Functional Annotation of LigU as a 1,3-Allylic Isomerase during the Degradation of Lignin in the Protocatechuate 4,5-Cleavage Pathway from the Soil Bacterium <i>Sphingobium</i> sp. SYK-6
作者:Tessily N. Hogancamp、Frank M. Raushel
DOI:10.1021/acs.biochem.8b00295
日期:2018.5.15
Sphingobium sp. SYK-6 is a Gram-negative soil bacterium that contributes to the degradation of lignin. Lignin provides structural support and protection to plants as a complex aromatic heteropolymer. The lignin degradation pathway of guaiacyl moieties leads to the intermediate, protocatechuate (PCA), which is further degraded via the 4,5-cleavage pathway in which PCA is ultimately metabolized to pyruvate and oxaloacetate. In this pathway, LigI has been shown to catalyze the hydrolysis of 2-pyrone-4,6-dicarboxylate to (4E)-oxalomesaconate (OMA). Here we have demonstrated, using 1H and 13C nuclear magnetic resonance spectroscopy, that LigU catalyzes the isomerization of the double bond between C4 and C5 in (4E)-OMA to (3Z)-2-keto-4-carboxy-3-hexenedioate (KCH), where the double bond has migrated to be between C3 and C4 via a 1,3-allylic isomerization. LigU is most closely related in amino acid sequence to methylaconitate isomerase (PrpF) from Shewanella oneidensis and methylitaconate-Δ-isomerase (Mii) from Eubacterium barkeri. The kinetic constants for the isomerization of OMA to KCH by LigU at pH 8.0 were determined to be 1300 ± 120 s–1 and (7.7 ± 1.5) × 106 M–1 s–1 for kcat and kcat/Km, respectively. We have also shown that the product of the LigU-catalyzed reaction is the preferred substrate for the LigJ hydratase. In this reaction, LigJ catalyzes the hydration of KCH to 4-carboxy-4-hydroxy-2-oxoadipate.
鞘氨醇单胞菌(Sphingobium sp. SYK-6)是一种革兰氏阴性土壤细菌,有助于木质素的降解。木质素是一种复杂的芳香杂多聚物,可为植物提供结构支撑和保护。木质素降解途径中的愈创木酚部分会形成中间产物原儿茶酸(PCA),后者会通过4,5-裂解途径进一步降解,最终代谢为丙酮酸和草酰乙酸。在此途径中,LigI已被证明可催化2-吡喃酮-4,6-二羧酸的水解反应,生成(4E)-草酰草酸(OMA)。在此,我们使用1H和13C核磁共振光谱法证明,LigU可催化(4E)-OMA中C4和C5之间的双键异构化为(3Z)-2-酮-4-羧基-3-己烯二酸(KCH),其中双键通过1,3-烯丙基异构化移动到C3和C4之间。在氨基酸序列上,LigU与单胞菌(Shewanella oneidensis)的甲基乙二酸异构酶(PrpF)和巴克氏真杆菌(Eubacterium barkeri)的甲基异丁酸-Δ-异构酶(Mii)最为接近。在pH 8.