution: Use the Continuous-Exchange Cell-Free (CECF) method. A dialysis membrane provides a constant supply of ATP and nutrients while removing inorganic phosphate, sustaining the reaction for complex protein folding.

• Challenge C: Maintaining Correct Orientation
  In vitro systems lack the natural “inside-outside” topology of living cells, so proteins may insert incorrectly into synthetic membranes.
  Solution: Adjust the physicochemical environment by tuning lipid ratios (e.g., phosphatidylethanolamine [PE] or phosphatidylglycerol [PG]) to encourage the Positive-Inside Rule. Supplement with purified chaperones (e.g., DnaK, GroEL) to keep proteins flexible until proper orientation is achieved.

7. 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.

   Common Reasons for Low Protein Yield in CFPS and Fixes

• Rapid Template Degradation
  Crude extracts tend to contain nucleases that degrade linear DNA templates before transcription.
  Fixes:

  • Add nuclease inhibitors (e.g., GamS to block RecBCD).
  • Switch to circular plasmid DNA, which is more resistant to degradation, but is slower to replicate.

• Magnesium Ion ($Mg^{2+}$) Imbalance
  Magnesium stabilizes ribosomes and enzymes, but its optimal range is narrow (8–15 mM). Too little causes ribosome collapse; too much causes mRNA aggregation. ATP breakdown also sequesters magnesium mid-reaction.
  Fixes:

  • Perform magnesium titration (e.g., 2 mM increments in 96-well plates).
  • Use stronger buffers (HEPES or Tris) to maintain pH and magnesium solubility.

• Inefficient Protein Folding or Aggregation
  This is usually the main culprit when complex proteins with disulfide bonds are involved. They may misfold or aggregate at high local concentrations.
  Fixes:

  • Lower reaction temperature (e.g., from 37°C to 25–20°C) to slow synthesis and allow proper folding.
  • Add molecular chaperones (e.g., DnaK, DnaJ, GroEL/ES) to prevent aggregation and assist folding.

Kate Adamala’s Question

• Design an example of a useful synthetic minimal cell:
  • Pick a function and describe it (input/output).
  • Could this function be realized by cell-free Tx/Tl alone, without encapsulation?
  • Could this function be realized by a genetically modified natural cell?
  • Describe the desired outcome of your synthetic cell operation.
  • Design all components (membrane composition, encapsulated molecules, Tx/Tl source organism).
  • How will your synthetic cell communicate with the environment?
  • Provide experimental details (lipids, genes, measurement method).

Designing a Microsynthetic Methanogen

1. Function, Input, and Output

• Function: Acetoclastic Methanogenesis (converting acetate to methane).
• Input: Acetate (from pre-processed biomass slurry). (I thought of going through just puliverized biomass. But the engineering becomes too complex or even unviable.)
• Output: ($CH_4$) and Carbon Dioxide ($CO_2$).

2. Can this be realized by cell-free Tx/Tl alone?

• No. Methanogenesis requires a proton motive force ($\Delta p$) across a lipid bilayer. In open Tx/Tl systems, ions dissipate and energy cycles fail.

3. Can this be realized by a genetically modified natural cell?

• Yes, but inefficient. Natural methanogens are slow-growing, strictly anaerobic, and spend energy on survival. A synthetic cell directs all flux toward gas production.

4. Desired Outcome

• A stable “biocatalytic bead” added to anaerobic digesters to accelerate acetate-to-methane conversion, bypassing microbial growth limitations.

5. Component Design & System Architecture

• Membrane

  • Composition: POPC (1‑Palmitoyl‑2‑Oleoyl‑sn‑Glycero‑3‑Phosphocholine) + DPPC (1,2‑Dipalmitoyl‑sn‑Glycero‑3‑Phosphocholine) hybrid.
  • Rationale: Semi-permeable bilayer mimicking archaeal stability, compatible with bacterial Tx/Tl.

• Tx/Tl Source

  • Source: E. coli (S30 Extract).
  • Rationale: Robust protein synthesis; archaeal genes translated efficiently with optimized RBS.

• Encapsulated Cargo

  • Machinery: Ribosomes, T7 RNA Polymerase, Amino Acids, NTPs.
  • Small Molecules: Coenzyme M (HS-CoM), Coenzyme B (HS-CoB) pre-loaded for methane production.

• Communication (Permeability)

  • Substrate Entry: Acetate transporter AatP.
  • Product Exit: Methane ($CH_4$) diffuses naturally through the bilayer.

6. Experimental Details

7. Measuring Function

• Gas Chromatography (GC): Quantify methane volume and purity from headspace.
• pH Fluorescent Probes: Encapsulate Pyranine (HPTS) to detect proton movement across the membrane.
• Radioactive Labeling ($^{14}C$): Track conversion of $^{14}C$-acetate into labeled methane for definitive proof.

Peter Nguyen’s Question

• Freeze-dried cell-free systems can be incorporated into materials. Choose one field (Architecture, Textiles/Fashion, or Robotics) and propose an application:
  1. Write a one-sentence pitch:
  A “living” Martian masonry system composed of regolith bricks held together by mycellium, embedded with freeze-dried, cell-free biosensors that detect structural micro-fractures and signal repair needs via bioluminescence before catastrophic failure occurs.

