Protein bioconjugation residues

Fig. 10.3-2 Common strategies for protein bioconjugation, targeting lysine, cysteine, aspartic acid, and glutamic acid residues. In most situations, only cysteine modification reactions are site selective.

In proteins, cysteine residues are not always readily accessible, since they are often involved in disulfide bridges within the complex three-dimensional biomacromolecular structures. Therefore, only a small number of cysteine residues can be used for bioconjugation reactions. The ligation of polymer bound dibromomaleimides takes advantage of this circumstance, as it allows for the functionalization of disulfide moieties. Haddleton and coworkers demonstrated the applicability of this reaction for bioconjugation of salmon calcitonin (sCT). ... 

Tyrosine residues are underutilized targets for bioconjugate preparation. As it is displayed with intermediate frequency on protein surfaces, tyrosine can often be modified with greater selectivity than other residues. In contrast to charged amino acids, tyrosine residues are often partially buried in the surface of the proteins owing to the amphipathic nature of the phenolic group, Fig. 10.3-3(a-d). This close association with the topography of protein... 

Fig. 10.3-3 Tyrosine residues as targets for bioconjugation, (a) In contrast to charged amino acid side chains, tyrosine residues (yellow) are more closely associated with the protein surface. The reactive 3- and 5-positions of the phenolic ring (indicated...

Cysteine residues are frequently addressed in biological molecules because they are rather rare in peptides and proteins. If accessible thiols are present, they can therefore allow for a site-specific polymer ligation, yielding well-defined bioconjugates. Classically, thioether formation by Michael... 

The thiolate residue must be fully scavenged by an equivalent of the Cu(I) and, as dictated by a balanced reaction, the presence of a full equivalent of a strongly bonding third valence for the -B(OH)2 fragment is required, in this case, the carboxylate. The mechanistic requirement of a stoichiometric quantity of a Cu(I) carboxylate for pH-neutral desulfltative cross-coupling will be incompatible with any desulfltative bioconjugative transformations of biomolecules, such as proteins, that must be carried out in water. 

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