4-Amino-4-deoxychorismate synthase (ADCS), a chorismate-utilizing enzyme, is composed of two subunits: PabA and PabB. PabA is a glutamine amidotransferase that hydrolyzes glutamine into glutamate and ammonia. PabB is an aminodeoxychorismate synthase that converts chorismate to 4-amino-4-deoxychorismate (ADC) using the ammonia produced by PabA. ADCS functions under allosteric regulation between PabA and PabB. However, the allosteric mechanism remains unresolved because the structure of the PabA–PabB complex has not been determined. Here, the crystal structure and characterization of PapA from Streptomyces venezuelae (SvPapA), a bifunctional enzyme comprising the PabA and PabB domains, is reported. SvPapA forms a unique dimer in which PabA and PabB domains from different monomers complement each other and form an active structure. The chorismate-bound structure revealed that recognition of the C1 carboxyl group by Thr501 and Gly502 of the 498-PIKTG-502 motif in the PabB domain is essential for the catalytic Lys500 to reach the C2 atom, a reaction-initiation site. SvPapA demonstrated ADCS activity in the presence of Mg²⁺ when glutamate or NH⁺₄ was used as the amino donor. The crystal structure indicated that the Mg²⁺-binding position changed depending on the binding of chorismate. In addition, significant structural changes were observed in the PabA domain depending on the presence or absence of chorismate. This study provides insights into the structural factors that are involved in the allosteric regulation of ADCS.

中文翻译：

---

4-氨基-4-脱氧分支酸合酶变构途径的结构基础

4-氨基-4-脱氧分支酸合酶 (ADCS) 是一种分支酸利用酶，由两个亚基组成：PabA 和 PabB。PabA 是一种谷氨酰胺酰胺转移酶，可将谷氨酰胺水解成谷氨酸和氨。PabB 是一种氨基脱氧分支酸合酶，可利用 PabA 产生的氨将分支酸转化为 4-氨基-4-脱氧分支酸 (ADC)。ADCS 在 PabA 和 PabB 之间的变构调节下发挥作用。然而，由于 PabA-PabB 复合物的结构尚未确定，变构机制仍未解决。在此，报道了来自委内瑞拉链霉菌(SvPapA)的 PapA 的晶体结构和表征，SvPapA 是一种包含 PabA 和 PabB 结构域的双功能酶。SvPapA 形成独特的二聚体，其中来自不同单体的 PabA 和 PabB 结构域相互补充并形成活性结构。分支酸结合结构表明，PabB 结构域中 498-PIKTG-502 基序的 Thr501 和 Gly502 对 C1 羧基的识别对于催化 Lys500 到达 C2 原子（反应起始位点）至关重要。当谷氨酸或NH ⁺ ₄用作氨基供体时，SvPapA 在 Mg ²⁺存在下表现出 ADCS 活性。晶体结构表明Mg ²⁺结合位置根据分支酸的结合而变化。此外，根据分支酸的存在与否，在 PabA 结构域中观察到显着的结构变化。这项研究提供了对 ADCS 变构调节所涉及的结构因素的见解。