BPS3021 Chap.2 Expression Systems and Protein Purification

Expression Systems and Protein Purification

Choose a host from product requirements, then recover, purify and characterise the recombinant protein.

What this chapter covers

• 01

  Bacterial, yeast, mammalian and cell-free hosts

• 02

  Inducible expression and codon optimisation

• 03

  Cell lysis and soluble fraction recovery

• 04

  Chromatography and affinity-tag cleavage

Worked example · free

Plan a His-tag purification

Key terms

Codon optimisation

Synonymous coding-sequence redesign to improve expression compatibility with a chosen host.

Affinity tag

An engineered sequence that enables selective capture through a known binding interaction.

Inclusion body

An insoluble intracellular aggregate that can contain highly expressed but improperly folded protein.

Expression Systems and Protein Purification FAQ

Exam move

Link host capability to the required product attribute, then diagnose expression, solubility, purity and function in order; Choosing an expression system is a product decision, not a race for the largest yield; Bacteria are fast, inexpensive and easy to scale, but they do not reproduce every eukaryotic folding or post-translational modification;

Yeast adds processing capacity and high productivity, while mammalian cells offer more human-like processing at greater cost and complexity; Cell-free systems offer control and rapid testing but bring their own scale and cost constraints;

A small, non-glycosylated protein may suit bacterial expression; A complex therapeutic whose activity, persistence or immunogenicity depends on glycosylation may require a eukaryotic system; The answer should connect a molecular attribute to a host capability;

“Mammalian is best” is not a justification; “mammalian processing is needed to produce the relevant glycan pattern” is; Sequence design also interacts with the host; A correct amino-acid sequence can still express poorly if the coding sequence relies on codons that are rare in the chosen organism;

Codon optimisation changes synonymous codons to better match host usage without changing the protein sequence; Inducible promoters such as lac-derived systems separate cell growth from high-level production: grow biomass first, then add inducer to redirect cellular resources toward the recombinant product;

A dense band is evidence of abundant protein at a particular apparent size, not proof that the molecule is folded, soluble or active; Soluble and insoluble fractions, an activity assay and identity testing answer different questions;

The examiner rewards a production plan that follows the chain from expression to recovery to characterisation, with each measurement tied to a decision; Purification begins by releasing proteins from cells through sonication, a French press or chemical lysis;

Clarification removes debris before chromatography; The available methods separate molecules by different properties: size-exclusion by hydrodynamic size, ion-exchange by charge, hydrophobic-interaction by exposed hydrophobicity, and affinity chromatography by a specific binding interaction;

An original recombinant enzyme carries a His tag and is expected to remain soluble; Lyse the cells under conditions that preserve activity, clarify the lysate, then load the soluble fraction onto nickel resin; Wash away weakly retained contaminants and elute the tagged protein with imidazole;

Collect fractions and assess identity, purity and activity rather than treating one chromatogram peak as a complete answer; The practice paper uses a GST fusion workflow because the tag can improve solubility and enable affinity purification under non-denaturing conditions;

A site-specific protease can then release the target from the carrier; The mark is not for reciting this history; it is for explaining why the fusion helps, what the column selects, and why cleavage and a further separation step may be necessary;

A bacterial culture has a strong promoter but little target protein, and the coding sequence contains many host-rare codons; Answer; Consider codon optimisation or a strain supplying the relevant tRNAs; Verify transcript and induction conditions before concluding that translation is the only problem;

A synonymous redesign should preserve the amino-acid sequence while improving compatibility with the host; The target appears mainly in the insoluble pellet after induction; Answer; Test lower induction temperature, weaker or later induction, an alternative host, or a solubility tag;

Compare soluble and insoluble fractions; If denaturing recovery is attempted, recognise that refolding and retained biological activity must be demonstrated rather than assumed; High-scoring answers name the bottleneck and choose a measurement that can locate it;

No band may mean failed induction, low transcription, poor translation or rapid degradation; A strong pellet band means expression succeeded but solubility failed; A pure-looking band can still represent an inactive conformation;

Each observation narrows the diagnosis without closing every alternative; Finish by linking host, process and assay; State why the host can make the required molecular features, why the chosen purification property discriminates the target, and which test establishes function;

This three-part chain earns more than an unranked list of technologies because it shows how a production decision is made; greater processing capability and costspeed / yieldbacteriayeastmammaliancell-freeHost selection balances manufacturing efficiency against the processing needed for a functional product.Sequence design also interacts with the host;

A correct amino-acid sequence can still express poorly if the coding sequence relies on codons that are rare in the chosen organism; Codon optimisation changes synonymous codons to better match host usage without changing the protein sequence;

Inducible promoters such as lac-derived systems separate cell growth from high-level production: grow biomass first, then add inducer to redirect cellular resources toward the recombinant product.Separate abundance from qualityA dense band is evidence of abundant protein at a particular apparent size, not proof that the molecule is folded, soluble or active;

Soluble and insoluble fractions, an activity assay and identity testing answer different questions; The examiner rewards a production plan that follows the chain from expression to recovery to characterisation, with each measurement tied to a decision.Host selection also affects downstream purification;

Secreted products may simplify recovery, whereas intracellular products require disruption and clarification; The recommended system should therefore be judged across the complete process, including product quality, recovery burden, scalability and the assay needed to confirm activity;

· · PURIFICATIONPurification methodExploit one property at a timeRelease the protein, capture it, polish it and test the resultPurification begins by releasing proteins from cells through sonication, a French press or chemical lysis;

Clarification removes debris before chromatography; The available methods separate molecules by different properties: size-exclusion by hydrodynamic size, ion-exchange by charge, hydrophobic-interaction by exposed hydrophobicity, and affinity chromatography by a specific binding interaction.EX 2Plan an affinity purificationAn original recombinant enzyme carries a His tag and is expected to remain soluble;