  2. Explain how the idea works in detail: The core of this application is the integration of freeze-dried cell-free (FD-CF) machinery into the binder of Martian 3D-printed regolith. During the manufacturing of these mycellium-based bricks, a stabilized E. coli lysate containing the genetic instructoins for Luciferase is mixed with a porous substrate. These components remain dormant in the hyper-arid, cold Martian environment. If a structural micro-crack forms, it allows a small amount of pressurized “habitat air” (containing water vapor and localized heat) to reach the FD-CF pocket. This moisture acts as the trigger, rehydrating the system. Upon activation, the machinery translates the bioluminescent protein, causing the crack to glow brightly against the dark Martian regolith, acting as an autonomous, self-powered alarm system for astronauts.

  4. Identify the societal challenge or market need addressed. The primary challenge in Martian architecture is human safety in extreme environments. Unlike Earth, a hairline crack on Mars can lead to explosive decompression or lethal radiation exposure. Current electronic sensors require extensive wiring, constant power, and are prone to radiation interference. There is a need for passive, zero-energy monitoring systems that are lightweight to transport. By using cell-free systems, we eliminate the need to keep biological organisms alive during the 7-month space transit, providing a “just-expose-to-moisture” safety net for the first Martian colonies.

  5. Discuss how you would overcome limitations of cell-free reactions (activation, stability, one-time use).

• Activation Control To prevent accidental activation from ambient habitat humidity, the FD-CF components are encapsulated in hygroscopic wax microspheres. These spheres only melt or dissolve when exposed to the specific temperature and moisture profile of a localized structural leak, ensuring the “bio-sensor” only fires when a true breach occurs.

• Stability and Longevity Space radiation is the biggest threat to biological molecules. We will try to address this by incorporating lyoprotectants (like trehalose) and polyphenolic antioxidants into the freeze-drying mix. Whether these can stabilize the protein machinery and DNA templates in a “glassy state,” allowing them to remain viable for years in the Martian crust without denaturing, must be validated experimentally.

• One-Time Use to Repeatable Use While a single cell-free reaction is typically “one-shot,” we can think of designing the material with modular “casings.” Each brick contains hundreds of isolated micro-pockets of extract. If one pocket is used to signal a crack and the crack is then patched, the surrounding unused pockets remain dormant. This “redundancy-by-design” ensures the material provides monitoring capabilities throughout the lifespan of the building, despite the one-time-use nature of each individual biochemical reaction. However, we can expect the cost to be very high, and must be adressed suitably.

Ally Huang’s Question

• Develop a mock Genes in Space proposal using BioBits® (and optionally miniPCR® and P51 viewer):

  1. Background information (≤100 words):
     Monitoring structural integrity in space-grown “myco-architecture” is vital for long-duration missions. While mycelium-regolith composites are promising, they face structural stress from internal pressurization and radiation. BioBits®—a freeze-dried cell-free (FD-CF) system—enables biological sensing without the logistical burden of keeping cells alive. This experiment, designed for the ISS, seeks to validate whether FD-CF machinery, embedded in a fungal-mineral matrix, can undergo autonomous rehydration and protein synthesis in microgravity. Proving this confirms the feasibility of using “living” bricks that glow to warn astronauts of pressure loss or radiation spikes.

  2. Molecular/genetic target (≤30 words):
     A DNA plasmid encoding T7 RNA Polymerase and the mCherry fluorescent protein under a T7 promoter, optimized for detection via the P51 viewer and miniPCR® validation.

  3. Relation to space biology challenge (≤100 words):
     The primary challenge is the stability of biological hardware in a high-radiation, microgravity environment. Traditional sensors rely on electronics that are heavy and sensitive to galactic cosmic rays. This project tests if BioBits® can survive the “launch-to-activation” timeline while embedded in a porous, fungal-regolith matrix. Validating this on the ISS addresses the need for low-mass, zero-power safety systems. It also explores how microgravity affects the diffusion of rehydrating fluids within the unique capillary structures of desiccated mycelium, a critical factor for sensor response time in orbit.

  4. Hypothesis/research goal with reasoning (≤150 words):
     Hypothesis: Microgravity will not significantly inhibit the rehydration-induced activation of BioBits® within a mycelium-regolith matrix, and the fungal chitin will provide a protective micro-environment against ISS-level ionizing radiation.

Reasoning: In microgravity, fluid dynamics are dominated by surface tension rather than gravity-driven flow. We can hypothesize that the natural porosity of the mycelium will facilitate uniform rehydration of the FD-CF pellets via capillary action. Annd also, the molecular density of the regolith and the melanin content in the fungal cell walls should shield the DNA and ribosomes from radiation damage during their “dormant” phase. The goal will be to compare the fluorescence kinetics (speed and brightness) of the space-activated samples against Earth-based controls to determine if the lack of convection in microgravity slows down the metabolic-like reaction of the cell-free system.

5. Experimental plan (samples, controls, data collection, ≤100 words): Samples: Three mycelium-regolith cubes containing embedded BioBits® pellets and the mCherry DNA circuit.

Controls: One “Dry” brick (unactivated) and one “Wet” brick (Earth-activated) as baselines.

Execution: Use a sealed MWA (Maintenance Work Area) to inject 100 μL of rehydration buffer into the bricks via syringe to simulate a localized atmospheric leak.

Data Collection: Cubes are placed into the P51 viewer; astronauts take time-lapse photos to track fluorescence development. Finally, miniPCR® will amplify the mCherry gene from the “Dry” brick to assess DNA degradation during the flight.

Part B: Individual Final Project

1. Decide and write down Aim 1 of your final project.
   Answer:

2. Add your chosen final project slide to the appropriate deck.
   (Attach or link slide here)

3. Submit the Final Project selection form if not already done.
   (Confirmation note here)

4. Begin planning documentation based on provided guidelines.
   (Notes here)

5. Prepare your first DNA order and place it in the correct Twist tab (deadline varies by group).
   (Details here)