Miyazaki Kentaro
Institute for Biological Resources and Functions, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8566, Japan.
Biochem Biophys Res Commun. 2005 May 27;331(1):341-6. doi: 10.1016/j.bbrc.2005.03.169.
Beta-decarboxylating dehydrogenases comprise 3-isopropylmalate dehydrogenase, isocitrate dehydrogenase, and homoisocitrate dehydrogenase. They share a high degree of amino acid sequence identity and occupy equivalent positions in the amino acid biosynthetic pathways for leucine, glutamate, and lysine, respectively. Therefore, not only the enzymes but also the whole pathways should have evolved from a common ancestral pathway. In Pyrococcus horikoshii, only one pathway of the three has been identified in the genomic sequence, and PH1722 is the sole beta-decarboxylating dehydrogenase gene. The organism does not require leucine, glutamate, or lysine for growth; the single pathway might play multiple (i.e., ancestral) roles in amino acid biosynthesis. The PH1722 gene was cloned and expressed in Escherichia coli and the substrate specificity of the recombinant enzyme was investigated. It exhibited activities on isocitrate and homoisocitrate at near equal efficiency, but not on 3-isopropylmalate. PH1722 is thus a novel, bifunctional beta-decarboxylating dehydrogenase, which likely plays a dual role in glutamate and lysine biosynthesis in vivo.
β-脱羧脱氢酶包括3-异丙基苹果酸脱氢酶、异柠檬酸脱氢酶和高异柠檬酸脱氢酶。它们具有高度的氨基酸序列同一性，并且分别在亮氨酸、谷氨酸和赖氨酸的氨基酸生物合成途径中占据等效位置。因此，不仅这些酶，而且整个途径都应该是从一个共同的祖先途径进化而来的。在嗜热栖热菌中，基因组序列中仅鉴定出这三条途径中的一条，而PH1722是唯一的β-脱羧脱氢酶基因。该生物体生长不需要亮氨酸、谷氨酸或赖氨酸；单一途径可能在氨基酸生物合成中发挥多种（即祖先的）作用。克隆了PH1722基因并在大肠杆菌中表达，并研究了重组酶的底物特异性。它对异柠檬酸和高异柠檬酸表现出几乎相同效率的活性，但对3-异丙基苹果酸没有活性。因此，PH1722是一种新型的双功能β-脱羧脱氢酶，可能在体内谷氨酸和赖氨酸生物合成中发挥双重作用。