Lyse the cells under conditions that preserve activity, clarify the lysate, then load the soluble fraction onto nickel resin; Wash away weakly retained contaminants and elute the tagged protein with imidazole; Collect fractions and assess identity, purity and activity rather than treating one chromatogram peak as a complete answer.Confirm the target is in the soluble fraction before loading;

Use binding, wash and elution conditions consistent with the tag-resin pair; Remove the tag with a suitable protease if the final product requires it, then separate the released tag; Characterise the final material with an orthogonal identity or activity assay;

· HOST SELECTION Production choices WEEK 6 The host shapes the product Speed, yield, folding and post-translational processing pull in different directions Choosing an expression system is a product decision, not a race for the largest yield;

Bacteria are fast, inexpensive and easy to scale, but they do not reproduce every eukaryotic folding or post-translational modification; Yeast adds processing capacity and high productivity, while mammalian cells offer more human-like processing at greater cost and complexity;

Cell-free systems offer control and rapid testing but bring their own scale and cost constraints; Match the host to the quality requirement A small, non-glycosylated protein may suit bacterial expression; A complex therapeutic whose activity, persistence or immunogenicity depends on glycosylation may require a eukaryotic system;

The answer should connect a molecular attribute to a host capability; “Mammalian is best” is not a justification; “mammalian processing is needed to produce the relevant glycan pattern” is; greater processing capability and cost speed / yield bacteria yeast mammalian cell-free Host selection balances manufacturing efficiency against the processing needed for a functional product;

Sequence design also interacts with the host; A correct amino-acid sequence can still express poorly if the coding sequence relies on codons that are rare in the chosen organism; Codon optimisation changes synonymous codons to better match host usage without changing the protein sequence;

Inducible promoters such as lac-derived systems separate cell growth from high-level production: grow biomass first, then add inducer to redirect cellular resources toward the recombinant product; Separate abundance from quality A dense band is evidence of abundant protein at a particular apparent size, not proof that the molecule is folded, soluble or active;

Soluble and insoluble fractions, an activity assay and identity testing answer different questions; The examiner rewards a production plan that follows the chain from expression to recovery to characterisation, with each measurement tied to a decision;

Host selection also affects downstream purification; Secreted products may simplify recovery, whereas intracellular products require disruption and clarification; The recommended system should therefore be judged across the complete process, including product quality, recovery burden, scalability and the assay needed to confirm activity;

· · PURIFICATION Purification method Exploit one property at a time Release the protein, capture it, polish it and test the result Purification begins by releasing proteins from cells through sonication, a French press or chemical lysis;

Clarification removes debris before chromatography; The available methods separate molecules by different properties: size-exclusion by hydrodynamic size, ion-exchange by charge, hydrophobic-interaction by exposed hydrophobicity, and affinity chromatography by a specific binding interaction;

EX 2 Plan an affinity purification An original recombinant enzyme carries a His tag and is expected to remain soluble; Lyse the cells under conditions that preserve activity, clarify the lysate, then load the soluble fraction onto nickel resin;

Wash away weakly retained contaminants and elute the tagged protein with imidazole; Collect fractions and assess identity, purity and activity rather than treating one chromatogram peak as a complete answer; Confirm the target is in the soluble fraction before loading;

Use binding, wash and elution conditions consistent with the tag-resin pair; Remove the tag with a suitable protease if the final product requires it, then separate the released tag; Characterise the final material with an orthogonal identity or activity assay;

PAIR THE CHEMISTRY His tag → nickel ions → imidazole elution; GST tag → glutathione binding → reduced-glutathione elution; The practice paper uses a GST fusion workflow because the tag can improve solubility and enable affinity purification under non-denaturing conditions;

A site-specific protease can then release the target from the carrier; The mark is not for reciting this history; it is for explaining why the fusion helps, what the column selects, and why cleavage and a further separation step may be necessary;

Fraction tracking protects against silent product loss; Retain samples from the load, flow-through, wash and elution steps, then compare them with a suitable analytical method; This reveals whether the target failed to bind, washed away or eluted with contaminants and directs the next optimisation;

· · TROUBLESHOOTING Troubleshooting expression Diagnose the first failed stage Do not optimise purification when the protein was never soluble P1 Low expression Practice A bacterial culture has a strong promoter but little target protein, and the coding sequence contains many host-rare codons;

Answer; Consider codon optimisation or a strain supplying the relevant tRNAs; Verify transcript and induction conditions before concluding that translation is the only problem; A synonymous redesign should preserve the amino-acid sequence while improving compatibility with the host;

P2 Inclusion bodies Practice The target appears mainly in the insoluble pellet after induction; Answer; Test lower induction temperature, weaker or later induction, an alternative host, or a solubility tag; Compare soluble and insoluble fractions;

If denaturing recovery is attempted, recognise that refolding and retained biological activity must be demonstrated rather than assumed; Yield is useful only when the molecule has the identity, structure and activity the product needs;

Production principle Marker-facing logic High-scoring answers name the bottleneck and choose a measurement that can locate it; No band may mean failed induction, low transcription, poor translation or rapid degradation; A strong pellet band means expression succeeded but solubility failed;

A pure-looking band can still represent an inactive conformation; Each observation narrows the diagnosis without closing every alternative; A defensible final recommendation Finish by linking host, process and assay; State why the host can make the required molecular features, why the chosen purification property discriminates the target, and which test establishes function;

This three-part chain earns more than an unranked list of technologies because it shows how a production decision is made; Always change one major process variable at a time when diagnosing a failure; Simultaneous changes to temperature, inducer, host and lysis conditions can improve the outcome while destroying the evidence needed to learn why; Controlled comparisons turn troubleshooting into an experiment; ·