Page 25 - Tyrosine-Based Bioconjugations - Jorick Bruins

Tyrosinases find many applications in the field of dye production, self-tanning agents, biosensors, tea production and more.94 The generation of quinones by tyrosinases can also be used for conjugation strategies, as their electron-poor nature allows for rapid Michael additions.95 Mushroom tyrosinase is generally used in these conjugation procedures, as it is an inexpensive and readily available enzyme.
1.3.1. Quinone chemistryi
The quinones formed by tyrosinases are highly electrophilic compounds that readily undergo attack by nucleophilic species like a thiol or an amine to afford a stable aromatic conjugate. Within the context of a protein, this means that quinones will react with the sidechains of cysteine, lysine, or histidine residues via Michael addition (Scheme 5A).96-98
Utilizing this concept, Liu et al. showed that nucleophilic amino acids are able to undergo pseudo-intramolecular cross-coupling when brought in close proximity by two short complementary peptide nucleic acid (PNA) strands.99 Thus, an in situ generated quinone from 3,4-dihydroxylphenylalanine (DOPA) using sodium periodate (Scheme 5B) underwent Michael addition with specific amino acid residues attached to the complementary PNA strand. Obviously, the hybridized PNA strands ensure close proximity of the intermediate quinone and the nucleophilic amino group on the complementary strand. First, it was found that the α-amino group of alanine was capable of performing this reaction (not depicted), but not when the amine was protected with an acetyl group. Similarly, Liu et al. detected products resulting from the cross coupling via the side-chains of lysine, histidine and cysteine (Scheme 5B), with the thiol of cysteine being the most reactive partner. Finally, lack of reactivity of the side-chains of Arg, Glu, Met, Ser, Thr, Trp, Tyr, Phe and Gln residues corroborated the ability of a free amine, imidazole, or thiol group for Michael addition to quinones, but not of alcohol, guanidine, thioether, indole, carboxylic acid, and amide.
General Introduction
i Subsections 1.4.1 – 1.4.4 was published as a review; Jorick J. Bruins, Bauke Albada, and Floris L. van Delft, Chem. - Eur. J., 2018, 24, 4749−4756.
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