VTT Technical Research Centre of Finland, Espoo, Finland.
Fungal Genet Biol. 2012 Feb;49(2):152-9. doi: 10.1016/j.fgb.2011.11.005. Epub 2011 Nov 30.
For the catabolism of D-galactose three different metabolic pathways have been described in filamentous fungi. Apart from the Leloir pathway and the oxidative pathway, there is an alternative oxido-reductive pathway. This oxido-reductive pathway has similarities to the metabolic pathway of L-arabinose, and in Trichoderma reesei (Hypocrea jecorina) and Aspergillus nidulans the same enzyme is employed for the oxidation of L-arabitol and galactitol. Here we show evidence that in Aspergillus niger L-arabitol dehydrogenase (LadA) is not involved in the D-galactose metabolism; instead another dehydrogenase encoding gene, ladB, is induced in response to D-galactose and galactitol and functions as a galactitol dehydrogenase. Deletion of ladB in A. niger results in growth arrest on galactitol and significantly slower growth on D-galactose supplemented with a small amount of D-xylose. D-galactose alone cannot be utilised by A. niger and the addition of D-xylose stimulates growth on D-galactose via transcriptional activation of the D-xylose-inducible reductase gene, xyrA. XyrA catalyses the first step of the D-galactose oxido-reductive pathway, the reduction to galactitol, which in turn seems to be an inducer of the downstream genes such as LadB. The deletion of xyrA results in reduced growth on D-galactose. The ladB gene was expressed in the heterologous host Saccharomyces cerevisiae and the tagged and purified enzyme characterised. LadB and LadA have similar in vitro activity with galactitol. It was confirmed that the reaction product of the LadB reaction from galactitol is L-xylo-3-hexulose as in the case of the T. reesei Lad1.
对于 D-半乳糖的分解代谢，丝状真菌中有三种不同的代谢途径。除了 Leloir 途径和氧化途径外，还有一种替代的氧化还原途径。这条氧化还原途径与 L-阿拉伯糖的代谢途径相似，在里氏木霉（Hypocrea jecorina）和构巢曲霉（Aspergillus nidulans）中，同一种酶用于氧化 L-阿拉伯糖醇和半乳糖醇。在这里，我们证明了在黑曲霉中，L-阿拉伯糖醇脱氢酶（LadA）不参与 D-半乳糖代谢；相反，另一个编码脱氢酶的基因 ladB，响应 D-半乳糖和半乳糖醇被诱导，并作为半乳糖醇脱氢酶发挥作用。在黑曲霉中缺失 ladB 会导致在半乳糖醇上生长停滞，并且在添加少量 D-木糖的 D-半乳糖上的生长速度明显较慢。单独的 D-半乳糖不能被黑曲霉利用，并且 D-木糖的添加通过转录激活 D-木糖诱导的还原酶基因 xyrA 来刺激 D-半乳糖的生长。XyrA 催化 D-半乳糖氧化还原途径的第一步，即还原为半乳糖醇，而半乳糖醇反过来似乎是下游基因如 LadB 的诱导剂。缺失 xyrA 会导致在 D-半乳糖上的生长减少。 ladB 基因在异源宿主酿酒酵母中表达，并对标记和纯化的酶进行了表征。LadB 和 LadA 对半乳糖醇具有相似的体外活性。确认了 LadB 对半乳糖醇反应的产物是 L-木糖-3-己酮糖，就像里氏木霉 Lad1 的情况一样。
