Smith Brian D, Sanders Jason L, Porubsky Patrick R, Lushington Gerald H, Stout C David, Scott Emily E
Department of Medicinal Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
J Biol Chem. 2007 Jun 8;282(23):17306-13. doi: 10.1074/jbc.M702361200. Epub 2007 Apr 11.
The human lung cytochrome P450 2A13 (CYP2A13) activates the nicotine-derived procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) into DNA-altering compounds that cause lung cancer. Another cytochrome P450, CYP2A6, is also present in human lung, but at much lower levels. Although these two enzymes are 93.5% identical, CYP2A13 metabolizes NNK with much lower K(m) values than does CYP2A6. To investigate the structural differences between these two enzymes the structure of CYP2A13 was determined to 2.35A by x-ray crystallography and compared with structures of CYP2A6. As expected, the overall CYP2A13 and CYP2A6 structures are very similar with an average root mean square deviation of 0.5A for the Calpha atoms. Like CYP2A6, the CYP2A13 active site cavity is small and highly hydrophobic with a cluster of Phe residues composing the active site roof. Active site residue Asn(297) is positioned to hydrogen bond with an adventitious ligand, identified as indole. Amino acid differences between CYP2A6 and CYP2A13 at positions 117, 300, 301, and 208 relate to different orientations of the ligand plane in the two protein structures and may underlie the significant variations observed in binding and catalysis of many CYP2A ligands. In addition, docking studies suggest that residues 365 and 366 may also contribute to differences in NNK metabolism.
人类肺细胞色素P450 2A13（CYP2A13）可将源自尼古丁的致癌物4-（甲基亚硝胺基）-1-（3-吡啶基）-1-丁酮（NNK）激活为可导致DNA改变的化合物，进而引发肺癌。另一种细胞色素P450，即CYP2A6，也存在于人类肺组织中，但含量要低得多。尽管这两种酶的同源性为93.5%，但CYP2A13代谢NNK的米氏常数（K(m)）值比CYP2A6低得多。为了研究这两种酶之间的结构差异，通过X射线晶体学测定了CYP2A13的结构，分辨率达到2.35埃，并与CYP2A6的结构进行了比较。正如预期的那样，CYP2A13和CYP2A6的整体结构非常相似，α碳原子的平均均方根偏差为0.5埃。与CYP2A6一样，CYP2A13的活性位点腔较小且高度疏水，有一簇苯丙氨酸残基构成活性位点顶部。活性位点残基Asn(297)的位置可与一种偶然配体形成氢键，该配体被鉴定为吲哚。CYP2A6和CYP2A13在第117、300、301和208位氨基酸的差异与两种蛋白质结构中配体平面的不同取向有关，这可能是许多CYP2A配体在结合和催化过程中观察到显著差异的基础。此外，对接研究表明，第365和366位残基也可能导致NNK代谢的差异。
