molecules Review Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control Irina I. Vlasova 1,2 1 Federal Research and Clinical Center of Physical-Chemical Medicine, Department of Biophysics, Malaya Pirogovskaya, 1a, Moscow 119435, Russia; [email protected]; Tel./Fax: +7-985-771-1657 2 Institute for Regenerative Medicine, Laboratory of Navigational Redox Lipidomics, Sechenov University, 8-2 Trubetskaya St., Moscow 119991, Russia Received: 28 August 2018; Accepted: 1 October 2018; Published: 8 October 2018 Abstract: The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Peroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes. Keywords: peroxidase activity; halogenating activity; reduction potential; myeloperoxidase; hemoglobin; Cyt c/cardiolipin complexes; cytoglobin 1. Introduction Reactive oxygen species (ROS) are involved in many physiological processes, including signal transduction, cell proliferation, gene expression, angiogenesis, and aging. However, excessive ROS production initiates oxidative stress, which has been implicated in pathophysiological processes like diabetes, cardiovascular and neurodegenerative diseases, pulmonary diseases, sepsis and cancer [1,2]. As such, a variety of special mechanisms exist in the body for controlling the concentration and activity of molecules capable of generating oxidants and ROS. The NADPH oxidase family of enzymes is a major source of ROS in vivo. The enzymes generate •– •– superoxide radicals (O2 ) that are the precursor of many ROS [1,3]. O2 can be spontaneously or catalytically (by superoxide dismutase) converted into hydrogen peroxide (H2O2). The major •– sources of O2 in plasma are NADPH oxidases on the surface of phagocytes and endothelial cells, xanthine oxidase bound to endothelial cells, and leakage from the mitochondrial respiratory chain [4–6]. Molecules 2018, 23, 2561; doi:10.3390/molecules23102561 www.mdpi.com/journal/molecules Molecules 2018, 23, 2561 2 of 27 A healthy physiological level of H2O2 in the plasma is 1–5 µM, but may be as high as 50 µM in inflammatory disease [6–8]. Hydrogen peroxide is one of the major in vivo oxidants. It is a two-electron acceptor with reduction potential Eo (H2O2/H2O) of 1.32 V. However, H2O2 reacts poorly or not at all with most biological molecules due the high activation energy barrier that must be overcome to release H2O2 oxidative power [9] (see Supplementary Materials, Section S1). The development of oxidative stress is mediated by transition metal ions, primarily with iron ions that promote H2O2 tunneling through the activation energy barrier. Various forms of iron react with hydrogen peroxide to form ROS. Table1 lists the reaction rate constants of various forms of iron with H2O2 and the reduction potentials of radicals and oxidants formed in the reactions. The rate constants of the reaction of H2O2 with free iron or free heme are low. Ligation usually stimulates Fenton reactions. Notably, iron ions are strongly controlled in vivo. A system of ceruloplasmin-transferrin is responsible for the transport of free iron ions in plasma; intracellular iron redox activity is arrested by ferritin [10]. Iron chaperons guide iron delivery in cells directly to their “protein clients” thus limiting non-enzymatic redox-cycling reactions [11]. In plasma, a special protein hemopexin (Hx) binds free heme and inhibits its peroxidase activity [12]. Table 1. Second order rate constants and oxidants produced in the reaction of different forms of iron with H2O2. Standard reduction potentials (Eo) are presented for oxidative couples: HOX/X-; • − • • • NO2/NO2 ; phenoxyl radical (Ph-O )/phenolic compound (Ph-OH); HO /H2O; HOO /H2O2. −1 −1 Form of Iron k, M s Reaction Products Eo, V pH 7 References hydroxyl radical, HO• 2.31 [13,14] 2+ 76 * free ferrous ion, Fe lipid alkoxyl radical, LO• ≤1.06 Lig-Fe2+ 102–104 HO•, LO• [15,16] hydroperoxyl radical, HOO• 1.06 [14,17,18] 3+ 0.01–0.02 free ferric ion, Fe lipid peroxyl radical, LOO• ≤1.00 phenoxyl radicals, Ph-O• 0.4–0.94 [19,20] Albumin-heme lipid radical, L• (LO•, LOO•) 0.6 [14] hypochlorous acid, HOCl 1.28 [21,22] hypobromous acid, HOBr 1.13 hypoiodous acid, HOI 0.78 True peroxidases (1.1–4.3) × 107 hypothiocyanous acid, HSCN 0.56 phenoxyl radicals, Ph-O• 0.4–0.94 [20] • nitrogen dioxide, NO2 1.04 [23] • • • • • Hemoglobin 