Question 3 of 6: Recombinant Therapeutic Proteins and Antibody Action

Question 3: Recombinant Therapeutic Proteins and Antibody Action (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) The two host systems differ fundamentally at the level of cell biology, and every downstream difference in the recombinant protein they produce traces back to this. E. coli is a prokaryote: it has no nucleus or membrane-bound organelles, a single circular chromosome plus (for recombinant work) a plasmid vector, and a comparatively simple protein-synthesis and quality-control machinery. CHO cells are a eukaryotic mammalian cell line, with a nucleus, endoplasmic reticulum, Golgi apparatus and the full complement of eukaryotic secretory-pathway machinery. Practically, this drives four major differences. First, growth and cost: E. coli doubles roughly every 20–30 minutes in cheap, chemically defined media, while CHO cells double roughly every 24 hours and require expensive, serum-supplemented (or specialized serum-free) media and sterile mammalian cell-culture conditions — so E. coli is far faster and cheaper to grow at scale. Second, post-translational modification: CHO cells possess the full eukaryotic ER/Golgi machinery for disulfide-bond formation with chaperone-assisted folding, N- and O-linked glycosylation, and proteolytic processing, all of which E. coli lacks (it has no glycosylation machinery at all and only a limited, non-eukaryotic capacity for disulfide bond formation in its periplasm); this makes CHO essential for any therapeutic that must be correctly glycosylated to be active, stable, or non-immunogenic (most antibodies and many other therapeutic glycoproteins). Third, folding and solubility: complex, multi-domain or disulfide-rich eukaryotic proteins expressed in E. coli frequently misfold into insoluble inclusion bodies requiring harsh denaturation/refolding downstream processing, whereas CHO cells' native eukaryotic folding machinery generally produces correctly folded, soluble, secreted protein directly into the culture medium, simplifying purification. Fourth, safety/regulatory profile: E. coli-derived product carries a risk of bacterial endotoxin (lipopolysaccharide) contamination that must be rigorously removed and tested for, while mammalian CHO product does not carry this particular contaminant, though it introduces its own concerns (viral contamination screening, host-cell protein/DNA clearance). In short, E. coli is chosen for simple, non-glycosylated proteins where speed and cost dominate (e.g. insulin, growth hormone), while CHO is the workhorse for complex glycosylated therapeutics, especially monoclonal antibodies.

(b)(i) An antibody (immunoglobulin, Ig) is a Y-shaped glycoprotein produced by B lymphocytes (as plasma cells) that is composed of two identical heavy chains and two identical light chains, held together by inter-chain disulfide bonds. Each chain has an N-terminal variable region (V_H/V_L), which together form the two identical antigen-binding sites (Fab arms) at the tips of the Y, and a C-terminal constant region; the paired heavy-chain constant domains form the Fc ("tail") region at the base of the Y.

(b)(ii) The primary function of an antibody is antigen recognition and specific binding — the variable-region Fab sites bind a specific epitope on a target antigen (e.g. a pathogen surface protein, a toxin, or in the therapeutic context a specific disease-associated receptor or protein) with high affinity and specificity, which directly neutralizes the target (e.g. blocking a toxin's active site or a receptor's ligand-binding site) or flags it for removal.

(b)(iii) A secondary function is Fc-mediated effector activity: once bound to its antigen, the antibody's Fc region can engage Fc receptors on immune effector cells (e.g. NK cells, macrophages) to trigger antibody-dependent cell-mediated cytotoxicity (ADCC) or phagocytosis (opsonization), or can activate the complement cascade (complement-dependent cytotoxicity, CDC) — recruiting the immune system to destroy the antigen-bearing cell rather than simply blocking the antigen directly.

(b)(iv) Glycosylation — specifically N-linked glycosylation of the Fc region (at a conserved asparagine, Asn297, in the heavy-chain constant domain) — is the key post-translational modification that aids therapeutic efficacy, because it can only be performed correctly by a eukaryotic host (which is a major reason CHO, not E. coli, is used to manufacture therapeutic antibodies).

(b)(v) During N-linked glycosylation, a pre-assembled oligosaccharide (a branched glycan built from N-acetylglucosamine, mannose and glucose residues) is transferred en bloc from a dolichol-phosphate lipid carrier onto the amide nitrogen of the target asparagine side chain (within the consensus sequence Asn-X-Ser/Thr, X ≠ Pro) as the nascent polypeptide is translocated into the endoplasmic reticulum; the glycan is then progressively trimmed and remodelled as the protein transits the ER and Golgi. The final glycan structure on the Fc region modulates the antibody's binding affinity for Fc receptors and complement, and therefore directly tunes its effector-function potency (e.g. afucosylated glycoforms bind FcγRIIIa more tightly and show enhanced ADCC), its serum half-life, and its overall stability and immunogenicity — all of which matter for a therapeutic antibody's efficacy and dosing.