Week 9 HW: Cell-Free Systems

Homework Part A: General and Lecturer-Specific 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.

Ans: Cell-free protein synthesis offers significant advantages over in vivo methods due to its open and controllable nature. It allows direct manipulation of reaction components, precise control over parameters such as pH and substrate concentration, and eliminates constraints related to cell viability. As a result, all system resources can be directed toward protein production, enabling rapid optimization and high-throughput experimentation.

CFPS is especially beneficial in cases such as (1) expression of toxic or difficult-to-express proteins, where cellular systems fail, and (2) high-throughput screening and synthetic biology applications, where rapid prototyping without cloning is required.

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

Ans: A cell-free protein synthesis (CFPS) system contains the essential molecular machinery required for transcription and translation outside living cells.

Key components and roles

a. Cell extract (lysate): Derived from organisms like E. coli, wheat germ, or rabbit reticulocytes

Contains:

Ribosomes → protein synthesis

tRNAs → amino acid delivery

Enzymes → transcription & translation

Role: Core machinery that performs protein production

b. DNA or mRNA template:Encodes the target protein

Can be plasmid DNA or PCR product

Role: Provides genetic instructions for protein synthesis

c. Amino acids

Role: Building blocks for protein formation

d. Energy source system:ATP, GTP + regeneration components

Role: Powers transcription and translation processes

e. Nucleotides (NTPs) ATP, GTP, CTP, UTP

Role: Required for mRNA synthesis during transcription

f. Cofactors and salts Mg²⁺, K⁺, etc.

Role: Maintain optimal enzyme activity and ribosome stability

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.

Ans: Protein synthesis is energy-intensive:

->ATP → transcription + tRNA charging

->GTP → translation (elongation steps)

Without regeneration: a.ATP is rapidly depleted

b.Reaction stops prematurely

c.Protein yield becomes very low

CFPS lacks metabolism, so no natural ATP recycling occurs

Method to ensure continuous ATP supply

Phosphocreatine–creatine kinase system

Addition of Phosphocreatine (energy reservoir) and creatine kinase enzyme

Mechanism:

Phosphocreatine donates phosphate → regenerates ATP from ADP

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

Ans: Below table shows the difference between the prokaryotic and eukaryotic cell-free expression systems.

Note: Included the major features for flexible comparison

Protein I choosed.

a. Prokaryotic system → GFP (Green Fluorescent Protein)

-> Simple, no complex modifications needed

-> High yield required

Reason: E. coli CFPS is fast, cheap, and efficient for simple proteins

b. Eukaryotic system → Antibodies

Production of antibodies requires:

-> Proper folding

-> Disulfide bonds

-> Sometimes post-translational processing

As Eukaryotic systems better mimic cellular conditions for complex proteins, one can use Eukaryotic system to produce antibodies.

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.

Ans: Challenges included in designing a cell-free experiment to optimize the expresion of a membrane protein:

Membrane proteins are:

-> Hydrophobic

-> Prone to aggregation

-> Difficult to fold correctly

Remedies of challenges:

a. Add membrane mimetics: Liposomes or nanodiscs - Detergents (mild, non-denaturing)

Purpose: Provide a membrane-like environment

b. Optimize reaction conditions : By adjusting Mg²⁺, temperature, and redox conditions.

c. Include chaperones: Assist folding and insertion

d. Continuous exchange system (dialysis CFPS): ->Removes toxic byproducts

-> Extends reaction time

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.

Ans: Low protein yield: causes and troubleshooting

Problem 1: Poor DNA template quality

Reason:

Degraded DNA or weak promoter

Solution:

a.Use high-quality plasmid

b.Optimize promoter and RBS

Problem 2: Energy depletion

Reason:

ATP runs out quickly

Solution:

a. Use efficient regeneration system (e.g., PEP or glucose-based)

b. Optimize energy substrate concentration

Problem 4 : Inhibitory byproducts

Reason:

Accumulation of phosphate or waste

Solution:

Use continuous exchange CFPS

Homework question from Kate Adamala