Greenblatt H M, Feinberg H, Tucker P A, Shoham G
Department of Inorganic and Analytical Chemistry, Hebrew University, Jerusalem, Israel, 91904.
Acta Crystallogr D Biol Crystallogr. 1998 May 1;54(Pt 3):289-305. doi: 10.1107/s0907444997010445.
The crystal structure of the zinc-containing exopeptidase bovine carboxypeptidase A (CPA) has been refined to high resolution, based on a data set collected from a single crystal, incorporating new sequence information based on cloning of the bovine gene. In addition, new refined structures are available for the zinc-removed form of the enzyme, apo-CPA, as well as the mercury-replaced form, Hg-CPA. The native structure reveals that the zinc-bound water molecule does not appear to more loosely bound than the rest of the zinc ligands, at least when B-factor values are considered. Nor is there any evidence for a secondary location of this water molecule. The apo-enzyme structure does not show any density in the place of the removed zinc ion. The only significant change appears to be a chi2 rotation of one zinc histidine ligand to form an ion-pair interaction with a glutamic acid side chain. The structure of Hg-CPA reveals a solvent Tris molecule bound to the mercury cation, as well as an unidentified cation bound to Glu270. The location of this citation agrees with previous proposals for the binding side of inhibitory zinc. These observations may explain some of the differences in kinetics observed in metal- replaced CPA.
基于从单晶收集的数据集，并结合基于牛基因克隆的新序列信息，含锌外肽酶牛羧肽酶A（CPA）的晶体结构已被精修至高分辨率。此外，该酶的去锌形式脱辅基CPA以及汞取代形式汞-CPA也有了新的精修结构。天然结构表明，至少从B因子值来看，与锌结合的水分子的结合似乎并不比其他锌配体更松散。也没有任何证据表明该水分子存在次要位置。脱辅基酶结构在去除锌离子的位置没有显示出任何密度。唯一显著的变化似乎是一个锌组氨酸配体的χ2旋转，以与一个谷氨酸侧链形成离子对相互作用。汞-CPA的结构显示一个溶剂三羟甲基氨基甲烷分子与汞阳离子结合，以及一个未鉴定的阳离子与Glu270结合。该引用的位置与先前关于抑制性锌结合位点的提议一致。这些观察结果可能解释了在金属取代的CPA中观察到的一些动力学差异。
