Rajapakse Shanuk, Lee Yuri, Jayawardana Samith B, Helms Joshua, Mondal Pritam, Singh Akhil, Pierce Brad S, Shafaat Hannah S, Wijeratne Gayan B
Department of Chemistry and Biochemistry, University of Alabama Tuscaloosa AL 35401 USA
Department of Chemistry and Biochemistry, University of California Los Angeles CA 90095 USA
Chem Sci. 2025 Apr 28. doi: 10.1039/d4sc08701a.
Mid-valent heme-oxygen intermediates are central to a medley of pivotal physiological transformations in humans, and such systems are increasingly becoming more relevant therapeutic targets for challenging disease conditions. Nonetheless, precise mechanistic details pertaining to mid-valent heme intermediates as well as key structure-activity relationships remain enigmatic. To this end, this study strives to describe the influence of heme proximal ligation on the nucleophilic reactivity patterns of heme peroxo intermediates. A functional model system in which organic oxime substrates are used as -hydroxy-l-arginine mimics reproduces the second mechanistic step of nitric oxide synthase. Our findings reveal that axial ligation of heme peroxo adducts escalates the rates of nucleophilic reactivity, wherein the anionic ligands exhibited the most pronounced "push effect". Coordination of these axial ligands are accompanied by distinct geometric and electronic perturbations, which are supported by complementary theoretical studies. Kinetic interrogations reveal that the axially ligated heme peroxo adducts presumably mediate oxime oxidation the same mechanism as the parent (, with only solvent ligation) heme peroxo adduct, where the initial nucleophilic attack from the peroxo moiety on the oxime substrate is rate-limiting. All reaction products, including the final ketone as well as NO, have been characterized in detail.
中价态血红素-氧中间体在人类一系列关键的生理转变中起着核心作用，并且这类体系越来越成为具有挑战性疾病状态下更具相关性的治疗靶点。尽管如此，关于中价态血红素中间体的精确机理细节以及关键的构效关系仍然不明确。为此，本研究致力于描述血红素近端配位对血红素过氧中间体亲核反应活性模式的影响。一个使用有机肟底物作为-羟基-L-精氨酸模拟物的功能模型系统重现了一氧化氮合酶的第二步反应机理。我们的研究结果表明，血红素过氧加合物的轴向配位提高了亲核反应速率，其中阴离子配体表现出最显著的“推动效应”。这些轴向配体的配位伴随着明显的几何和电子扰动，这得到了互补理论研究的支持。动力学研究表明，轴向配位的血红素过氧加合物可能介导肟氧化，其机理与母体（仅溶剂配位）血红素过氧加合物相同，其中过氧部分对肟底物的初始亲核攻击是限速步骤。所有反应产物，包括最终的酮以及一氧化氮，都已得到详细表征。
