Department of Food Science, University of Wisconsin, Madison, WI, United States.
Department of Microbiology & Molecular Genetics, University of California, Davis, CA, United States.
Adv Microb Physiol. 2023;82:205-266. doi: 10.1016/bs.ampbs.2023.01.001. Epub 2023 Feb 11.
Purines are abundant among organic nitrogen sources and have high nitrogen content. Accordingly, microorganisms have evolved different pathways to catabolize purines and their metabolic products such as allantoin. Enterobacteria from the genera Escherichia, Klebsiella and Salmonella have three such pathways. First, the HPX pathway, found in the genus Klebsiella and very close relatives, catabolizes purines during aerobic growth, extracting all four nitrogen atoms in the process. This pathway includes several known or predicted enzymes not previously observed in other purine catabolic pathways. Second, the ALL pathway, found in strains from all three species, catabolizes allantoin during anaerobic growth in a branched pathway that also includes glyoxylate assimilation. This allantoin fermentation pathway originally was characterized in a gram-positive bacterium, and therefore is widespread. Third, the XDH pathway, found in strains from Escherichia and Klebsiella spp., at present is ill-defined but likely includes enzymes to catabolize purines during anaerobic growth. Critically, this pathway may include an enzyme system for anaerobic urate catabolism, a phenomenon not previously described. Documenting such a pathway would overturn the long-held assumption that urate catabolism requires oxygen. Overall, this broad capability for purine catabolism during either aerobic or anaerobic growth suggests that purines and their metabolites contribute to enterobacterial fitness in a variety of environments.
嘌呤是有机氮源中含量丰富且含氮量高的物质。因此，微生物进化出不同的途径来分解嘌呤及其代谢产物，如尿囊素。属于埃希氏菌属、克雷伯氏菌属和沙门氏菌属的肠杆菌有三种这样的途径。首先，在克雷伯氏菌属和非常近缘的菌属中发现的 HPX 途径，在有氧生长过程中分解嘌呤，在此过程中提取所有四个氮原子。该途径包括几个以前在其他嘌呤分解途径中没有观察到的已知或预测的酶。其次，ALL 途径存在于来自三个物种的菌株中，在无氧生长期间通过支链途径分解尿囊素，该途径还包括乙醛酸同化。最初在革兰氏阳性菌中对这种尿囊素发酵途径进行了表征，因此它分布广泛。第三，在来自埃希氏菌属和克雷伯氏菌属的菌株中发现的 XDH 途径，目前尚未明确，但可能包括在无氧生长期间分解嘌呤的酶。关键是，该途径可能包括一种用于无氧尿酸分解的酶系统，这是以前没有描述过的现象。记录这样的途径将推翻尿酸分解需要氧气的长期假设。总的来说，这种在有氧或无氧生长过程中广泛分解嘌呤的能力表明，嘌呤及其代谢物在各种环境中对肠杆菌的适应性有贡献。
