Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of L-cysteine, DQ immediately binds to the –SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. L-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of L-cysteine thiolate (Cys–S−) to DQ. Interestingly, the C2–S bonded intermediate was less energetically stable than the C6–S bonded case. Furthermore, the most pre-ferred Cys–S−-attacked intermediate is at the carbon-carbon bridge between the two carbonyls (C3– C4 bridge site) but not on the C5 site. This structure allows the Cys–S− to migrate onto the adjacent C5 or C2 with small activation energies. Further simulation demonstrated a possible conversion pathway of the C5–S (and C2–S) intermediate into 5-S-cysteinyldopa (and 2-S-cysteinyldopa), which is the experimentally identified major (and minor) product. Based on the results, we propose that the binding of Cys–S− to DQ proceeds via the following path: (i) coordination of Cys–S− to C3–C4 bridge, (ii) migration of Cys–S− to C5 (C2), (iii) proton rearrangement from cysteinyl –NH3+ to O4 (O3), and (iv) proton rearrangement from C5 (C2) to O3 (O4).
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- コンピュータ サイエンスの応用