Density Functional Theory-Based Calculation Shed New Light on the Bizarre Addition of Cysteine Thiol to Dopaquinone.
Int J Mol Sci 2021;
22:ijms22031373. [PMID:
33573055 PMCID:
PMC7866380 DOI:
10.3390/ijms22031373]
[Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2020] [Revised: 01/24/2021] [Accepted: 01/27/2021] [Indexed: 01/03/2023] Open
Abstract
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 preferred 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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