Loch Joanna I, Imiolczyk Barbara, Sliwiak Joanna, Wantuch Anna, Bejger Magdalena, Gilski Miroslaw, Jaskolski Mariusz
Department of Crystal Chemistry and Crystal Physics, Faculty of Chemistry, Jagiellonian University, Krakow, Poland.
Institute of Bioorganic Chemistry, Polish Academy of Sciences, Poznan, Poland.
Nat Commun. 2021 Nov 18;12(1):6717. doi: 10.1038/s41467-021-27105-x.
Rhizobium etli, a nitrogen-fixing bacterial symbiont of legume plants, encodes an essential L-asparaginase (ReAV) with no sequence homology to known enzymes with this activity. High-resolution crystal structures of ReAV show indeed a structurally distinct, dimeric enzyme, with some resemblance to glutaminases and β-lactamases. However, ReAV has no glutaminase or lactamase activity, and at pH 9 its allosteric asparaginase activity is relatively high, with K for L-Asn at 4.2 mM and k of 438 s. The active site of ReAV, deduced from structural comparisons and confirmed by mutagenesis experiments, contains a highly specific Zn binding site without a catalytic role. The extensive active site includes residues with unusual chemical properties. There are two Ser-Lys tandems, all connected through a network of H-bonds to the Zn center, and three tightly bound water molecules near Ser48, which clearly indicate the catalytic nucleophile.
费氏中华根瘤菌是豆科植物的一种固氮细菌共生体，它编码一种必需的L-天冬酰胺酶（ReAV），该酶与具有这种活性的已知酶没有序列同源性。ReAV的高分辨率晶体结构显示它确实是一种结构独特的二聚体酶，与谷氨酰胺酶和β-内酰胺酶有一些相似之处。然而，ReAV没有谷氨酰胺酶或内酰胺酶活性，在pH 9时其变构天冬酰胺酶活性相对较高，L-天冬酰胺的K值为4.2 mM，k值为438 s⁻¹。通过结构比较推导并经诱变实验证实，ReAV的活性位点包含一个没有催化作用的高度特异性锌结合位点。广泛的活性位点包括具有异常化学性质的残基。有两个丝氨酸-赖氨酸串联结构，它们都通过氢键网络与锌中心相连，并且在Ser48附近有三个紧密结合的水分子，这清楚地表明了催化亲核试剂。
