Zheng Y J, Bruice T C
Department of Chemistry, University of California, Santa Barbara, CA 93106, USA.
Proc Natl Acad Sci U S A. 1998 Apr 14;95(8):4158-63. doi: 10.1073/pnas.95.8.4158.
Scytalone dehydratase (EC 4.2.1.94) catalyzes the dehydration of two important intermediates in the biosynthesis of melanin, and it functions without metal ions or any cofactors. Using molecular orbital theory, we have examined the role of a critical water molecule in the mechanism of scytalone dehydratase. The water, together with an internal hydrogen bonding, contributes significantly to the stabilization of the transition state (or the enolate intermediate). The role of two active site tyrosines (Tyr-50 and Tyr-30) is (i) to hold the critical water in place so that it may stabilize the transition state without much structural rearrangement during the catalytic reaction, and (ii) to polarize the water, making it a better general acid. The stereochemistry of the scytalone dehydratase-catalyzed dehydration is also discussed.
剑松素脱水酶(EC 4.2.1.94)催化黑色素生物合成中两种重要中间体的脱水反应,其作用无需金属离子或任何辅因子。利用分子轨道理论,我们研究了关键水分子在剑松素脱水酶作用机制中的作用。该水分子与内部氢键一起,对过渡态(或烯醇盐中间体)的稳定有显著贡献。两个活性位点酪氨酸(Tyr-50和Tyr-30)的作用是:(i)固定关键水分子,使其在催化反应过程中无需太多结构重排就能稳定过渡态;(ii)使水分子极化,使其成为更好的广义酸。本文还讨论了剑松素脱水酶催化脱水反应的立体化学。