Ottosson Jenny, Fransson Linda, Hult Karl
Department of Biotechnology, Royal Institute of Technology, Stockholm, Sweden.
Protein Sci. 2002 Jun;11(6):1462-71. doi: 10.1110/ps.3480102.
The temperature dependence of the enantioselectivity of Candida antarctica lipase B for 3-hexanol, 2-butanol, 3-methyl-2-butanol, 3,3-dimethyl-2-butanol, and 1-bromo-2-butanol revealed that the differential activation entropy, deltaR-SdeltaS, was as significant as the differential activation enthalpy, DeltaR-SdeltaH, to the enantiomeric ratio, E. 1-Bromo-2-butanol, with isosteric substituents, displayed the largest deltaR-SdeltaS. 3-Hexanol displayed, contrary to other sec-alcohols, a positive deltaR-SdeltaS. In other words, for 3-hexanol the preferred R-enantiomer is not only favored by enthalpy but also by entropy. Molecular dynamics (MD) simulations and systematic search calculations of the substrate accessible volume within the active site revealed that the (R)-3-hexanol transition state (TS) accessed a larger volume within the active site than the (S)-3-hexanol TS. This correlates well with the higher TS entropy of (R)-3-hexanol. In addition, this enantiomer did also yield a higher number of allowed conformations, N, from the systematic search routines, than did the S-enantiomer. The substrate accessible volume was greater for the enantiomer preferred by entropy also for 2-butanol. For 3,3-dimethyl-2-butanol, however, neither MD-simulations nor systematic search calculations yielded substrate accessible volumes that correlate to TS entropy. Ambiguous results were achieved for 3-methyl-2-butanol.
南极假丝酵母脂肪酶B对3-己醇、2-丁醇、3-甲基-2-丁醇、3,3-二甲基-2-丁醇和1-溴-2-丁醇的对映选择性与温度的关系表明，对映体比例E的微分活化熵ΔR-SΔS与微分活化焓ΔR-SΔH同样重要。具有等排取代基的1-溴-2-丁醇表现出最大的ΔR-SΔS。与其他仲醇相反，3-己醇表现出正的ΔR-SΔS。换句话说，对于3-己醇，优先的R-对映体不仅在焓方面有利，在熵方面也有利。分子动力学（MD）模拟以及对活性位点内底物可及体积的系统搜索计算表明，（R）-3-己醇的过渡态（TS）在活性位点内可及的体积比（S）-3-己醇的TS更大。这与（R）-3-己醇较高的TS熵很好地相关。此外，从系统搜索程序来看，该对映体产生的允许构象数N也比S-对映体多。对于2-丁醇，熵优先的对映体的底物可及体积也更大。然而，对于3,3-二甲基-2-丁醇，MD模拟和系统搜索计算都没有得到与TS熵相关的底物可及体积。对于3-甲基-2-丁醇，结果不明确。
