Uracil phosphoribosyltransferase (UPRT) is a member of
a large family of salvage and biosynthetic enzymes, the
phosphoribosyltransferases, and catalyzes the transfer of ribose
5-phosphate from
α-
d
-5-phosphoribosyl-1-pyrophosphate (PRPP) to the N1
nitrogen of uracil. The UPRT from the opportunistic pathogen
Toxoplasma gondii
represents a promising target for
rational drug design, because it can create intracellular, lethal
nucleotides from subversive substrates. However, the development of
such compounds requires a detailed understanding of the catalytic
mechanism. Toward this end we determined the crystal structure of the
T. gondii
UPRT bound to uracil and cPRPP, a
nonhydrolyzable PRPP analogue, to 2.5-Å resolution. The structure
suggests that the catalytic mechanism is substrate-assisted, and a
tetramer would be the more active oligomeric form of the enzyme.
Subsequent biochemical studies revealed that GTP binding, which has
been suggested to play a role in catalysis by other UPRTs, causes a
6-fold activation of the
T. gondii
enzyme and strikingly
stabilizes the tetramer form. The basis for stabilization was revealed
in the 2.45-Å resolution structure of the UPRT–GTP complex, whereby
residues from three subunits contributed to GTP binding. Thus, our
studies reveal an allosteric mechanism involving nucleotide
stabilization of a more active, higher order oligomer. Such regulation
of UPRT could play a role in the balance of purine and pyrimidine
nucleotide pools in the cell.
尿
嘧啶磷酸核糖转移酶(
UPRT)是一个大型救赎和
生物合成酶家族的成员,催化将
核糖5-
磷酸从α-d-5-
磷酸核糖-1-
焦磷酸(PRPP)转移至尿
嘧啶的N1氮。来自机会性病原体弓形虫的
UPRT代表了理性药物设计的一个有前途的靶点,因为它可以从颠覆性底物中创建细胞内致死核苷酸。然而,这种化合物的开发需要对催化机制有详细的了解。为此,我们确定了弓形虫
UPRT与尿
嘧啶和cPRPP(一种不可
水解的PRPP类似物)结合的晶体结构,分辨率为2.5埃。该结构表明,催化机制是底物辅助的,四聚体将是酶的更活跃的寡聚体形式。随后的生化研究揭示,GTP结合,这被认为在其他
UPRT中在催化中起作用,导致弓形虫酶的6倍活化,并显着稳定四聚体形式。稳定的基础在
UPRT-GTP复合物的2.45埃分辨率结构中揭示,其中来自三个亚基的残基贡献于GTP结合。因此,我们的研究揭示了一种涉及核苷酸稳定更活跃的高级别寡聚体的变构机制。
UPRT的这种调节可能在细胞内
嘌呤和
嘧啶核苷酸池的平衡中发挥作用。