Mechanistic investigation for neutralization of toxic heme by histidine rich protein (HRP2) of malaria parasite using a series of HRP2 model peptides

恶性疟原虫富组氨酸蛋白2（HRP 2）的一系列模型肽，9 - 27-mer。在4.2K下，用光学吸收光谱和电子自旋共振（ESR）研究了这些多肽与meso-卟啉铁配合物（Fe（III）MP）的配位反应。基于光谱滴定法，测定了血红素-肽复合物的离解常数。最长的肽27-R（HRP 2的相互序列的三个重复序列; Ala-His-His-Ala-His-His-Ala-Ala-Asp），Ala-His-His-Ala-His-Ala-His-Ala-Ala-His-His-Ala-Ala-Asp-Ala-His-His-Ala-His-His-Ala-Ala-Asp显示出显著高的Kd值，为0.97 μM，与天然存在的HRP 2（0.77 μM）相对应。随着His残基数目的减少，肽的Kd值也随之降低，例如，最短肽10 R的Kd值（Ala-His-His-Ala-Ala-Asp-Ala-His-His-Ala）的最大吸收峰为77 mM。27 R-和10 R-Fe（III）MP络合物的最大吸收峰相同（405、533和564 nm）的光致发光，归属于六配位铁卟啉配合物。结合ESR谱的结果，确定了配合物的两个轴向配体均为组氨酸咪唑部分的氮原子。

A series of model peptides, 9 to 27-mer, of Plasmodium Falciparum histidine rich protein 2(HRP2). The coordination reactions occurring between these peptides and ferric iron complex of meso-porphyrin (Fe(III)MP) were studied by using optical absorption measurements and electron spin resonance spectroscopy at 4.2 K. Based on the optical spectrometric titration, the dissociation constants for the heme-peptide complexes were evaluated. The longest peptide 27-R (three repeat of reciprocal sequence of HRP2 ; Ala-His-His-Ala-His-His-Ala-Ala-Asp), Ala-His-His-Ala-His-His-Ala-Ala-Asp-His-His-Ala-His-His-Ala-His-His-Ala-Ala-Asp-Ala-His-His-Ala-His-His-Ala-Ala-Asp, showed significantly high Kd value as 0.97 μM, which correspond to that of naturally occurring HRP2 (0.77 μM). The Kd values of the peptides revealed concomitant decrease with decrease of the number of His residue, for example, the Kd value of the shortest prptide 10R (Ala-His-His-Ala-Ala-Asp-Ala-His-His-Ala) was evaluated to be 77 mM. The both 27R- and 10R-Fe(III)MP complexes gave an identical absorption maxima (405, 533, and 564 nm) ascribed to be the six coordinate iron porphyrin complex. Taking into account the results of ESR spectroscopy, the both axial ligands of these complexes were identified to be nitrogen atoms of imidazole moiety of His.