Martini R, Murray M
Department of Medicine, University of Sydney, Westmead Hospital, New South Wales, Australia.
Chem Res Toxicol. 1996 Jan-Feb;9(1):268-76. doi: 10.1021/tx950106l.
The rat hepatic microsomal aldehyde dehydrogenase (mALDH) metabolizes aliphatic and aromatic aldehydes to the corresponding acids with NAD as the optimal cofactor. However, dehydrogenation of the aliphatic compounds is substantially more efficient. In the present study, a series of aromatic aldehydes was evaluated as substrates of the purified mALDH so that the physicochemical factors that contribute to substrate affinity could be evaluated. Substitution of the aromatic system in the 3- and 4-positions produced relatively good substrates, but 2-substituted congeners did not undergo dehydrogenation. However, aldehydes with hydrophilic substituents in the 3- or 4-positions and those with extremely bulky substituents at both positions (e.g., 3,4-dibenzyloxy) were also poor substrates for the enzyme and dehydrogenation was undetectable. A quantitative structure-activity relationship was determined that related the logarithm of the Michaelis constants for 27 substituted aromatic aldehydes with the zero-order connectivity function of the molecule (0 chi), the shapes of the 3- and 4-substituents (kappa), and the electronic nature of the 4-substituent (sigma). In this equation, 81% of the data variance was explained. From a consideration of the dimensions of 3-phenoxybenzaldehyde, which was a relatively good substrate, the mALDH possesses a narrow cleft within the active site that is at least 7.5 angstroms wide and extends at least 12 angstroms from the catalytic residue (probably cysteine). Previously established relationships between connectivity functions and molecular polarizability suggest that dipolar interactions within the active site, as well as dispersion forces, may play a role in substrate specificity. Although optimal shapes for carbocyclic substituents were not provided by the analysis, the unfavorable effect on dehydrogenation from hydrophilic and large substituents suggests that the active site of the mALDH is relatively rigid and that the orientation of the substrate in relation to the catalytic cysteine and the cofactor binding site is critical.
大鼠肝微粒体醛脱氢酶（mALDH）以NAD作为最佳辅因子，将脂肪族和芳香族醛代谢为相应的酸。然而，脂肪族化合物的脱氢效率要高得多。在本研究中，评估了一系列芳香醛作为纯化的mALDH的底物，以便评估有助于底物亲和力的物理化学因素。在3位和4位取代芳香体系产生了相对较好的底物，但2位取代的同系物未发生脱氢反应。然而，在3位或4位带有亲水性取代基的醛以及在两个位置都带有极大取代基的醛（如3,4-二苄氧基）也是该酶的不良底物，未检测到脱氢反应。确定了一种定量构效关系，该关系将27种取代芳香醛的米氏常数的对数与分子的零级连接性函数（0χ）、3位和4位取代基的形状（κ）以及4位取代基的电子性质（σ）相关联。在这个方程中，81%的数据方差得到了解释。通过考虑相对较好的底物3-苯氧基苯甲醛的尺寸，mALDH在活性位点内具有一个狭窄的裂隙，其宽度至少为7.5埃，并且从催化残基（可能是半胱氨酸）延伸至少12埃。先前建立的连接性函数与分子极化率之间的关系表明，活性位点内的偶极相互作用以及色散力可能在底物特异性中起作用。尽管分析未提供碳环取代基的最佳形状，但亲水性和大取代基对脱氢的不利影响表明，mALDH的活性位点相对刚性，并且底物相对于催化半胱氨酸和辅因子结合位点的取向至关重要。
