Oliveira M A, Carroll D, Davidson L, Momany C, Hackert M L
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, USA.
Biochemistry. 1997 Dec 23;36(51):16147-54. doi: 10.1021/bi970605g.
A nucleotide effector site of the biodegradative form of ornithine decarboxylase from Lactobacillus 30a (OrnDC L30a) has been identified. OrnDC L30a activity at pH 8.0, where the enzyme is normally inactive, is stimulated by GTP and dGTP and to a lesser extent by GDP but not by ATP, CTP, or UTP. The pH profile indicates that activation by GTP is reflected by an increase in kcat/KM,orn (above pH 6.8), while Vmax remains constant over the pH range 4.0-9. 0. Scatchard plot analysis shows that GTP binds to OrnDC L30a at both pH 5.8 (KD = 0.11 microM) and pH 8.0 (KD = 1.6 microM), but unexpectedly, half-site binding is observed at the higher pH. The OrnDC L30a dodecamer dissociates into dimers at high pH in the presence or absence of GTP. The GTP binding site was located in difference electron density maps using low-resolution X-ray data. This represents a new type of GTP binding site. A model explaining the activation of OrnDC L30a by GTP is presented.
已鉴定出来自乳酸杆菌30a（OrnDC L30a）的鸟氨酸脱羧酶生物降解形式的核苷酸效应位点。在pH 8.0时，该酶通常无活性，但GTP和dGTP可刺激OrnDC L30a的活性，GDP在较小程度上也有刺激作用，而ATP、CTP或UTP则无此作用。pH曲线表明，GTP的激活表现为kcat/KM,orn增加（pH高于6.8），而在4.0 - 9.0的pH范围内Vmax保持恒定。Scatchard图分析表明，GTP在pH 5.8（KD = 0.11 microM）和pH 8.0（KD = 1.6 microM）时均与OrnDC L30a结合，但出乎意料的是，在较高pH下观察到半位点结合。在存在或不存在GTP的情况下，OrnDC L30a十二聚体在高pH下会解离成二聚体。利用低分辨率X射线数据，在差分电子密度图中定位了GTP结合位点。这代表了一种新型的GTP结合位点。本文提出了一个解释GTP激活OrnDC L30a的模型。
