Rzepecki L M, Waite J H
College of Marine Studies, University of Delaware, Lewes 19958.
Arch Biochem Biophys. 1991 Feb 15;285(1):27-36. doi: 10.1016/0003-9861(91)90324-c.
The amino acid L-3,4-dihydroxyphenylalanine (DOPA), when present in the primary sequence of proteins, does not form melanin upon oxidation to the quinone, since its amine moiety participates in a peptide bond and cannot undergo internal cyclization. Instead, peptidyl DOPA quinone is available for other reactions. We have investigated the oxidation chemistry of a low molecular weight peptidyl DOPA analog, N-acetylDOPA ethyl ester (NAcDEE), and have shown that a major product of oxidation is an unsaturated DOPA derivative, N-acetyl-alpha,beta-dehydroDOPA ethyl ester (NAc delta DEE) (see companion paper, Rzepecki et al., Arch. Biochem. Biophys. (1991) 285, 17-26). In the present study, we have explored kinetic and mechanistic features of the conversion of NAcDEE to NAc delta DEE and found that the reaction requires: (i) oxidation of NAcDEE to the quinone, (ii) the presence of a Lewis base as a catalyst (phosphate anion was the best of those tried in the pH range 6.0-8.0), and (iii) prevention of competing reactions such as Michael additions. Conversion efficiencies in the presence of Lewis bases ranged between 12 and 19% at pH 8.0 and 35 and 90% at pH 6.0. At least two separate reaction mechanisms appeared necessary to explain the kinetic data: (i) a pseudo-first-order mechanism at pH 6.0 and above, and (ii) an additional second-order mechanism at higher pH which involved both NAcDEE catechol and quinone. The apparent pseudo-first-order rate constants increased with pH from 2.36 X 10(-4) s-1 at pH 6.0 to about 30 X 10(-4) s-1 at pH 8.0 in 0.1 M sodium phosphate. Tautomerization of DOPA quinone to dehydroDOPA may thus be a factor in the sclerotization of natural structures incorporating DOPA containing proteins.
氨基酸L-3,4-二羟基苯丙氨酸（DOPA），当存在于蛋白质的一级序列中时，氧化成醌后不会形成黑色素，因为其胺基参与肽键形成，无法进行分子内环化。相反，肽基多巴醌可用于其他反应。我们研究了一种低分子量肽基多巴类似物N-乙酰多巴乙酯（NAcDEE）的氧化化学，结果表明氧化的主要产物是一种不饱和多巴衍生物，N-乙酰-α,β-脱氢多巴乙酯（NAcδDEE）（见配套论文，Rzepecki等人，《生物化学与生物物理学报》（1991年）285卷，第17 - 26页）。在本研究中，我们探讨了NAcDEE转化为NAcδDEE的动力学和机理特征，发现该反应需要：（i）将NAcDEE氧化成醌，（ii）存在路易斯碱作为催化剂（在pH值6.0 - 8.0范围内尝试的物质中，磷酸根阴离子效果最佳），以及（iii）防止诸如迈克尔加成等竞争反应。在路易斯碱存在下，pH值为8.0时转化效率在12%至19%之间，pH值为6.0时在35%至90%之间。至少需要两种不同的反应机理来解释动力学数据：（i）pH值6.0及以上时的准一级反应机理，以及（ii）更高pH值时涉及NAcDEE儿茶酚和醌的二级反应机理。在0.1 M磷酸钠中，表观准一级速率常数随pH值从pH 6.0时的2.36×10⁻⁴ s⁻¹增加到pH 8.0时的约30×10⁻⁴ s⁻¹。因此，多巴醌互变异构为脱氢多巴可能是含有多巴的蛋白质参与天然结构硬化的一个因素。
