Synthesis of Protein Bioconjugates via Cysteine-maleimide Chemistry

Bioconjugation involves covalently linking one biomolecule with another or with a synthetic molecule such as a dye, drug or a polymer. Protein bioconjugation methods are now extensively used in many chemistry, biology and nanotechnology research groups with applications ranging from fluorescent dye labelling^1,2, making of protein (antibody)-prodrugs³ (antibody drug conjugates — ADCs) synthesis of protein dimers^4,5, through to self-assembling protein-polymer hybrids^6,7 used in nanomedicine⁸ and systems chemistry⁹.

Specificity of the chemistry used for bioconjugation, while not always critical, is of utmost importance for most functional protein bioconjugates, so as to not interfere with the active site of the target protein. The ideal bioconjugation reaction needs to fulfill several criteria, including: i) targeting rare or unique sites on the protein of interest, ii) be selective towards this target, iii) proceed under non-denaturing conditions to avoid protein unfolding and iv) be high-yielding as the target protein is usually only available at sub-millimolar concentration. The maleimide – cysteine Michael addition comes close to fulfilling all these criteria, and has for that reason long claimed a special status in the field of bioconjugate chemistry¹⁰. This is because i) many proteins containing only one cysteine residue on their surface can be genetically engineered there, ii) at the correct pH the reaction is highly selective towards cysteine, iii) it proceeds smoothly in aqueous buffers and iv) it is very fast with the second order rate constant of maleimides to cysteine-containing proteins reported to exceed 5,000 M^-1 sec^-1 in some cases¹¹. Provided the protein of interest can tolerate a small (≈ 5-10%) amount of organic co-solvent¹², almost any maleimide-functionalized dye, polymer, surface or another protein can be linked to proteins. In addition, maleimides are more specific for cysteines on proteins than iodoacetamides, which are more prone to reacting with other nucleophiles at elevated pH; and more stable than disulfide-based conjugations which need to be kept at acidic pH to prevent disulfide exchange¹³.

Here we report a generic protocol for the conjugation of maleimide-functionalized molecules to a protein containing a single cysteine residue using the reaction between a Ru(II)-based chromophore and the redox protein cytochrome c as an example. This protocol is equally applicable to most other proteins containing an accessible surface cysteine residue and the corresponding maleimide-functionalized target, be it another protein, a fluorescent dye, a chromophore or a synthetic polymer.