After a protein is synthesized by the ribosome, the job is not done. The protein still has to undergo a series of chemical alterations known as post-translational modifications (PTMs).
Nowhere is this more critical than in antibody development and complex biologic drug development.
Understanding PTMs is essential for any protein engineer. These modifications can be the difference between a potent, stable therapeutic and a heterogeneous, ineffective, or even immunogenic product.
Glycosylation: A Regulator of Function and Stability
N-linked Glycosylation occurs on the nitrogen atom of an asparagine (N) residue, typically within the sequon N-X-S/T (where X is any amino acid except proline). For antibody development, the conserved N-linked glycan in the Fc region is a critical regulator of effector function.
O-linked Glycosylation occurs on the oxygen atom of serine (S) or threonine (T) residues.
Unintended glycosylation at a newly created N-X-S/T site (e.g., in a CDR loop) is a major developability risk that can impair antigen binding.
Disulfide Bond Formation: The Architect of Structure
For a typical IgG antibody, multiple cysteine residues must be correctly paired to create the iconic Y-shape.
Incorrect disulfide bond pairing leads to misfolding, loss of function, and often triggers the cell’s quality control machinery to degrade the protein. The presence of unpaired cysteines on a final product is a major developability red flag.
Proteolytic Cleavage: Essential Maturation and a Source of Heterogeneity
Signal Peptide Removal: Incomplete cleavage can lead to a heterogeneous product with a different N-terminus than intended.
C-terminal Clipping: For antibodies, clipping of the C-terminal lysine residue on the heavy chain is a common modification that must be characterized and controlled.
”Problematic” PTMs: The Chemical Liability Hotspots
- Deamidation: Spontaneous conversion of asparagine (N) into aspartic acid or isoaspartic acid. Particularly common at N-G and N-S motifs.
- Oxidation: Most commonly affects methionine (M) and tryptophan (W). Oxidation of a methionine residue within a CDR can dramatically reduce an antibody’s binding affinity.
- Isomerization: Conversion of aspartic acid (D) into isoaspartate, altering the backbone geometry.
Conclusion: PTMs as a Critical Quality Attribute
Post-translational modifications are not an afterthought in antibody development and protein production; they are a central determinant of a biologic’s safety, efficacy, and viability as a drug. Thinking about PTMs from day one is key to success.
Related Ranomics services
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- Antibody engineering: PTM-aware antibody development including glycoform control.