Gajhede M, Schuller D J, Henriksen A, Smith A T, Poulos T L
Department of Chemistry, University of Copenhagen, Denmark.
Nat Struct Biol. 1997 Dec;4(12):1032-8. doi: 10.1038/nsb1297-1032.
The crystal structure of horseradish peroxidase isozyme C (HRPC) has been solved to 2.15 A resolution. An important feature unique to the class III peroxidases is a long insertion, 34 residues in HRPC, between helices F and G. This region, which defines part of the substrate access channel, is not present in the core conserved fold typical of peroxidases from classes I and II. Comparison of HRPC and peanut peroxidase (PNP), the only other class III (higher plant) peroxidase for which an X-ray structure has been completed, reveals that the structure in this region is highly variable even within class III. For peroxidases of the HRPC type, characterized by a larger FG insertion (seven residues relative to PNP) and a shorter F' helix, we have identified the key residue involved in direct interactions with aromatic donor molecules. HRPC is unique in having a ring of three peripheral Phe residues, 142, 68 and 179. These guard the entrance to the exposed haem edge. We predict that this aromatic region is important for the ability of HRPC to bind aromatic substrates.
辣根过氧化物酶同工酶C（HRPC）的晶体结构已解析至2.15埃分辨率。III类过氧化物酶独有的一个重要特征是在F螺旋和G螺旋之间有一个长插入片段，在HRPC中为34个残基。该区域定义了底物进入通道的一部分，在I类和II类过氧化物酶典型的核心保守折叠中不存在。HRPC与花生过氧化物酶（PNP）（唯一另一种已完成X射线结构解析的III类（高等植物）过氧化物酶）的比较表明，即使在III类中，该区域的结构也高度可变。对于具有较大FG插入片段（相对于PNP多七个残基）和较短F'螺旋的HRPC类型过氧化物酶，我们确定了与芳香族供体分子直接相互作用的关键残基。HRPC的独特之处在于有一个由三个外围苯丙氨酸残基（142、68和179）组成的环。这些残基守护着暴露的血红素边缘的入口。我们预测这个芳香区域对于HRPC结合芳香族底物的能力很重要。
