A cytosolic ferredoxin-independent hydrogenase possibly mediates hydrogen uptake in Trichomonas vaginalis

Abstract

Trichomonads, represented by the highly prevalent sexually transmitted human parasite Trichomonas vaginalis, are anaerobic eukaryotes with hydrogenosomes in the place of the standard mitochondria. Hydrogenosomes form indispensable FeS clusters, synthesize ATP, and release molecular hydrogen as a waste product. Hydrogen formation is catalyzed by [FeFe] hydrogenase, the hallmark enzyme of all hydrogenosomes found in various eukaryotic anaerobes. Eukaryotic hydrogenases were originally thought to be exclusively localized within organelles, but today few eukaryotic anaerobes are known that possess hydrogenase in their cytosol. We identified a so far unknown hydrogenase in T. vaginalis cytosol which cannot utilize ferredoxin as a redox partner but can use cytochrome b5 as an electron acceptor. Trichomonads overexpressing the cytosolic hydrogenase, while maintaining the carbon flux through hydrogenosomes, show decreased excretion of hydrogen and increased excretion of methylated alcohols, suggesting that the cytosolic hydrogenase utilizes the hydrogen gas as a source of reducing power for the reactions occurring in the cytoplasm and thus accounts for the overall redox balance. This is the first evidence of hydrogen uptake in a eukaryote, although further work is needed to confirm it. Assembly of the catalytic center of [FeFe] hydrogenases (H-cluster) requires the activity of three dedicated maturases, and these proteins in T. vaginalis are exclusively localized in hydrogenosomes, where they participate in the maturation of organellar hydrogenases. Despite the different subcellular localization of cytosolic hydrogenase and maturases, the H-cluster is present in the cytosolic enzyme, suggesting the existence of an alternative mechanism of H-cluster assembly.

摘要

以传播广泛的人体性传播寄生虫阴道毛滴虫（Trichomonas vaginalis）为代表的毛滴虫类，是一类以氢化酶体（hydrogenosomes）替代标准线粒体的厌氧真核生物。氢化酶体可合成不可或缺的铁硫簇（FeS clusters）、生成三磷酸腺苷（ATP），并将分子氢作为代谢副产物释放。氢分子的生成由[FeFe]氢化酶催化，该酶是各类厌氧真核生物体内氢化酶体的标志性酶类。此前学界普遍认为真核生物的氢化酶仅定位于细胞器内，但截至目前，已知胞质中含有氢化酶的厌氧真核生物数量极少。本研究在阴道毛滴虫的胞质中发现了一种此前未被报道的氢化酶，该酶无法以铁氧还蛋白作为氧化还原伴侣，却可利用细胞色素b5作为电子受体。过表达该胞质氢化酶的毛滴虫，在维持通过氢化酶体的碳通量的同时，其氢分子排泄量降低，而甲基化醇类的排泄量升高，这一现象表明，胞质氢化酶可将氢分子作为胞质内反应的还原力来源，从而参与维持细胞整体的氧化还原平衡。这是真核生物中存在氢摄取现象的首个直接证据，不过仍需开展后续研究进一步验证该结论。

[FeFe]氢化酶催化中心（H簇）的组装需要三种专属成熟酶的参与，而阴道毛滴虫体内的这类成熟酶仅定位于氢化酶体中，参与细胞器内氢化酶的成熟过程。尽管胞质氢化酶与成熟酶的亚细胞定位存在显著差异，但该胞质酶中仍检测到H簇的存在，这表明存在一套全新的H簇组装机制。