antioxidants Review Human Paraoxonase-2 (PON2): Protein Functions and Modulation Giuseppe Manco *, Elena Porzio and Teresa Maria Carusone * Institute of Biochemistry and Cell Biology (IBBC, CNR), National Research Council, 80131 Naples, Italy; [email protected] * Correspondence: [email protected] (G.M.); [email protected] (T.M.C.); Fax: +30-0816132277 or +39-0816132296 (G.M.) Abstract: PON1, PON2, and PON3 belong to a family of lactone hydrolyzing enzymes endowed with various substrate specificities. Among PONs, PON2 shows the highest hydrolytic activity toward many acyl-homoserine lactones (acyl-HL) involved in bacterial quorum-sensing signaling. Accord- ingly, defense against pathogens, such as Brevundimonas aeruginosa (B. aeruginosa), was postulated to be the principal function of PON2. However, recent findings have highlighted the importance of PON2 in oxidative stress control, inhibition of apoptosis, and the progression of various types of malignancies. This review focuses on all of these aspects of PON2. Keywords: PON2; catalytic activity; lactonase; antioxidant; bacterial infections; inflammation; cancer; isoforms; SNPs; post-translational modifications; PON2 regulation Citation: Manco, G.; Porzio, E.; 1. Introduction Carusone, T.M. Human Paraoxonase 2 (PON2) is the oldest member and the most potent quorum quencher Paraoxonase-2 (PON2): Protein of the paraoxonase family, nevertheless it is less studied than PON1. Its intracellular Functions and Modulation. localization, in contrast to PON1 and PON3 secreted extracellularly, makes PON2 studies Antioxidants 2021, 10, 256. https:// more challenging. In fact in cells functional assays to measure its activity in different doi.org/10.3390/antiox10020256 compartments are still not available and the lack of the 3D structure does not allow one to clarify reaction mechanisms. The most common PON2 polymorphisms are associated with Academic Editors: C. Henrique Alves, its decreased lactonase activity and with a higher risk for coronary artery disease (CAD) Peter M.J. Quinn and António and Alzheimer’s disease. From 2010 it was highlighted PON2’s ability to reduce oxidative Francisco Ambrósio stress in mitochondria and to prevent apoptosis in the endoplasmic reticulum [1–4] with a Received: 4 January 2021 still unclarified mechanism suggested to be independent from the lactonase activity [1]. Accepted: 2 February 2021 From here on scientists explored PON2 antioxidant effects, its role in preventing heart Published: 7 February 2021 failure [5] and its involvement in any type of tumor [6]. Scientists are focusing on individual aspects of PON2. Of the 996 PON2 papers pro- Publisher’s Note: MDPI stays neutral duced from 1998, 792 were published in the last 10 years. This huge amount of information with regard to jurisdictional claims in published maps and institutional affil- needs to be collected and summarized allowing scientists to look at the whole picture of iations. PON2 functions and roles while continuing to elucidate single mechanisms. This review updates new discoveries and involvements of PON2 in diseases. In addition, it provides a concrete analysis of PON2 structure and functions on the basis of PON1 data, a new perspective on PON2 modulation based on post-translational modifications identified in our last paper and a connection of PON2 lactonase and antioxidant activities with the Copyright: © 2021 by the authors. reported diseases. Licensee MDPI, Basel, Switzerland. For a more comprehensive analysis of PON1 structure and mechanism readers can This article is an open access article distributed under the terms and refer to ref. [7]. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Antioxidants 2021, 10, 256. https://doi.org/10.3390/antiox10020256 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, x FOR PEER REVIEW 2 of 29 Antioxidants 2021, 10, 256 2 of 28 2.1. Gene and Localization 2. PON2Based Structure on a phylogenetic and Function analysis, PON2 emerges as the oldest member of this family,2.1. Gene with and PON1 Localization and PON3 evolving from PON2 [2,8]. Their genes, which reside on the sameBased cluster on a phylogenetic on chromosome analysis, 7 [2], PON2 share emergesabout 70% as thesequence oldest memberidentity. ofIt thisis worth fam- ily,noting with a PON1structural and similarity PON3 evolving of PONs from me PON2mbers [with2,8]. Theirthe endoplasmic genes, which reticulum reside on (ER)- the sameresident cluster molecular on chromosome chaperone 7 [2MEC-6], share [9]. about At 70%the sequencegenomic identity.structure It islevel, worth PON2 noting is aarranged structural in similaritynine exons of encoding PONs members a protein with of 355 the amino endoplasmic acids, approximately reticulum (ER)-resident 40–43 kDa molecularin mass. PON2 chaperone displays MEC-6 a 66% [9]. sequence At the genomic identity structure and 81% level, similarity PON2 with is arranged PON1, at in least nine exonsconsidering encoding the amost protein abundant of 355 form amino of acids, PON2 approximately [10]. PON1 and 40–43 PON3 kDa are in mass.extracellular PON2 proteinsdisplays asecreted 66% sequence in plasma identity and and bound 81% similarityto high-density with PON1, lipoproteins at least considering(HDLs). Their the expressionmost abundant is predominant form of PON2 in [ 10the]. PON1liver and and kidneys PON3 are [11–147]. extracellular PON2, proteins in contrast, secreted is ina plasmaubiquitously and bound expressed to high-density intracellular lipoproteins protein [1 (HDLs).5]. It localizes Their expression in the perinuclear is predominant region, in the liverendoplasmic and kidneys reticulum [11–14]. PON2,(ER), inmitochondria contrast, is a ubiquitously[3], and associates expressed with intracellular plasma proteinmembrane [15 ].fractions It localizes [16]. in In the summary, perinuclear althou region,gh PONs the endoplasmic are very similar reticulum in their (ER), amino mito- acidchondria sequences, [3], and they associates have different with plasma functions membrane and are fractions found at [16 different]. In summary, locations. although For a recentPONs review are very more similar focused in their on aminoPON1 structure acid sequences, and mechanism they have see different [7]. functions and are found at different locations. For a recent review more focused on PON1 structure and 2.2.mechanism PON2 Model see [ 7]. 2.2. PON2PON1 Model was the first HDL-associated protein and the only PON family member for which the structure has been elucidated [17] (PDB ID: 1V04). The first crystallized PON1 is a recombinantPON1 wasthe variant first from HDL-associated rabbit, highly protein similar and in thesequence only PON to human family PON1 member (Figure for which the structure has been elucidated [17] (PDB ID: 1V04). The first crystallized PON1 is 1). a recombinant variant from rabbit, highly similar in sequence to human PON1 (Figure1). Figure 1. PON1 protein structure. Side view of the six-bladed propeller-like structure of PON1. The top left side of the structure (molecular surfaceFigure colored 1. by PON1 atoms protein and ribbon structure. view Side in transparency) view of the six-bladed shows the retainedpropeller-like N-terminal structure helix of A PON1. and The top left side of the structure (molecular surface colored by atoms and ribbon view in helix B taking contact with a schematic lipid surface (phospholipids of a high-density lipoprotein (HDL) particle; membrane transparency) shows the retained N-terminal helix A and helix B taking contact with a schematic in the case of PON2). The N and C termini and the two calcium atoms (green balls) localize in the central tunnel of the lipid surface (phospholipids of a high-density lipoprotein (HDL) particle; membrane in the case of propeller. The ribbon of PON1PON2). molecule The N is and colored C termini by chain and progression. the two calcium Residues atoms interacting (green balls) with localize the two in the calcium central ions tunnel and phosphate are shown withof the line propeller. representation The ribbon (inset) of. Red PON1 balls molecule are water is colored molecules. by chain Picture progression. was drawn Residues by the Pymol program (The PyMOL Molecularinteracting Graphics with System, the two Version calcium 2.0. ions Schrödinger, and phosph LLC.,ate are New shown York, with NY, line USA). representation (inset). Red balls are water molecules. Picture was drawn by the Pymol program (The PyMOL Molecular GraphicsThe System, overall Version architecture 2.0. Schrödinger, of PON1 is LLC.). a β-propeller with six blades and a central tunnel; each blade consists of four β-sheets. A disulfide bridge between Cys42 and Cys353 forms a covalentThe closureoverall betweenarchitecture the Nof and PON1 C termini; is a β the-propeller two Cys with are conservedsix blades throughout and a central the tunnel;PON family each[ 10blade,17]. Twoconsists calcium of four ions, β one-sheets. at the A top disulfide of the structure bridge between (Ca1) and Cys42 one in and the Cys353central tunnelforms (Ca2),a covalent are present closure at abetween distance ofthe 7.4 N Å. and The calciumC termini; at thethe top, two considered Cys are conservedto be the catalyticthroughout calcium, the PON interacts family with [10,17]. the sideTwo chaincalcium oxygens ions, one of Asn224,at the top Asn270, of the Asn168, Asp269, and Glu53 (line representation in Figure1 inset). The central calcium Antioxidants 2021, 10, x FOR PEER