Reassignment of the human aldehyde dehydrogenase ALDH8A1 (ALDH12) to the kynurenine pathway in tryptophan catabolism
Reassignment of the human aldehyde dehydrogenase ALDH8A1 (ALDH12) to the kynurenine pathway in tryptophan catabolism
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DOI：
10.1074/jbc.ra118.003320
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发表时间：
2018-06-22
影响因子：
4.8
通讯作者：
Liu, Aimin
中科院分区：
文献类型：
--
作者：
Davis, Ian;Yang, Yu;Liu, Aimin
犬尿氨酸途径是哺乳动物l-色氨酸降解的主要途径。该途径的中间体和副产物参与免疫反应和神经退行性疾病。这使得研究犬尿氨酸途径的酶，特别是那些来自哺乳动物的酶是有价值的。最近对该途径的细菌版本酶，2-氨基戊酸半醛（2-AMS）脱氢酶（AMSDH）的研究提供了对催化机制的详细了解，并确定了对氨基戊酸半醛识别和激活的保守残基。细菌酶的发现促使人们重新考虑先前鉴定的人类醛脱氢酶ALDH8A1（或ALDH12）的功能，该酶被注释为基于其优先氧化9-顺式视网膜而不是反式视网膜的能力的视网膜脱氢酶。在这里，我们提供了令人信服的生物信息学和实验证据，证明人类ALDH8A1应该被重新分配到缺失的2-AMS脱氢酶的犬尿氨酸代谢途径。我们发现ALDH8A1催化2-AMS的NAD（+）依赖性氧化，其催化效率与荧光假单胞菌的AMSDH相当。细菌酶中底物识别、结合和异构化所需的活性位点残基的替代导致人类ALDH8A1变体的K-m增加160倍或没有可检测到的活性。总之，这项分子研究在一个重要的人类色氨酸分解代谢途径中建立了一个额外的酶促步骤。
The kynurenine pathway is the primary route for l-tryptophan degradation in mammals. Intermediates and side products of this pathway are involved in immune response and neurodegenerative diseases. This makes the study of enzymes, especially those from mammalian sources, of the kynurenine pathway worthwhile. Recent studies on a bacterial version of an enzyme of this pathway, 2-aminomuconate semialdehyde (2-AMS) dehydrogenase (AMSDH), have provided a detailed understanding of the catalytic mechanism and identified residues conserved for muconate semialdehyde recognition and activation. Findings from the bacterial enzyme have prompted the reconsideration of the function of a previously identified human aldehyde dehydrogenase, ALDH8A1 (or ALDH12), which was annotated as a retinal dehydrogenase based on its ability to preferentially oxidize 9-cis-retinal over trans-retinal. Here, we provide compelling bioinformatics and experimental evidence that human ALDH8A1 should be reassigned to the missing 2-AMS dehydrogenase of the kynurenine metabolic pathway. For the first time, the product of the semialdehyde oxidation by AMSDH is also revealed by NMR and high-resolution MS. We found that ALDH8A1 catalyzes the NAD(+)-dependent oxidation of 2-AMS with a catalytic efficiency equivalent to that of AMSDH from the bacterium Pseudomonas fluorescens. Substitution of active-site residues required for substrate recognition, binding, and isomerization in the bacterial enzyme resulted in human ALDH8A1 variants with 160-fold increased K-m or no detectable activity. In conclusion, this molecular study establishes an additional enzymatic step in an important human pathway for tryptophan catabolism.