Simpson Paul J, Tantitadapitak Chonticha, Reed Anna M, Mather Owen C, Bunce Christopher M, White Scott A, Ride Jon P
School of Biosciences, University of Birmingham, Edgbaston, Birmingham, UK.
J Mol Biol. 2009 Sep 18;392(2):465-80. doi: 10.1016/j.jmb.2009.07.023. Epub 2009 Jul 15.
Aldo-keto reductases (AKRs) are widely distributed in nature and play numerous roles in the metabolism of steroids, sugars, and other carbonyls. They have also frequently been implicated in the metabolism of exogenous and endogenous toxicants, including those stimulated by stress. Although the Arabidopsis genome includes at least 21 genes with the AKR signature, very little is known of their functions. In this study, we have screened the Arabidopsis thaliana genomic sequence for genes with significant homology to members of the mammalian AKR1 family and identified four homologues for further study. Following alignment of the predicted protein sequences with representatives from the AKR superfamily, the proteins were ascribed not to the AKR1 family but to the AKR4C subfamily, with the individual designations of AKR4C8, AKR4C9, AKR4C10, and AKR4C11. Expression of two of the genes, AKR4C8 and AKR4C9, has been shown to be coordinately regulated and markedly induced by various forms of stress. The genes have been overexpressed in bacteria, and recombinant proteins have been purified and crystallized. Both enzymes display NADPH-dependent reduction of carbonyl compounds, typical of the superfamily, but will accept a very wide range of substrates, reducing a range of steroids, sugars, and aliphatic and aromatic aldehydes/ketones, although there are distinct differences between the two enzymes. We have obtained high-resolution crystal structures of AKR4C8 (1.4 A) and AKR4C9 (1.25 A) in ternary complexes with NADP(+) and acetate. Three extended loops, present in all AKRs and responsible for defining the cofactor- and substrate-binding sites, are shorter in the 4C subfamily compared to other AKRs. Consequently, the crystal structures reveal open and accommodative substrate-binding sites, which correlates with their broad substrate specificity. It is suggested that the primary role of these enzymes may be to detoxify a range of toxic aldehydes and ketones produced during stress, although the precise nature of the principal natural substrates remains to be determined.
醛酮还原酶（AKRs）在自然界中广泛分布，在类固醇、糖类和其他羰基化合物的代谢中发挥着多种作用。它们也经常参与外源性和内源性毒物的代谢，包括那些由应激刺激产生的毒物。尽管拟南芥基因组中至少有21个具有AKR特征的基因，但对其功能却知之甚少。在本研究中，我们在拟南芥基因组序列中筛选了与哺乳动物AKR1家族成员具有显著同源性的基因，并鉴定出四个同源基因用于进一步研究。将预测的蛋白质序列与AKR超家族的代表序列进行比对后，这些蛋白质被归类为AKR4C亚家族，分别命名为AKR4C8、AKR4C9、AKR4C10和AKR4C11，而不是AKR1家族。已证明其中两个基因AKR4C8和AKR4C9的表达受到协同调控，并在各种应激形式下显著诱导。这些基因已在细菌中过表达，重组蛋白已被纯化和结晶。这两种酶都表现出NADPH依赖的羰基化合物还原作用，这是该超家族的典型特征，但它们能接受非常广泛的底物，可还原多种类固醇、糖类以及脂肪族和芳香族醛/酮，不过这两种酶之间存在明显差异。我们获得了AKR4C8（1.4 Å）和AKR4C9（1.25 Å）与NADP(+)和乙酸盐形成的三元复合物的高分辨率晶体结构。在所有AKR中都存在且负责定义辅因子和底物结合位点的三个延伸环，在4C亚家族中比其他AKR更短。因此，晶体结构揭示了开放且适应性强的底物结合位点，这与其广泛的底物特异性相关。有人认为，这些酶的主要作用可能是清除应激过程中产生的一系列有毒醛和酮，尽管主要天然底物的确切性质仍有待确定。
