Maté M J, Sevinc M S, Hu B, Bujons J, Bravo J, Switala J, Ens W, Loewen P C, Fita I
CID (Consejo Superior de Investigaciones Cietifícas) Jordi Girona 18-26, 08034 Barcelona, Spain.
J Biol Chem. 1999 Sep 24;274(39):27717-25. doi: 10.1074/jbc.274.39.27717.
The three-dimensional structures of two HPII variants, V169C and H392Q, have been determined at resolutions of 1.8 and 2.1 A, respectively. The V169C variant contains a new type of covalent bond between the sulfur atom of Cys(169) and a carbon atom on the imidazole ring of the essential His(128). This variant enzyme has only residual catalytic activity and contains heme b. The chain of water molecules visible in the main channel may reflect the organization of the hydrogen peroxide substrates in the active enzyme. Two alternative mechanisms, involving either compound I or free radical intermediates, are presented to explain the formation of the Cys-His covalent bond. The H392Q and H392E variants exhibit 75 and 25% of native catalytic activity, respectively. The Gln(392) variant contains only heme b, whereas the Glu(392) variant contains a mixture of heme b and cis and trans isomers of heme d, suggesting of a role for this residue in heme conversion. Replacement of either Gln(419) and Ser(414), both of which interact with the heme, affected the cis:trans ratio of spirolactone heme d. Implications for the heme oxidation mechanism and the His-Tyr bond formation in HPII are considered.
已分别以1.8埃和2.1埃的分辨率测定了两种HPII变体V169C和H392Q的三维结构。V169C变体在半胱氨酸（169）的硫原子与必需组氨酸（128）的咪唑环上的一个碳原子之间含有一种新型共价键。这种变体酶仅具有残余催化活性，且含有血红素b。主通道中可见的水分子链可能反映了活性酶中过氧化氢底物的组织情况。提出了两种涉及化合物I或自由基中间体的替代机制来解释半胱氨酸 - 组氨酸共价键的形成。H392Q和H392E变体分别表现出天然催化活性的75%和25%。谷氨酰胺（392）变体仅含有血红素b，而谷氨酸（392）变体含有血红素b以及血红素d的顺式和反式异构体的混合物，这表明该残基在血红素转化中起作用。与血红素相互作用的谷氨酰胺（419）和丝氨酸（414）的替换影响了螺旋内酯血红素d的顺式：反式比例。文中考虑了对HPII中血红素氧化机制和组氨酸 - 酪氨酸键形成的影响。
