Trickey P, Wagner M A, Jorns M S, Mathews F S
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, 660 S. Euclid Ave, St. Louis, MO 63110, USA.
Structure. 1999 Mar 15;7(3):331-45. doi: 10.1016/s0969-2126(99)80043-4.
Monomeric sarcosine oxidases (MSOXs) are among the simplest members of a recently recognized family of eukaryotic and prokaryotic enzymes that catalyze similar oxidative reactions with various secondary or tertiary amino acids and contain covalently bound flavins. Other members of this family include heterotetrameric sarcosine oxidase, N-methyltryptophan oxidase and pipecolate oxidase. Mammalian sarcosine dehydrogenase and dimethylglycine dehydrogenase may be more distantly related family members.
The X-ray crystal structure of MSOX from Bacillus sp. B-0618, expressed in Escherichia coli, has been solved at 2.0 A resolution by multiwavelength anomalous dispersion (MAD) from crystals of the selenomethionine-substituted enzyme. Fourteen selenium sites, belonging to two MSOX molecules in the asymmetric unit, were used for MAD phasing and to define the local twofold symmetry axis for electron-density averaging. The structures of the native enzyme and of two enzyme-inhibitor complexes were also determined.
MSOX is a two-domain protein with an overall topology most similar to that of D-amino acid oxidase, with which it shares 14% sequence identity. The flavin ring is located in a very basic environment, making contact with sidechains of arginine, lysine, histidine and the N-terminal end of a helix dipole. The flavin is covalently attached through an 8alpha-S-cysteinyl linkage to Cys315 of the catalytic domain. Covalent attachment is probably self-catalyzed through interactions with the positive sidechains and the helix dipole. Substrate binding is probably stabilized by hydrogen bonds between the substrate carboxylate and two basic sidechains, Arg52 and Lys348, located above the re face of the flavin ring.
单体肌氨酸氧化酶（MSOXs）是最近被认识的真核和原核酶家族中最简单的成员之一，该家族能催化各种仲胺或叔胺的类似氧化反应，并含有共价结合的黄素。这个家族的其他成员包括异源四聚体肌氨酸氧化酶、N-甲基色氨酸氧化酶和哌啶酸氧化酶。哺乳动物的肌氨酸脱氢酶和二甲基甘氨酸脱氢酶可能是关系更远的家族成员。
通过对硒代甲硫氨酸取代酶晶体进行多波长反常散射（MAD），已在2.0埃分辨率下解析了在大肠杆菌中表达的来自芽孢杆菌属B-0618的MSOX的X射线晶体结构。属于不对称单元中两个MSOX分子的14个硒位点用于MAD定相，并确定用于电子密度平均的局部二重对称轴。还测定了天然酶和两种酶-抑制剂复合物的结构。
MSOX是一种双结构域蛋白，其整体拓扑结构与D-氨基酸氧化酶最相似，它们的序列同一性为14%。黄素环位于非常碱性的环境中，与精氨酸、赖氨酸、组氨酸的侧链以及螺旋偶极子的N末端接触。黄素通过8α-S-半胱氨酰键与催化结构域的Cys315共价连接。共价连接可能通过与正侧链和螺旋偶极子的相互作用自催化。底物结合可能通过底物羧酸盐与位于黄素环re面上方的两个碱性侧链Arg52和Lys348之间的氢键而稳定。
