Krahn J M, Kim J H, Burns M R, Parry R J, Zalkin H, Smith J L
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA.
Biochemistry. 1997 Sep 16;36(37):11061-8. doi: 10.1021/bi9714114.
Activation of gluatmine phosphoribosylpyrophosphate (RPPP) amidotransferase (GPATase) by binding of a PRPP substrate analog results in the formation of a 20 A channel connecting the active site for glutamine hydrolysis in one domain with the PRPP site in a second domain. This solvent-inaccessible channel permits transfer of the NH3 intermediate between the two active sites. Tunneling of NH3 may be a common mechanism for glutamine amidotransferase-catalyzed nitrogen transfer and for coordination of catalysis at two distinct active sites in complex enzymes. The 2.4 A crystal structure of the active conformer of GPATase also provides the first description of an intact active site for the phosphoribosyltransferase (PRTase) family of nucleotide synthesis and salvage enzymes. Chemical assistance to catalysis is provided primarily by the substrate and secondarily by the enzyme in the proposed structure-based mechanism. Different catalytic and inhibitory modes of divalent cation binding to the PRTase active site are revealed in the active conformer of the enzyme and in a feedback-inhibited GMP complex.
谷氨酰胺磷酸核糖焦磷酸（RPPP）酰胺转移酶（GPATase）与PRPP底物类似物结合而激活，导致形成一个20埃的通道，该通道将一个结构域中谷氨酰胺水解的活性位点与另一个结构域中的PRPP位点相连。这个溶剂不可及的通道允许NH3中间体在两个活性位点之间转移。NH3的隧道效应可能是谷氨酰胺酰胺转移酶催化的氮转移以及复杂酶中两个不同活性位点催化作用协调的常见机制。GPATase活性构象体的2.4埃晶体结构也首次描述了核苷酸合成和补救酶的磷酸核糖转移酶（PRTase）家族完整的活性位点。在所提出的基于结构的机制中，催化的化学辅助主要由底物提供，其次由酶提供。在该酶的活性构象体和反馈抑制的GMP复合物中揭示了二价阳离子与PRTase活性位点结合的不同催化和抑制模式。
