22.2 Synthetic Cell Energy Currencies

Synthetic Cell Energy Currencies refers to the specific molecular pools that a synthetic cell uses to store and transfer usable chemical energy, encompassing nucleotide triphosphates, related nucleotide forms, phosphate-containing molecules, reducing equivalents, and ion gradients, each serving as a distinct medium through which energy is made available to power cellular processes.

The Adenylate System

ATP, ADP, and AMP Pools

The adenosine triphosphate pool represents the primary charged energy currency in most cellular systems, releasing usable energy when converted to adenosine diphosphate, which can be further converted to adenosine monophosphate, with each step representing a progressively lower energy state of the same underlying molecule.

Adenylate Pool

Taken together, the combined total of adenosine triphosphate, diphosphate, and monophosphate forms constitutes the adenylate pool, and the relative proportion of these forms provides an indicator of the overall energy charge available within the synthetic cell at any given time.

Other Nucleotide Triphosphate Currencies

GTP and GDP Pools

Guanosine triphosphate and its diphosphate counterpart serve as a distinct energy currency from the adenylate system, commonly powering specific processes such as certain synthesis and signaling reactions rather than serving as the general-purpose currency that adenosine triphosphate typically provides.

UTP and CTP Pools

Uridine triphosphate and cytidine triphosphate represent additional nucleotide currencies, each preferentially used to drive specific classes of biosynthetic reactions distinct from those powered by the adenylate or guanylate systems.

Nucleoside Triphosphate Pool

Collectively, these various charged nucleotide forms constitute the broader nucleoside triphosphate pool, representing the full set of high-energy nucleotide currencies available to the synthetic cell across its different functional needs.

Supporting Phosphate Pools

Inorganic Phosphate and Pyrophosphate Pools

Free inorganic phosphate, released when a triphosphate is hydrolyzed to a diphosphate, and pyrophosphate, a linked pair of phosphate groups released during certain synthesis reactions, together represent supporting phosphate pools whose availability directly influences the equilibrium of phosphate-transfer reactions throughout the cell.

High-Energy Phosphate Donors

Certain reactive intermediates carry a phosphate group at an even higher energy level than standard nucleotide triphosphates, allowing them to directly regenerate depleted nucleotide currencies through substrate-level phosphorylation.

Non-Phosphate Currencies

Reducing Equivalent Pool

Beyond phosphate-based currencies, the synthetic cell maintains a pool of reducing equivalents, molecules capable of donating or accepting electrons, which store and transfer energy through changes in oxidation state rather than through phosphate bond chemistry.

Ion-Motive Energy Reservoir

An ion concentration gradient maintained across the membrane represents a physically distinct energy reservoir, storing energy in the form of an electrochemical gradient that can later be converted into phosphate-based or other chemical currencies.

Choosing Among Currencies

Energy Currency Selection

Designing a synthetic cell's energy system requires selecting which combination of these currencies to implement, balancing the specific energy needs of the cell's intended processes against the added complexity of maintaining multiple interconverting energy pools simultaneously.

Summary

Synthetic Cell Energy Currencies encompasses the adenylate system, other nucleotide triphosphate pools, supporting phosphate species, reducing equivalents, and ion-motive reservoirs that together provide the diverse molecular media through which a synthetic cell stores and transfers usable chemical energy. Selecting an appropriate combination of these currencies is a foundational design decision underlying the cell's broader energy regeneration system.