The autotrophic 3-hydroxypropionate/4-hydroxybutyrate (HP/HB) cycle functions in thermoacidophilic, (micro)aerobic, hydrogen-oxidizing Crenarchaeota of the order Sulfolobales as well as in mesophilic, aerobic, ammonia-oxidizing Thaumarchaeota. Notably, the HP/HB cycle evolved independently in these two archaeal lineages, and crenarchaeal and thaumarchaeal versions differ regarding their enzyme properties and phylogeny. These differences result in altered energetic efficiencies between the variants. Compared to the crenarchaeal HP/HB cycle, the thaumarchaeal variant saves two ATP equivalents per turn, rendering it the most energy-efficient aerobic pathway for carbon fixation. Characteristically, the HP/HB cycle includes two enoyl coenzyme A (CoA) hydratase reactions: the 3-hydroxypropionyl-CoA dehydratase reaction and the crotonyl-CoA hydratase reaction. In this study, we show that both reactions are catalyzed in the aforementioned archaeal groups by a promiscuous 3-hydroxypropionyl-CoA dehydratase/crotonyl-CoA hydratase (Msed_2001 in crenarchaeon Metallosphaera sedula and Nmar_1308 in thaumarchaeon Nitrosopumilus maritimus). Although these two enzymes are homologous, they are closely related to bacterial enoyl-CoA hydratases and were retrieved independently from the same enzyme pool by the ancestors of Crenarchaeota and Thaumarchaeota, despite the existence of multiple alternatives. This striking similarity in the emergence of enzymes involved in inorganic carbon fixation from two independently evolved pathways highlights that convergent evolution of autotrophy could be much more widespread than anticipated.

中文翻译：

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Crenarchaeota 和 Thaumarchaeota 中混杂的 3-羟基丙酰-CoA 脱水酶/巴豆酰-CoA 水合酶的趋同进化

自养 3-羟基丙酸盐/4-羟基丁酸盐 (HP/HB) 循环在嗜热嗜酸、（微）有氧、氢氧化Crenarchaeota的Sulfolobales以及中温、有氧、氨氧化奇古菌中起作用。值得注意的是，HP/HB 循环在这两个古菌谱系中独立进化，并且 crenarchaeal 和 thaumarchaeal 版本在酶特性和系统发育方面有所不同。这些差异导致变体之间的能量效率改变。与 crenarchaeal HP/HB 循环相比，thaumarchaeal 变体每转可节省两个 ATP 当量，使其成为最节能的碳固定有氧途径。典型地，HP/HB 循环包括两个烯酰辅酶 A (CoA) 水合酶反应：3-羟基丙酰辅酶 A 脱水酶反应和巴豆酰辅酶 A 水合酶反应。在这项研究中，我们表明在上述古菌群中，这两种反应都是由混杂的 3-羟基丙酰辅酶 A 脱水酶/巴豆酰辅酶 A 水合酶催化的（Msed_2001 in crenarchaeonMetallosphaera sedula和 Nmar_1308 在 thaumarchaeon Nitrosopumilus maritimus 中）。尽管这两种酶是同源的，但它们与细菌烯酰辅酶 A 水合酶密切相关，尽管存在多种替代方案，但它们仍由Crenarchaeota和Thaumarchaeota的祖先从同一酶库中独立回收。从两个独立进化的途径中参与无机碳固定的酶的出现这一惊人的相似性突出表明，自养的趋同进化可能比预期的要广泛得多。