Buchanan B B, Arnon D I
Division of Molecular Plant Biology, University of California, Berkeley 94720, USA.
Photosynth Res. 1990;24:47-53.
The Krebs cycle (citric acid or tricarboxylic acid cycle), the final common pathway in aerobic metabolism for the oxidation of carbohydrates, fatty acids and amino acids, is known to be irreversible. It liberates CO2 and generates NADH whose aerobic oxidation yields ATP but it does not operate in reverse as a biosynthetic pathway for CO2 assimilation. In 1966, our laboratory described a cyclic pathway for CO2 assimilation (Evans, Buchanan and Arnon 1966) that was unusual in two respects: (i) it provided the first instance of an obligate photoautotroph that assimilated CO2 by a pathway different from Calvin's reductive pentose phosphate cycle (Calvin 1962) and (ii) in its overall effect the new cycle was a reversal of the Krebs cycle. Named the 'reductive carboxylic acid cycle' (sometimes also called the reductive tricarboxylic acid cycle) the new cycle appeared to be the sole CO2 assimilation pathway in Chlorobium thiosulfatophilum (Evans et al. 1966) (now known as Chlorobium limicola forma thiosulfatophilum). Chlorobium is a photosynthetic green sulfur bacterium that grows anaerobically in an inorganic medium with sulfide and thiosulfate as electron donors and CO2 as an obligatory carbon source. In the ensuing years, the new cycle was viewed with skepticism. Not only was it in conflict with the prevailing doctrine that the 'one important property ... shared by all (our emphasis) autotrophic species is the assimilation of CO2 via the Calvin cycle' (McFadden 1973) but also some of its experimental underpinnings were challenged. It is only now that in the words of one of its early skeptics (Tabita 1988) 'a long and tortuous controversy' has ended with general acceptance of the reductive carboxylic acid cycle as a photosynthetic CO2 assimilation pathway distinct from the pentose cycle. (Henceforth, to minimize repetitiveness, the reductive pentose phosphate cycle will often be referred to as the pentose cycle and the reductive carboxylic acid cycle as the carboxylic acid cycle.) Aside from photosynthetic pathways which are the focus of this article, CO2 assimilation is also known to sustain autotrophic growth via the acetyl-CoA pathway (Wood et al. 1986). Our aim here is to discuss (i) the findings that led our group to the discovery of the reductive carboxylic acid cycle, (ii) the nature and resolution of the controversy that followed, and (iii) the possible evolutionary implications of the cycle as an ancient mechanism for photosynthetic CO2 assimilation that preceded the pentose cycle and served as a precursor of the Krebs cycle in aerobic metabolism.
三羧酸循环（柠檬酸循环或三羧酸循环）是碳水化合物、脂肪酸和氨基酸有氧代谢的最终共同途径，已知是不可逆的。它释放二氧化碳并生成NADH，NADH的有氧氧化产生ATP，但它不会作为二氧化碳同化的生物合成途径逆向运行。1966年，我们实验室描述了一条二氧化碳同化的循环途径（埃文斯、布坎南和阿农，1966年），该途径在两个方面不同寻常：（i）它首次提供了一个专性光合自养生物的例子，该生物通过一条不同于卡尔文还原戊糖磷酸循环（卡尔文，1962年）的途径同化二氧化碳；（ii）从总体效果来看，新循环是三羧酸循环的逆向过程。这个新循环被命名为“还原性羧酸循环”（有时也称为还原性三羧酸循环），它似乎是嗜硫绿菌（埃文斯等人，1966年）（现称为嗜硫栖热硫化叶菌）中唯一的二氧化碳同化途径。嗜硫绿菌是一种光合绿色硫细菌，在以硫化物和硫代硫酸盐作为电子供体、二氧化碳作为必需碳源的无机培养基中厌氧生长。在随后的几年里，这个新循环受到了质疑。它不仅与当时盛行的学说相冲突，即“所有（我们强调）自养物种共有的一个重要特性是通过卡尔文循环同化二氧化碳”（麦克法登，1973年），而且其一些实验依据也受到了挑战。直到现在，用其早期怀疑者之一（塔比塔，1988年）的话说，“一场漫长而曲折的争论”才结束，还原性羧酸循环作为一条不同于戊糖循环的光合二氧化碳同化途径得到了普遍认可。（此后，为了尽量减少重复，还原戊糖磷酸循环通常将被称为戊糖循环，还原性羧酸循环将被称为羧酸循环。）除了本文重点讨论的光合途径外，已知二氧化碳同化还通过乙酰辅酶A途径维持自养生长（伍德等人，1986年）。我们这里的目的是讨论：（i）促使我们小组发现还原性羧酸循环的研究结果；（ii）随后争论的性质和解决情况；（iii）该循环作为一种古老的光合二氧化碳同化机制的可能进化意义，它先于戊糖循环，并且是有氧代谢中三羧酸循环的前身。
