Glycolate oxidase (GOX) is a crucial enzyme of plant photorespiration. The encoding gene is thought to have originated from endosymbiotic gene transfer between the eukaryotic host and the cyanobacterial endosymbiont at the base of plantae. However, animals also possess GOX activities. Plant and animal GOX belong to the gene family of (L)-2-hydroxyacid-oxidases ((L)-2-HAOX). We find that all (L)-2-HAOX proteins in animals and archaeplastida go back to one ancestral eukaryotic sequence; the sole exceptions are green algae of the chlorophyta lineage. Chlorophyta replaced the ancestral eukaryotic (L)-2-HAOX with a bacterial ortholog, a lactate oxidase that may have been obtained through the primary endosymbiosis at the base of plantae; independent losses of this gene may explain its absence in other algal lineages (glaucophyta, rhodophyta, and charophyta). We also show that in addition to GOX, plants possess (L)-2-HAOX proteins with different specificities for medium- and long-chain hydroxyacids (lHAOX), likely involved in fatty acid and protein catabolism. Vertebrates possess lHAOX proteins acting on similar substrates as plant lHAOX; however, the existence of GOX and lHAOX subfamilies in both plants and animals is not due to shared ancestry but is the result of convergent evolution in the two most complex eukaryotic lineages. On the basis of targeting sequences and predicted substrate specificities, we conclude that the biological role of plantae (L)-2-HAOX in photorespiration evolved by co-opting an existing peroxisomal protein.
乙二酸氧化酶(GOX)是植物光呼吸的重要酶。编码
基因被认为起源于真核宿主与蓝藻内共生体在植物基部的内共生
基因转移。然而,动物也具有GOX活性。植物和动物的GOX属于(L)-2-羟基酸氧化酶((L)-2-HAOX)
基因家族。我们发现,动物和原核
生物中的所有(L)-2-HAOX蛋白都可以追溯到一种原始真核序列;唯一的例外是绿藻门。绿藻门用细菌同源物(
乳酸氧化酶)取代了原始真核(L)-2-HAOX,这种
乳酸氧化酶可能是在植物基部的主要内共生过程中获得的;这种
基因的独立丢失可以解释为什么其他藻类(蓝藻门、红藻门和绿藻门)中没有这种
基因。我们还发现,除了GOX,植物还拥有(L)-2-HAOX蛋白,对中链和长链羟基酸具有不同的特异性(lHAOX),可能参与
脂肪酸和蛋白质的分解代谢。脊椎动物的lHAOX蛋白作用于与植物lHAOX相似的底物;然而,植物和动物中GOX和lHAOX亚家族的存在并不是由于共同的祖先,而是两个最复杂真核
生物谱系趋同进化的结果。根据靶向序列和预测的底物特异性,