Ternes Chad M, Schönknecht Gerald
Department of Botany, Oklahoma State University.
Department of Botany, Oklahoma State University
Genome Biol Evol. 2014 Sep;6(9):2335-49. doi: 10.1093/gbe/evu185.
NAD(+) is an essential molecule for life, present in each living cell. It can function as an electron carrier or cofactor in redox biochemistry and energetics, and serves as substrate to generate the secondary messenger cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate. Although de novo NAD(+) biosynthesis is essential, different metabolic pathways exist in different eukaryotic clades. The kynurenine pathway starting with tryptophan was most likely present in the last common ancestor of all eukaryotes, and is active in fungi and animals. The aspartate pathway, detected in most photosynthetic eukaryotes, was probably acquired from the cyanobacterial endosymbiont that gave rise to chloroplasts. An evolutionary analysis of enzymes catalyzing de novo NAD(+) biosynthesis resulted in evolutionary trees incongruent with established organismal phylogeny, indicating numerous gene transfers. Endosymbiotic gene transfers probably introduced the aspartate pathway into eukaryotes and may have distributed it among different photosynthetic clades. In addition, several horizontal gene transfers substituted eukaryotic genes with bacterial orthologs. Although horizontal gene transfer is accepted as a key mechanism in prokaryotic evolution, it is supposed to be rare in eukaryotic evolution. The essential metabolic pathway of de novo NAD(+) biosynthesis in eukaryotes was shaped by numerous gene transfers.
NAD(+)是生命必需的分子，存在于每个活细胞中。它在氧化还原生物化学和能量学中可作为电子载体或辅助因子，并作为生成第二信使环ADP核糖和烟酰胺腺嘌呤二核苷酸磷酸的底物。尽管从头合成NAD(+)至关重要，但不同的真核生物类群存在不同的代谢途径。以色氨酸为起始的犬尿氨酸途径很可能存在于所有真核生物的最后一个共同祖先中，并且在真菌和动物中活跃。在大多数光合真核生物中检测到的天冬氨酸途径可能是从产生叶绿体的蓝藻内共生体获得的。对催化从头合成NAD(+)的酶进行的进化分析产生了与已确立的生物系统发育不一致的进化树，表明存在大量基因转移。内共生基因转移可能将天冬氨酸途径引入真核生物，并可能将其分布在不同的光合类群中。此外，一些水平基因转移用细菌直系同源基因取代了真核基因。虽然水平基因转移被认为是原核生物进化的关键机制，但在真核生物进化中它被认为是罕见的。真核生物中从头合成NAD(+)的基本代谢途径是由大量基因转移形成的。
