Zhu X, Tang G, Galili G
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100, Israel.
Biochem J. 2000 Oct 1;351(Pt 1):215-20. doi: 10.1042/0264-6021:3510215.
Whereas plants and animals use the alpha-aminoadipic acid pathway to catabolize lysine, yeast and fungi use the very same pathway to synthesize lysine. These two groups of organisms also possess structurally distinct forms of two enzymes in this pathway, namely lysine-oxoglutarate reductase (lysine-ketoglutarate reductase; LKR) and saccharopine dehydrogenase (SDH): in plants and animals these enzymes are linked on to a single bifunctional polypeptide, while in yeast and fungi they exist as separate entities. In addition, yeast LKR and SDH possess bi-directional activities, and their anabolic function is regulated by complex transcriptional and post-transcriptional controls, which apparently ascertain differential accumulation of intermediate metabolites; in plants, the regulation of the catabolic function of these two enzymes is not known. To elucidate the regulation of the catabolic function of plant bifunctional LKR/SDH enzymes, we have used yeast as an expression system to test whether a plant LKR/SDH also possesses bi-directional LKR and SDH activities, similar to the yeast enzymes. The Arabidopsis enzyme complemented a yeast SDH, but not LKR, null mutant. Identical results were obtained when deletion mutants encoding only the LKR or SDH domains of this bifunctional polypeptide were expressed individually in the yeast cells. Moreover, activity assays showed that the Arabidopsis LKR possessed catabolic, but not anabolic, activity, and its uni-directional activity stems from its structure rather than its linkage to SDH. Our results suggest that the uni-directional activity of LKR plays an important role in regulating the catabolic function of the alpha-amino adipic acid pathway in plants.
植物和动物利用α-氨基己二酸途径分解代谢赖氨酸，而酵母和真菌则利用相同的途径合成赖氨酸。这两类生物体在该途径中还拥有两种结构不同形式的酶，即赖氨酸-氧代戊二酸还原酶（赖氨酸-酮戊二酸还原酶；LKR）和酵母氨酸脱氢酶（SDH）：在植物和动物中，这些酶连接在单一的双功能多肽上，而在酵母和真菌中，它们以独立的实体形式存在。此外，酵母LKR和SDH具有双向活性，其合成代谢功能受复杂的转录和转录后控制调节，这显然确定了中间代谢物的差异积累；在植物中，这两种酶分解代谢功能的调节尚不清楚。为了阐明植物双功能LKR/SDH酶分解代谢功能的调节机制，我们以酵母作为表达系统，测试植物LKR/SDH是否也具有与酵母酶类似的双向LKR和SDH活性。拟南芥酶补充了酵母SDH缺陷型突变体，但不能补充LKR缺陷型突变体。当仅编码该双功能多肽的LKR或SDH结构域的缺失突变体在酵母细胞中单独表达时，也得到了相同的结果。此外，活性测定表明，拟南芥LKR具有分解代谢活性，但不具有合成代谢活性，其单向活性源于其结构而非与SDH的连接。我们的结果表明，LKR的单向活性在调节植物α-氨基己二酸途径的分解代谢功能中起重要作用。
