van den Hoff M J, Jonker A, Beintema J J, Lamers W H
Department of Anatomy and Embryology, University of Amsterdam, The Netherlands.
J Mol Evol. 1995 Dec;41(6):813-32. doi: 10.1007/BF00173161.
Carbamoylphosphate is a common intermediate in the metabolic pathways leading to the biosynthesis of arginine and pyrimidines. The amino acid sequences of all available proteins that catalyze the formation of carbamoylphosphate were retrieved from Genbank and aligned to estimate their mutual phylogenetic relations. In gram-negative bacteria carbamoylphosphate is synthesized by a two-subunit enzyme with glutaminase and carbamoylphosphate synthetase (CPS) activity, respectively. In gram-positive bacteria and lower eukaryotes this two-subunit CPS has become dedicated to arginine biosynthesis, while in higher eukaryotes the two subunits fused and subsequently lost the glutaminase activity. The CPS dedicated to pyrimidine synthesis is part of a multifunctional enzyme (CPS II), encoding in addition dihydroorotase and aspartate transcarbamoylase. Evidence is presented to strengthen the hypothesis that the two "kinase" subdomains of all CPS isozymes arose from a duplication of an ancestral gene in the progenote. A further duplication of the entire CPS gene occurred after the divergence of the plants and before the divergence of the fungi from the eukaryotic root, generating the two isoenzymes involved in either the synthesis of arginine or that of pyrimidines. The mutation rate was found to be five- to tenfold higher after the duplication than before, probably reflecting optimization of the enzymes for their newly acquired specialized function. We hypothesize that this duplication arose from a need for metabolic channeling for pyrimidine biosynthesis as it was accompanied by the tagging of the CPS gene with the genes for dihydroorotase and aspartate transcarbamoylase, and as the duplication occurred independently also in gram-positive bacteria. Analysis of the exon-intron organization of the two "kinase" subdomains in CPS I and II suggests that ancient exons may have comprised approx. 19 amino acids, in accordance with the prediction of the "intron-early" theory.
氨甲酰磷酸是精氨酸和嘧啶生物合成代谢途径中的常见中间体。从Genbank检索出所有催化氨甲酰磷酸形成的现有蛋白质的氨基酸序列，并进行比对以估计它们之间的系统发育关系。在革兰氏阴性菌中，氨甲酰磷酸由一种双亚基酶合成，该酶分别具有谷氨酰胺酶和氨甲酰磷酸合成酶（CPS）活性。在革兰氏阳性菌和低等真核生物中，这种双亚基CPS已专门用于精氨酸生物合成，而在高等真核生物中，这两个亚基融合并随后失去了谷氨酰胺酶活性。专门用于嘧啶合成的CPS是一种多功能酶（CPS II）的一部分，该酶还编码二氢乳清酸酶和天冬氨酸转氨甲酰酶。有证据支持这样的假说：所有CPS同工酶的两个“激酶”亚结构域源自原核生物中一个祖先基因的复制。在植物分化后且真菌从真核生物根部分化之前，整个CPS基因又发生了一次复制，产生了参与精氨酸合成或嘧啶合成的两种同工酶。发现复制后的突变率比之前高五到十倍，这可能反映了这些酶为其新获得的特定功能进行的优化。我们推测这种复制源于嘧啶生物合成中代谢通道化的需求，因为它伴随着CPS基因与二氢乳清酸酶和天冬氨酸转氨甲酰酶基因的标记，并且这种复制在革兰氏阳性菌中也独立发生。对CPS I和II中两个“激酶”亚结构域的外显子 - 内含子组织分析表明，古老的外显子可能包含约19个氨基酸，这与“内含子早期”理论的预测一致。
