Microbial Enzymology Group, Department of Applied Sciences, Northumberland Building, University of Northumbria, Newcastle Upon Tyne, UK.
GENZ-Group of Research on Enzymology, Department of Biochemistry and Molecular Biology-A, Regional Campus of International Excellence "Campus Mare Nostrum,", University of Murcia, Espinardo, Murcia, Spain.
Biotechnol Appl Biochem. 2021 Aug;68(4):823-831. doi: 10.1002/bab.1998. Epub 2020 Aug 18.
Tyrosinase starts melanogenesis and determines its course, catalyzing the oxidation by molecular oxygen of tyrosine to dopa, and that of dopa to dopaquinone. Then, nonenzymatic coupling reactions lead to dopachrome, which evolves toward melanin. Recently, it has been reported that d-tyrosine acts as tyrosinase inhibitor and depigmenting agent. The action of tyrosinase on the enantiomers of tyrosine (l-tyrosine and d-tyrosine) and dopa (l-dopa and d-dopa) was studied for the first time focusing on quantitative transient phase kinetics. Post-steady-state transient phase studies revealed that l-dopachrome is formed more rapidly than d-dopachrome. This is due to the lower values of Michaelis constants for l-enantiomers than for d-enantiomers, although the maximum rates are equal for both enantiomers. A deeper analysis of the inter-steady-state transient phase of monophenols demonstrated that the enantiomer d-tyrosine causes a longer lag period and a lower steady-state rate, than l-tyrosine at the same concentration. Therefore, d-melanogenesis from d-tyrosine occurs more slowly than does l-melanogenesis from l-tyrosine, which suggests the apparent inhibition of melanin biosynthesis by d-tyrosine. As conclusion, d-tyrosine acts as a real substrate of tyrosinase, with low catalytic efficiency and, therefore, delays the formation of d-melanin.
酪氨酸酶启动黑色素生成并决定其进程，催化分子氧将酪氨酸氧化为多巴，将多巴氧化为多巴醌。然后，非酶偶联反应导致多巴色素的形成，进而演变为黑色素。最近有报道称，D-酪氨酸可作为酪氨酸酶抑制剂和退色剂。本研究首次针对酪氨酸（L-酪氨酸和 D-酪氨酸）和多巴（L-多巴和 D-多巴）的对映异构体，聚焦于定量瞬态相动力学，研究了酪氨酸酶的作用。准稳态瞬态相研究表明，L-多巴色素的形成速度快于 D-多巴色素。这是由于 L-对映体的米氏常数低于 D-对映体，尽管两种对映体的最大速率相等。对单酚的准稳态瞬态相的更深入分析表明，在相同浓度下，D-酪氨酸比 L-酪氨酸引起更长的滞后期和更低的稳态速率。因此，D-酪氨酸生成 D-黑色素的速度比 L-酪氨酸生成 L-黑色素的速度慢，这表明 D-酪氨酸对黑色素生物合成有明显的抑制作用。综上所述，D-酪氨酸可作为酪氨酸酶的真实底物，其催化效率较低，因此会延迟 D-黑色素的形成。
