9.2 Cell-Free System Classes

Cell-Free System Classes refers to the distinct categories into which cell-free systems can be organized based on how their biochemical machinery was prepared and structurally arranged, spanning crude lysates, clarified extracts, extract-based systems generally, fully reconstituted systems, hybrid combinations, systems derived from a single organism versus multiple sources, and compartmentalized versus noncompartmentalized arrangements, culminating in the practical process of selecting among these classes for a given application.

Crude Lysate Cell-Free System

Minimally Processed Cellular Contents

A crude lysate cell-free system uses the contents of broken-open cells with minimal further processing, retaining a broad and largely uncharacterized mixture of cellular components alongside the specific machinery of interest.

Trade-Off Between Simplicity and Definition

This approach offers a relatively fast and low-effort route to a functional system, but at the cost of compositional definition, since many unidentified components remain present alongside the intended functional machinery.

Clarified Extract Cell-Free System

Removing Debris While Retaining Soluble Machinery

A clarified extract cell-free system removes larger cellular debris, such as membrane fragments and unbroken cellular structures, from a crude lysate, retaining primarily the soluble molecular machinery suspended in the resulting mixture.

An Intermediate Level of Processing

This class occupies an intermediate position between minimally processed crude lysates and more extensively purified reconstituted systems, offering somewhat improved consistency and clarity of composition without the extensive work required for full reconstitution.

Extract-Based Cell-Free System

The Broader Category Encompassing Lysates and Clarified Extracts

Extract-based cell-free system refers to the broader category encompassing both crude lysates and clarified extracts, unified by their shared reliance on bulk cellular material rather than individually purified components.

General Characteristics of This Category

Systems in this broader category generally offer practical advantages in terms of cost and preparation time, while sharing the common limitation of containing some unidentified or uncharacterized components alongside the functional machinery of interest.

Reconstituted Cell-Free System

Assembled From Individually Purified Parts

A reconstituted cell-free system is built by combining individually purified molecular components, such as specific enzymes and ribosomal machinery, in defined quantities to achieve a precisely known composition.

Advantages of Full Compositional Definition

This class offers the greatest degree of compositional definition among cell-free system classes, allowing researchers to know exactly which components are present and in what amounts, at the cost of greater preparation effort and expense compared to extract-based approaches.

Hybrid Cell-Free System

Combining Extract-Based and Purified Elements

A hybrid cell-free system combines a bulk extract or lysate with one or more individually purified components added to supplement or enhance the system's function, blending characteristics of extract-based and reconstituted approaches.

A Practical Compromise Between Two Approaches

This hybrid class offers a practical compromise, retaining some of the convenience and cost advantages of extract-based systems while gaining improved control over at least the specific supplemented components.

Organism-Specific Cell-Free System

Machinery Derived From a Single Source Organism

An organism-specific cell-free system derives all of its biological machinery from a single source organism, ensuring internal consistency among the various components since they originate from the same biological context.

Value of Single-Source Consistency

This consistency can improve compatibility among the system's components, since machinery derived from the same organism is more likely to interact correctly than components combined from mismatched biological sources.

Multi-Source Cell-Free System

Machinery Combined From Different Organisms

A multi-source cell-free system combines biological machinery derived from more than one source organism, such as ribosomes from one organism paired with enzymes purified from another.

Motivations and Risks of Combining Sources

This approach can be motivated by the need to access a specific desirable property found only in a particular organism's machinery, but it introduces greater risk of component incompatibility compared to a single-source system.

Compartmentalized Cell-Free System

Cell-Free Machinery Enclosed Within a Boundary

A compartmentalized cell-free system encloses its biochemical machinery within a physical boundary, such as a lipid vesicle, introducing a defined internal environment separated from the external medium.

Relationship to Bottom-Up Synthetic Cell Construction

This class overlaps directly with bottom-up synthetic cell construction, since enclosing cell-free machinery within a compartment is a common strategy for building a cell-like system from the ground up.

Noncompartmentalized Cell-Free System

Reactions Occurring in Open or Bulk Solution

A noncompartmentalized cell-free system carries out its biochemical reactions in open or bulk solution, without any enclosing boundary separating the reaction mixture from its surrounding environment.

Simpler Preparation at the Cost of Cell-Like Organization

This class is generally simpler to prepare and analyze than a compartmentalized system, though it forgoes the spatial organization and selective boundary properties that compartmentalization would otherwise provide.

Cell-Free System Class Selection

Choosing the Appropriate Class for a Given Goal

Cell-free system class selection is the practical process of choosing which of the above classes best serves a specific research goal, weighing considerations such as required compositional precision, cost, preparation time, and whether compartmentalization is needed for the intended application.

Balancing Trade-Offs Across the Available Classes

This selection process requires balancing trade-offs across the classes described above, since no single class is universally superior, and the most appropriate choice depends on the specific priorities and constraints of a given project.