Medical Structural Genomics of Pathogenic Protozoa Consortium, University of Washington, Seattle, WA 98195, USA.
J Mol Biol. 2010 Mar 12;396(5):1244-59. doi: 10.1016/j.jmb.2010.01.013. Epub 2010 Jan 11.
Purine nucleoside phosphorylases (PNPs) and uridine phosphorylases (UPs) are closely related enzymes involved in purine and pyrimidine salvage, respectively, which catalyze the removal of the ribosyl moiety from nucleosides so that the nucleotide base may be recycled. Parasitic protozoa generally are incapable of de novo purine biosynthesis; hence, the purine salvage pathway is of potential therapeutic interest. Information about pyrimidine biosynthesis in these organisms is much more limited. Though all seem to carry at least a subset of enzymes from each pathway, the dependency on de novo pyrimidine synthesis versus salvage varies from organism to organism and even from one growth stage to another. We have structurally and biochemically characterized a putative nucleoside phosphorylase (NP) from the pathogenic protozoan Trypanosoma brucei and find that it is a homodimeric UP. This is the first characterization of a UP from a trypanosomal source despite this activity being observed decades ago. Although this gene was broadly annotated as a putative NP, it was widely inferred to be a purine nucleoside phosphorylase. Our characterization of this trypanosomal enzyme shows that it is possible to distinguish between PNP and UP activity at the sequence level based on the absence or presence of a characteristic UP-specificity insert. We suggest that this recognizable feature may aid in proper annotation of the substrate specificity of enzymes in the NP family.
嘌呤核苷磷酸化酶（PNP）和尿苷磷酸化酶（UP）是密切相关的酶，分别参与嘌呤和嘧啶的补救途径，催化核苷中核糖部分的去除，以便核苷酸碱基可以被回收利用。寄生原生动物通常不能从头合成嘌呤；因此，嘌呤补救途径具有潜在的治疗意义。关于这些生物体中嘧啶生物合成的信息要有限得多。尽管所有生物体似乎都至少携带来自每个途径的一组酶，但对从头嘧啶合成与补救的依赖程度因生物体而异，甚至因生长阶段而异。我们已经从致病原生动物布氏锥虫中结构和生化表征了一种假定的核苷磷酸化酶（NP），并发现它是一种同源二聚体 UP。尽管几十年前就观察到了这种活性，但这是首次从锥虫来源表征 UP。尽管该基因被广泛注释为假定的 NP，但广泛推断它是一种嘌呤核苷磷酸化酶。我们对这种锥虫酶的表征表明，基于 UP 特异性插入的存在或不存在，有可能在序列水平上区分 PNP 和 UP 活性。我们建议，这个可识别的特征可能有助于正确注释 NP 家族中酶的底物特异性。
