Week 9 HW: Cell Free Systems

Part A: General and Lecturer-Specific Questions

General Homework Questions

Q1. 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) is more flexible than in vivo expression because it removes cell growth and viability constraints and allows the reaction environment to be adjusted directly. In a cell-free system, researchers can independently tune DNA template concentration, salts, cofactors, energy substrates, pH, redox conditions, crowding agents, and added folding or membrane-assisting components. This makes it easier to study protein production as a controllable biochemical process.

CFPS is especially beneficial when:

  • (1) Producing toxic proteins or peptides, because the product does not need to accumulate inside a living host.

  • (2) When producing complex proteins such as membrane proteins or post-translationally modified proteins, because the reaction can be supplemented with vesicles, microsomes, chaperones or glycosylation machinery.

Q2. Describe the main components of a cell-free expression system and explain the role of each component.

A cell-free expression system usually contains:

  • (1) Cell extract or purified translation/transcription machinery: this provides ribosomes, translation factors, tRNAs, enzymes, and sometimes endogenous metabolic activities needed for protein synthesis.
  • (2) DNA or RNA template: this encodes the target protein and includes the regulatory elements required for transcription and/or translation.
  • (3) RNA polymerase and nucleotides: RNA polymerase transcribes DNA into mRNA, and nucleotides supply the building blocks for RNA synthesis.
  • (4) Amino acids, tRNAs, and translation factors: these are required to decode mRNA and assemble the protein chain.
  • (5) Energy system: substrates such as PEP, creatine phosphate, pyruvate, glucose, or maltodextrin support ATP regeneration for sustained transcription and translation.
  • (6) Salts, buffer, and cofactors: magnesium, potassium, ammonium, NAD, CoA, and buffering agents maintain ionic balance, enzyme activity, and pH stability.
  • (7) Optional additives such as chaperones, redox agents, detergents, liposomes, nanodiscs, or microsomes can be added for difficult proteins.

Q3. 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 regeneration is critical because transcription and translation consume large amounts of ATP and GTP, and a batch cell-free reaction quickly loses productivity if energy is depleted or inhibitory byproducts accumulate. Without regeneration, protein yield drops because the system can no longer support RNA synthesis, amino acid activation, and ribosome function. One practical method is to use a continuous-exchange or fed-batch setup together with an ATP-regenerating substrate such as PEP or maltodextrin. In this design, the reaction chamber is supplied with fresh small molecules from a feeding solution while waste products diffuse away, which prolongs reaction lifetime and maintains ATP production.

Q4. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.

A prokaryotic cell-free system, especially E. coli-based CFPS, is usually cheaper, faster, and higher-yielding, but it has limited native capacity for complex eukaryotic post-translational modifications. I would choose sfGFP or a simple bacterial enzyme for an E. coli system because these proteins do not require glycosylation and are ideal for rapid, low-cost, high-yield expression.

A eukaryotic cell-free system, such as CHO, wheat germ, or tobacco BY-2 extract, is more suitable for proteins that need more complex folding or post-translational processing. I would choose a full-length IgG antibody for a CHO-based system because CHO extracts can support production of functional antibodies and are better suited for proteins that benefit from eukaryotic folding and glycosylation-related machinery.

Q5. 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.

To optimize a membrane protein, I would set up a screening experiment in parallel conditions using the same DNA template but different membrane-mimicking environments. For example, I would compare nanodiscs, liposomes, mild detergents, and, if available, microsome-containing extracts. I would also vary temperature, magnesium concentration, and lipid composition, then measure both total yield and functional activity.

The main challenges are aggregation of hydrophobic transmembrane domains, incorrect insertion or orientation, and loss of activity due to the absence of a natural membrane environment. I would address these by adding membrane mimics during expression, choosing lipid compositions that better match the protein’s hydrophobic thickness, and using chaperones or lower-temperature conditions to improve folding. For activity testing, I would use ligand binding or transport assays rather than relying only on total protein yield, because a membrane protein can be expressed but still be nonfunctional.

Q6. 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.

  • Reason 1: Poor template quality or template degradation. Linear DNA can be degraded by nucleases, and low-quality DNA can reduce transcription efficiency. A good troubleshooting strategy is to switch to a plasmid template, improve template purification, or protect linear DNA with strategies such as GamS-like inhibition or terminal protection.

  • Reason 2: Energy limitation or inhibitory byproduct accumulation. If ATP regeneration is insufficient, or if phosphate and acidic byproducts accumulate, transcription and translation will slow down. I would troubleshoot this by optimizing the energy substrate, adjusting magnesium and buffer conditions, or moving from a simple batch reaction to fed-batch or continuous exchange.

