Sugumaran M, Dali H, Semensi V
Department of Biology, University of Massachusetts, Boston 02125.
Biochem J. 1991 Aug 1;277 ( Pt 3)(Pt 3):849-53. doi: 10.1042/bj2770849.
Mushroom tyrosinase, which is known to catalyse the conversion of o-diphenols into o-benzoquinones, has been shown to catalyse the oxidative decarboxylation of 3,4-dihydroxymandelic acid [Sugumaran (1986) Biochemistry 25, 4489-4492]. To account for this unusual reaction, a quinone methide intermediate has been proposed. Since all attempts to trap this intermediate ended in vain, mechanistic studies were designed to support the formation of this transient product. Replacement of the alpha-proton in 3,4-dihydroxymandelic acid with a methyl group generates alpha-(3,4-dihydroxyphenyl)-lactic acid, the enzymic oxidation of which should produce 3,4-dihydroxyacetophenone as the end product if the oxidative decarboxylation proceeds through the quinone methide intermediate. Accordingly, chemically synthesized alpha-(3,4-dihydroxyphenyl)-lactic acid on enzymic oxidation produced 3,4-dihydroxyacetophenone as the major isolatable product. Non-steady-state kinetic analysis of the enzyme reaction attested to the transient formation of the conventional quinone product. Thus the enzymic oxidation of alpha-(3,4-dihydroxyphenyl)-lactic acid seems to generate the conventional quinone, which, owing to its instability, is rapidly decarboxylated to yield the transient quinone methide. The coupled dieneonephenol re-arrangement and ketol-enol tautomerism transforms the quinone methide into 3,4-dihydroxyacetophenone.
已知蘑菇酪氨酸酶可催化邻二酚转化为邻苯醌，研究表明它还能催化3,4 - 二羟基扁桃酸的氧化脱羧反应[苏古马兰（1986年），《生物化学》25卷，4489 - 4492页]。为解释这一异常反应，有人提出了醌甲基化物中间体。由于所有捕获该中间体的尝试均以失败告终，因此设计了机理研究来支持这种瞬态产物的形成。将3,4 - 二羟基扁桃酸中的α - 质子用甲基取代，生成α -（3,4 - 二羟基苯基） - 乳酸，如果氧化脱羧反应通过醌甲基化物中间体进行，那么其酶促氧化反应应以3,4 - 二羟基苯乙酮作为终产物。相应地，化学合成的α -（3,4 - 二羟基苯基） - 乳酸经酶促氧化后，产生了3,4 - 二羟基苯乙酮作为主要可分离产物。酶反应的非稳态动力学分析证实了传统醌产物的瞬态形成。因此，α -（3,4 - 二羟基苯基） - 乳酸的酶促氧化似乎生成了传统的醌，由于其不稳定性，该醌迅速脱羧生成瞬态醌甲基化物。醌甲基化物通过二烯酮 - 酚重排和酮醇 - 烯醇互变异构转化为3,4 - 二羟基苯乙酮。