42–43.6 Ph-O , NO2,L , (LO , LOO )[24,25] Cyt c/cardiolipin ~46.4 Ph-O•,L•, (LO•, LOO•)[26,27] Cyt c/cardiolipin + ~5 × (103–105) [27] FFA-OOH 2+ 3+ • − * Fe + H2O2 = Fe + OH + OH —Fenton reaction; Lig—a ligand (ATP, ADP, UTP, [Fe-S] clusters); L—lipid; FFA-OOH—free fatty acid hydroperoxides. Another group of molecules that interact with H2O2 are peroxidases. Peroxidases consume H2O2 and catalyze the one- or two-electron oxidation of a diverse array of Compounds called peroxidase substrates [1]. Heme-peroxidases are enzymes with a heme prosthetic group in the catalytic site where the iron is bound by four coordination bonds to a protoporphyrin IX derivative [28]. The rate constants of the interaction of heme in the active center of peroxidases with H2O2 are several orders of magnitude higher than that of the Fenton reaction [21] (Table1). In the presence of H 2O2, heme-peroxidases are activated to highly reactive intermediates. Lipid hydroperoxides also react with the peroxidase catalytic site. Except for cytochrome c (Cyt c)/cardiolipin (CL) complexes [27], the rate of their reaction with heme is lower than that of H2O2 [29–31]. Molecules 2018, 23, 2561 3 of 27 The mandatory requirements for the peroxidase activity of a heme-containing protein (i.e., its ability to form reactive intermediates) are: (1) The peroxidases operate having iron in ferric form (Fe(3+) or Fe(III)). Molecules 2018, 23, x FOR PEER REVIEW 3 of 27 (2) The heme iron of a peroxidase has five coordination bonds: four with nitrogens of tetrapyrrole ring,of histidine and the residue fifth heme linking ligand the on heme the proximal to the protein. heme side On isth ae highlydistal conservedheme side, imidazole the iron binds ring of a histidine residue linking the heme to the protein. On the distal heme side, the iron binds a water water molecule. This H2O molecule is replaced by H2O2 upon activation of a peroxidase [28,32]. (3) molecule.The peroxidases This H 2areO moleculecharacterized is replaced by the by specific H2O2 uponlocation activation of amino of aacids peroxidase in the [active28,32]. center (3) Thefor peroxidaseseffective coordination are characterized and byuse the of specific H2O2 location. On the of aminodistal acids heme in thesite, active a conserved center for effectivehistidine-arginine coordination couple and is use involved of H2O in2. a On hydrogen the distal peroxide heme site, network a conserved [28,33]. histidine-arginine couple is involved in a hydrogen peroxide network [28,33]. If a catalytic center of a hemoprotein meets the requirements listed above, H2O2 is specifically positionedIf a catalytic in the centerperoxidase of a hemoprotein catalytic center meets and the transfers requirements its high listed oxidation above, potential H2O2 is to specifically the heme. positionedTwo electrons in the are peroxidase transferred catalytic from the center enzyme and to transfers H2O2, which its high is reduced oxidation into potential water, towhereby the heme. the Twoheme electrons is oxidized are to transferred Compound from I (Figure the enzyme 1). Comp to Hound2O2, I which has two is reducedoxidizing into equivalents: water, whereby one in thethe hemeoxyferryl is oxidized heme center to Compound and the Isecond (Figure is1 ).spre Compoundad over the I has porphyrin two oxidizing ring as equivalents: a porphyrin one π in-cation the oxyferrylradical. heme center and the second is spread over the porphyrin ring as a porphyrin π-cation radical. Figure 1. Peroxidase and halogenation cycles of human true peroxidases [34–36]. The interaction Figure 1. Peroxidase and halogenation cycles of human true peroxidases [34–36]. The interaction between hydrogen peroxide and native ferric peroxidase heme leads to the reduction of H2O2 to water between hydrogen peroxide and native ferric peroxidase heme leads to the reduction of H2O2 to with formation of Compound I which is oxoferryl porphyrin-π-cationic radical. Compound I can be water with formation of Compound I which is oxoferryl porphyrin-π-cationic radical. Compound I sequentially reduced to the native enzyme via formation of Compound II by two one-electron oxidations can be sequentially reduced to the native enzyme via formation of Compound II by two caused by a number of simple compounds, peroxidase substrates (peroxidase cycle).