  • Reason 3: Protein-specific folding or solubility problems. Some proteins misfold, aggregate, or require special environments such as oxidizing conditions, chaperones, or membranes. I would troubleshoot this by lowering the reaction temperature, adding folding chaperones, adjusting redox conditions, or supplementing membrane mimics if the target is membrane-associated. If the protein is highly complex, I would also consider switching from a prokaryotic to a eukaryotic cell-free system.

Homework question from Peter Nguyen

Freeze-dried cell-free systems can be incorporated into all kinds of materials as biological sensors or as inducible enzymes to modify the material itself or the surrounding environment. Choose one application field — Architecture, Textiles/Fashion, or Robotics — and propose an application using cell-free systems that are functionally integrated into the material. Answer each of these key questions for your proposal pitch:

  • Write a one-sentence summary pitch sentence describing your concept. A topology-optimized architectural substrate is coated with a biopolymer-based surface layer made from freeze-dried cell-free manufactured ingredients to improve durability while adding a lower-carbon functional finish.

  • How will the idea work, in more detail? Write 3-4 sentences or more. The base material would first be designed to use less material while increasing surface area for coating adhesion. A freeze-dried cell-free system would be used off-site to produce a dry biopolymer ingredient, such as a polysaccharide- or protein-based additive. This ingredient would then be blended into a coating and applied to the surface by spraying, dipping, or brushing. After curing, the coating would help improve adhesion, bridge microcracks, and create a tougher protective layer on top of the structural substrate.

  • What societal challenge or market need will this address? This idea addresses the need for more sustainable building materials and longer-lasting surface finishes. Many architectural coatings and repair layers rely on carbon-intensive ingredients and still fail through cracking, abrasion, or moisture damage. A bio-derived coating could reduce maintenance frequency, extend service life, and support lower-carbon construction without requiring a complete change in structural material systems.

  • How do you envision addressing the limitation of cell-free reactions (e.g., activation with water, stability, one-time use)? The cell-free system would be used only during the manufacturing stage to produce the biopolymer ingredient for the coating. Water activation would take place in a controlled setting off-site, where the freeze-dried reaction can be rehydrated, run once, and converted into a stable dry product. That finished biopolymer would then be blended into the coating formula, so the final architectural coating would function as a passive material layer and would not depend on ongoing biological activity after application. This approach improves stability, avoids maintenance challenges linked to repeated activation, and makes one-time use acceptable because the reaction is completed before the material is installed on the building.

Homework question from Ally Huang

How does deep-space radiation affect the integrity of stored DNA templates, and can that damage reduce the reliability of freeze-dried cell-free protein production systems such as BioBits® in future space missions?

  • Background information Deep-space radiation damages DNA and is a major risk for astronauts beyond Earth’s protective atmosphere. Future missions may also depend on stored DNA templates and freeze-dried cell-free systems to manufacture sensors, medicines, or enzymes on demand with minimal equipment. My proposal asks whether radiation exposure degrades those DNA instructions enough to reduce later protein production. This matters for human exploration because damaged DNA stockpiles could weaken in-flight diagnostics and biomanufacturing, and it is scientifically interesting because it tests how the space environment affects the information layer of synthetic biology, not only living cells.

  • Molecular or genetic target A GFP reporter DNA cassette used as a model stored genetic template for later cell-free protein production in space.

  • How the target relates to the challenge The reporter DNA is a stand-in for any DNA blueprint that astronauts might store and later load into BioBits. If radiation introduces strand breaks or base damage, PCR amplification should become less efficient and the cell-free system should produce less fluorescent protein. Measuring expression from the same cassette after different exposure conditions turns DNA integrity into a simple functional readout. This connects a molecular target to a practical space question: can freeze-dried biomanufacturing remain reliable after DNA templates spend time in the space environment?

  • Hypothesis or research goal My hypothesis is that DNA templates exposed to higher space-radiation dose or lower shielding will show lower PCR recoverability and lower GFP output in BioBits than protected templates. The reasoning is simple: ionizing radiation can damage DNA, and BioBits needs DNA instructions plus water to synthesize protein. If this is true, future missions should store critical DNA libraries behind better shielding or refresh them periodically. If expression remains stable despite exposure, that would support the idea that freeze-dried DNA-plus-cell-free kits are robust enough for long missions. Either result would be useful because it would test a key bottleneck between DNA storage and on-demand biomanufacturing in space.

  • Experimental plan I would test identical dried GFP DNA templates stored in two conditions: shielded and minimally shielded. Matching ground controls would stay on Earth. After exposure, each sample would be amplified with miniPCR and then added to BioBits; the P51 viewer would measure endpoint GFP fluorescence. Controls would include a fresh positive-control template, a no-DNA negative control, and non-flown matched DNA. The main data would be fluorescence intensity, PCR success or yield, and expression level normalized across conditions.