A pharmaceutical company wants to produce erythropoietin (EPO), a human hormone that is heavily glycosylated and requires glycan chains for its biological activity and plasma half-life. Which expression system should they use?
AE. coli with a strong T7 promoter — it is the fastest-growing and cheapest production host
BA mammalian cell line or glycosylation-competent yeast such as Pichia pastoris — because E. coli lacks the glycosylation machinery needed for EPO to fold and function correctly
CE. coli, but with co-expression of human glycosyltransferase genes to add the missing glycosylation
DAny expression system works equally well since the gene sequence is the same; glycosylation is added chemically afterward
This is the central caveat of microbial biotechnology: E. coli is the default workhorse for simple recombinant proteins, but it has no N-linked glycosylation machinery. For proteins like EPO where glycosylation is essential for correct folding, secretion, receptor binding, or serum half-life, E. coli will produce non-functional or unstable protein. Mammalian cell lines (CHO cells) perform human-like glycosylation; yeast such as Pichia pastoris can glycosylate proteins though with somewhat different glycan patterns. Option C is not feasible in practice — the complexity of the human glycosylation pathway makes co-expression of a few enzymes insufficient.
Question 2 Multiple Choice
What distinguishes metabolic engineering from simply inserting a single biosynthetic gene into a microbial host?
AMetabolic engineering always requires CRISPR, while single-gene insertion uses traditional cloning
CMetabolic engineering is only applied to yeast and fungi, while single-gene insertion is used exclusively in bacteria
DThe difference is purely one of scale: metabolic engineering produces more protein than single-gene insertion
Inserting a single gene is often insufficient because the target molecule may require multiple enzymatic steps, and those steps compete with the host's existing metabolic demands for substrates and cofactors. Metabolic engineering is a systems-level challenge: every enzyme in the introduced pathway must be expressed at the right level (bottlenecks and overexpression both cause problems), competing pathways that divert carbon must be downregulated or deleted, cofactor pools (NADPH, ATP) must be balanced, and toxic intermediates must not accumulate to inhibitory levels. The artemisinic acid example — requiring transplantation of the entire mevalonate pathway plus plant-specific enzymes — illustrates this complexity.
Question 3 True / False
E. coli can produce any human protein at high yield if the correct human gene is inserted into an expression vector with a sufficiently strong promoter.
TTrue
FFalse
Answer: False
E. coli is an excellent host for many proteins but fails for those requiring eukaryotic post-translational modifications. Glycosylation (the most common issue) is entirely absent in bacteria. Complex disulfide bonds that require specific isomerases (found in the eukaryotic ER) often result in inclusion bodies — insoluble aggregates — that must be denatured and refolded, with highly variable success. Multi-domain proteins and proteins with signal sequences for secretion may also be misfolded or mistargeted. These limitations are why yeast, insect, and mammalian cell expression systems exist and are widely used despite being more expensive and slower than E. coli.
Question 4 True / False
Synthetic biology treats genetic elements such as promoters, ribosome binding sites, and terminators as standardized, interchangeable components that can be assembled into programmable genetic circuits.
TTrue
FFalse
Answer: True
This modular design philosophy is the defining conceptual framework of synthetic biology, distinguishing it from earlier genetic engineering that relied on natural, context-specific regulatory elements. By standardizing parts (as in the Registry of Standard Biological Parts), synthetic biologists can assemble toggle switches, oscillators, logic gates, and biosensors from predictable modules — similar to electronic circuit design. CRISPR-based tools have further accelerated this by enabling rapid, precise genome edits that would have required months of traditional cloning. The payoff is a dramatically shorter design-build-test cycle for engineering microbial strains with desired behaviors.
Question 5 Short Answer
A team inserts the correct coding sequence for a human protein into E. coli, induces high-level expression, and recovers large amounts of the protein — but when tested, it has no biological activity. Give two mechanistic reasons why a correctly sequenced, highly expressed recombinant protein might be non-functional when produced in bacteria.
Think about your answer, then reveal below.
Model answer: First, the protein may require glycosylation that E. coli cannot provide — many human proteins depend on glycan chains for proper folding, stability, or receptor binding. Second, complex disulfide bonds may not form correctly: the bacterial cytoplasm is reducing, and the disulfide bond isomerase machinery found in the eukaryotic ER is absent. Proteins that form inclusion bodies (insoluble aggregates) under high expression require denaturation and refolding, a process that often fails to recover native activity. Additionally, some human proteins require chaperones not present in bacteria, or have sequences that cause premature termination in the bacterial translation system.
These limitations explain why the choice of expression system is as important as the gene itself in recombinant protein production. The decision tree typically starts with E. coli (cheapest, fastest), and escalates to yeast (for glycosylation, secretion), insect cells (for complex eukaryotic folding), or mammalian cells (for human-type glycosylation and complex assembly) based on the protein's specific structural requirements.