Week 9 HW: Cell Free Systems

Part A: General and Lecturer-Specific Questions

General homework questions

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: is a protein expression approach that enables the production of a target protein without the use of living cells.
- In vivo methods: it is also a protein expression approach that uses living cells such as bacteria (e. coli, most common), yeast, insect cells, and mammalian cells.

Here is a chart that makes a summary about differences between both methods:

Cell-free protein synthesis vs. traditional in vivo methods

As it is shown, cell-free protein synthesis has several advantages compared to traditional in vivo methods, mainly because everything happens outside of a living cell. This makes the system much more flexible, since you can directly control things like the amount of DNA, enzymes, and other components without worrying about how the cell will react. In contrast to cell-dependent methods, in which limitations are high due to metabolism, regulation, and survival.

Another important advantage is that cell-free systems allow the production of proteins that might be toxic to cells, while in in vivo methods, these types of proteins can kill or damage the host organism, making them difficult or impossible to produce. In cell-free expression, this is not a problem because there are no living cells involved.

In terms of speed, it is faster (cell-free), since it does not need to spend time growing cells or transforming them. This makes it easier to quickly test different DNA sequences or protein variants.

There are several situations where the cell-free method is more useful. For example: when producing toxic proteins that cannot be expressed in cells, or for rapid prototyping applications, such as synthetic biology circuits or screening multiple protein variants in a short time. It is important to add that while cell-free methods are better with rapid process and protein expression in a controled-medium size, in vivo methods can handle massive production and low-cost protein production

New England Biolabs. (2026). Neb.com. https://www.neb.com/en/applications/protein-expression/cell-free-protein-expression?srsltid=AfmBOorKmWZBUknZgtYzmBfbx0IiXqMcRlLcgWd8oi4EKcYBFwv4sudk

‌Mason, E. (2023, March 23). Advantages of Cell-Free Protein Expression. Biocompare.com. https://www.biocompare.com/Editorial-Articles/594727-Advantages-of-Cell-Free-Protein-Expression/

‌Cui, Y., Chen, X., Wang, Z., & Lu, Y. (2022). Cell-Free PURE System: Evolution and Achievements. BioDesign Research, 2022. https://doi.org/10.34133/2022/9847014

Describe the main components of a cell-free expression system and explain the role of each component.
Cell-free expression systems are made up of several components that work together to produce proteins outside living cells:
- Cell extract: contains the molecular machinery needed for protein synthesis. This extract usually comes from broken cells (like bacteria) and provides ribosomes, tRNAs, enzymes, and other factors required for transcription and translation.
- DNA template: this is the gene that encodes the protein of interest. The system uses this DNA to produce mRNA and then translates it into the desired protein.
- Amino acids: they are the building blocks of proteins. These are added to the system so that ribosomes can assemble them into a protein based on the sequence of the mRNA.
- NTPs (nucleoside triphosphates): such as ATP, GTP, CTP, and UTP. These molecules are essential both for building the mRNA during transcription and for providing energy during translation.
- Energy source: protein synthesis requires a lot of energy, so the system needs molecules like ATP and other energy-regenerating compounds to keep the reaction running.
- Cofactors and salts: this helps the stability of the chemical environment and helps enzymes to function correctly. Cofactors and salts ensure that the system remains stable and efficient.

Kim, W., Han, J., Chauhan, S., & Lee, J. W. (2025). Cell-free protein synthesis and vesicle systems for programmable therapeutic manufacturing and delivery. Journal of Biological Engineering, 19(1). https://doi.org/10.1186/s13036-025-00523-x

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 provision regeneration is critical in cell-free systems because protein synthesis is a highly energy-demanding process. Both transcription and translation require large amounts of ATP and GTP. Without the continuous energy supply, the reaction stops. Because of the absence of a cellular metabolism to naturally generate energy, the cell-free system would run out of ATP very fast. Without it, protein yields would be very low, and the reaction would only last a short period.
There are some interesting pathways to produce continuous energy in cell-free systems, some of them are:
- Glucose and sugar metabolism: systems that are frequently derived from E.coli or yeast, enable high-yield, in vitro protein production by utilizing metabolic pathways to break down glucose, which can improve cost-efficiency.
- Maltodextrin metabolism: it is a low-cost secondary energy compound for CFPS. It produces higher levels of protein than PEP, glucose, and glucose-6 phosphate. The enhancement of protein synthesis was largely attributed to be better-controlled phosphate levels ( recycling of inorganic phosphate) and a more homeostatic reaction environment.
- Electric-generated power: in cells, ATP is synthethized through a rather complicated process involving several membrane-bound redox protein complexes. Electrons are transferred along different redox centers, creating a proton motive force across the membrane, which is subsequently harvested for ATP synthesis.

System	How it generates ATP	Advantages	Disadvantages	When to use it
Glucose and sugar metabolism	Uses glucose or other sugars (e.g., glycolysis) to produce ATP	Low cost; biologically natural; supports longer reactions	Can produce acidic byproducts; less control over conditions	When a cost-effective and stable system is needed
Maltodextrin metabolism	Uses maltodextrin, slowly broken down to generate ATP	More stable energy release; fewer inhibitory byproducts	Requires optimization; depends on enzymatic activity	When higher stability and consistency are required
Electricity-powered generation	Uses electrical energy to drive redox reactions	Precise control; real-time adjustment; reduced byproducts	Technically complex; requires specialized equipment	Advanced research or tight energy control systems

Wang Y, Zhang YH. Cell-free protein synthesis energized by slowly-metabolized maltodextrin. BMC Biotechnol. 2009 Jun 28;9:58. doi: 10.1186/1472-6750-9-58. PMID: 19558718; PMCID: PMC2716334.

Luo, S., Adam, D., Giaveri, S., Barthel, S., Cestellos-Blanco, S., Hege, D., Paczia, N., Castañeda-Losada, L., Klose, M., Arndt, F., Heider, J., & Erb, T. J. (2023). ATP production from electricity with a new-to-nature electrobiological module. Joule, 7(8), 1745-1758. https://doi.org/10.1016/j.joule.2023.07.012

Calhoun, Kara & Swartz, James. (2005). Energizing cell-free protein synthesis with glucose metabolism. Biotechnology and bioengineering. 90. 606-13. 10.1002/bit.20449.

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