Week 3 HW: Cell-Free Protocols

A. Compare cell-free reaction compositions

1. Why does the concentration of potassium glutamate and magnesium glutamate differ between compositions? Which is more sensitive?

The concentrations of potassium glutamate and magnesium glutamate differ between formulations because cell-free protein synthesis (CFPS) performance depends on the overall chemical environment, so the reagent concentrations have to be optimized together rather than one at a time. The glutamate is only the counter-ion; what matters is the K⁺ and Mg²⁺ it delivers. K⁺ mainly sets the ionic strength of the reaction, while Mg²⁺ stabilizes the ribosome and is required for ATP and other nucleotide triphosphates to be usable by the enzymes. Because NTPs, ATP and the inorganic phosphate released from PEP all bind Mg²⁺, a formulation with a different energy and nucleotide system needs a different total amount of Mg²⁺ to keep the right amount free. The lysate and the fixed base buffer also contribute their own ions.

Magnesium is the more sensitive of the two. Its concentration may need to be re-optimized for different lysate batches, and protein synthesis depends nonlinearly on free Mg²⁺ and nucleotide concentrations, so a small deviation can reduce yield sharply. Potassium tolerates a much wider range. This is also visible in the reference formulations on the coopetition site: K-glutamate ranges from 200 to 345 mM (about 1.7-fold), whereas Mg-glutamate stays within roughly 8 to 10 mM (about 1.2-fold), which points to a narrow optimum for Mg²⁺.

2. Explain how a PEP+NTP reaction formulation is different from a Ribose+NMP reaction formulation.

Both formulations must supply nucleotides to build mRNA and energy to regenerate ATP, but they do it in different ways.

A PEP+NTP formulation supplies the nucleotides already phosphorylated (ATP, CTP, GTP and UTP, each with three phosphate groups), which makes them ready to use. PEP is added as the secondary energy substrate: it donates its phosphate to ADP to regenerate ATP. This works well, but NTPs are among the most expensive reagents in the menu, and PEP releases inorganic phosphate that binds Mg²⁺ and can acidify the reaction. These formulations also usually carry extra cofactors such as NAD, CoA, folinic acid, polyamines and oxalate. PEP is also needed at high concentration (about 30 mM), so it takes up more than half of the 11 µL of variable volume.

A Ribose+NMP formulation supplies the nucleotides as monophosphates (AMP, CMP, GMP and UMP, with only one phosphate group), which are cheaper but not ready to use. Ribose is a simple sugar that serves as the starting material for energy and carbon. Because the lysate still contains the metabolic machinery of the original cells, its enzymes can convert these simpler starting materials into the forms needed for protein synthesis, phosphorylating the NMPs up to NTPs. Olsen et al. (2026) report that their optimized formulation can produce NTPs from nucleotides and ribose. This removes NTPs, PEP, NAD, CoA and folinic acid from the recipe, and the formulation relies instead on nicotinamide and phosphate buffering.

In terms of cost, using the reagent prices given in the assignment, CTP, GTP and UTP at about 175 nL each of a 100 mM stock cost roughly $0.015 to $0.020 per 20 µL reaction, and CoA (about 100 nL at about $104/mL) costs roughly $0.010. Together they already match or exceed the lysate cost (5 µL at $4/mL, or $0.020). The trade-off is that Ribose+NMP formulations depend more on the activity of the lysate and on good pH buffering, while PEP+NTP formulations depend more on directly supplied components.