Minireviews ChemCatChem doi.org/10.1002/cctc.202000575 1 2 3 Biological Pincer Complexes 4 + [a] + [b] [a, b] [b, c] 5 Jorge L. Nevarez , Aiko Turmo , Jian Hu,* and Robert P. Hausinger* 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 ChemCatChem 2020, 12, 4242–4254 4242 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Freitag, 21.08.2020 2017 / 169458 [S. 4242/4254] 1 Minireviews ChemCatChem doi.org/10.1002/cctc.202000575 1 At least two types of pincer complexes are known to exist in Binding of metal to PQQ yields an ONO-type pincer complex. 2 biology. A metal-pyrroloquinolone quinone (PQQ) cofactor was More recently, a nickel-pincer nucleotide (NPN) cofactor was 3 first identified in bacterial methanol dehydrogenase, and later discovered in lactate racemase, LarA. This cofactor derives from 4 also found in selected short-chain alcohol dehydrogenases of nicotinic acid adenine dinucleotide via action of a carboxylase/ 5 other microorganisms. The PQQ-associated metal can be hydrolase, sulfur transferase, and nickel insertase, resulting in 6 calcium, magnesium, or a rare earth element depending on the an SCS-type pincer complex. The NPN cofactor likely occurs in 7 enzyme sequence. Synthesis of this organic ligand requires a selected other racemases and epimerases of bacteria, archaea, 8 series of accessory proteins acting on a small peptide, PqqA. and a few eukaryotes. 9 10 1. Introduction purified and structurally defined as PQQ by x-ray 11 crystallography.[3] The crystal structures of methanol dehydro- 12 This special collection of articles on pincer chemistry and genases from Methylophilus methylotrophus and Methylophilus 13 catalysis focuses primarily on the properties and reactivities of W3A1 at 2.6 Å resolution provided the first glimpses of how 14 pincer complexes synthesized by inorganic chemists. In this PQQ binds to this enzyme’s active site.[4] The cofactor is co- 15 contribution, we describe two types of pincer complexes planar with a tryptophan residue and located in a funnel- 16 identified in biological systems: the metal-pyrroloquinoline shaped channel. Various residues interact with the three 17 quinone (PQQ) cofactor, containing an ONO-type pincer ligand, carboxyl groups of PQQ and an arginine residue is positioned 18 found in particular dehydrogenases, and the nickel-pincer near the quinone moiety. Some, but not all, PQQ-dependent 19 nucleotide (NPN) cofactor, with an SCS-type pincer ligand, of enzymes also contain metal ions and other distinguishing 20 selected racemases and epimerases. Both biological cofactors features. For example, structural studies of methanol dehydro- 21 possess a planar organic ligand that tri-coordinates a metal ion genase from Methylorubrum (formerly Methylobacterium) extor- 22 (Figure 1) – the hallmark of a pincer complex. The following quens culminated in a 1.2 Å resolution structure (PDB ID 1W6S) 23 sections briefly review the discovery of each cofactor, summa- revealing a disulfide bridge nearby the cofactor and showing 24 rize their biosynthetic pathways, detail how they function in that PQQ binds a calcium ion using its O5, N6, and O7 atoms, 25 catalysis, and compare their properties to the synthetic systems. with additional metal coordination by Glu177 (bidentate) and 26 Asn261 side chains.[5] Asp303 is located near the calcium ion in 27 this structure, and a similarly positioned aspartic acid serves as 28 2. Metal-PQQ complexes an additional metal ligand in Ca-PQQ methanol dehydrogen- 29 ases from several other microorganisms including M. meth- 30 A novel prosthetic group was found in glucose dehydrogenase ylotrophus W3A1, Hyphomicrobium denitrificans, and Methano- 31 from Bacterium anitratum in 1964,[1] and three years later was coccus capsulatus strain Bath.[6] In studies of methanol 32 also reported to be present in methanol dehydrogenase of dehydrogenase from Methylacidiphilum fumariolicum SoIV, the 33 Pseudomonas sp. M27.[2] Using cell-free extracts of the facul- metal speciation of the PQQ pincer complex was expanded to 34 tative methylotroph Pseudomonas TP1, the cofactor was include several rare earth elements (lanthanum, cerium, neo- 35 dymium, praseodymium, samarium, europium, or gadolinium).[7] 36 The structures of the cerium-PQQ and europium-PQQ methanol 37 + [a] J. L. Nevarez, Prof. J. Hu dehydrogenases were shown to be very similar to the earlier 38 Department of Chemistry described Ca-PQQ enzymes with the lanthanides tricoordinated 39 578 South Shaw Lane Michigan State University to PQQ, but with four side chain metal ligands: a glutamic acid, 40 East Lansing an asparagine, and two aspartic acids.[7–8] A lanthanide-PQQ 41 Michigan 48824 (USA) cofactor also was structurally characterized from Meth- 42 E-mail: [email protected] [b] A. Turmo,+ Prof. J. Hu, Prof. R. P. Hausinger ylmicrobium buryatense 5GB1C methanol dehydrogenase,[9] 43 Department of Biochemistry and Molecular Biology whereas a magnesium-PQQ cofactor was identified in the 44 603 Wilson Road, Room 212 enzyme from Methylophaga aminisulfidivorans.[10] M. extorquens 45 Michigan State University East Lansing AM1 is notable in containing a Ca-PQQ methanol dehydrogen- 46 Michigan 48824 (USA) ase (encoded by mxaF), two lanthanide-PQQ methanol dehy- 47 E-mail: [email protected] drogenases (encoded by xoxF1 and xoxF2), and a lanthanide- 48 [c] Prof. R. P. Hausinger Department of Microbiology and Molecular Genetics PQQ ethanol dehydrogenase (encoded by exaF).[11] Structural 49 567 Wilson Road studies of M. extorquens XoxF1 combined with mutagenesis, 50 2215 Biomedical Physical Sciences metal-binding, and activity assays of this protein and M. 51 Michigan State University East Lansing extorquens ExaF provide evidence that the fourth metal ligand, 52 Michigan 48824 (USA) an aspartate residue, determines whether a lanthanide is bound 53 Homepage: https://twitter.com/msu_mmg and functionally active in these proteins.[12] Lanthanide-PQQ 54 [+] These authors contributed equally to this work. alcohol dehydrogenases are also found in Pseudomonas putida 55 This publication is part of a joint Special Collection with EurJIC on “Pincer KT 2440.[13] Recent evidence suggests this microorganism 56 Chemistry & Catalysis”. Please follow the link for more articles in the col- lection. possesses two dehydrogenases that convert the glycerol 57 ChemCatChem 2020, 12, 4242–4254 www.chemcatchem.org 4243 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Freitag, 21.08.2020 2017 / 169458 [S. 4243/4254] 1 Minireviews ChemCatChem doi.org/10.1002/cctc.202000575 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Figure 1. Biological pincer complexes. (A) Methanol dehydrogenase of Methylorubrum extorquens (PDB ID 1W6S) with its two protein subunits (yellow and 29 cyan ribbons) shown in cartoon mode, PQQ in stick mode (magenta carbon atoms, blue nitrogen atoms, and red oxygen atoms), Glu177 (bidentate) and Asn261 metal-binding residues and the nearby Asp303 (a comparable residue is used as a metal ligand in other methanol dehydrogenases) in stick mode, and 30 calcium as a grey sphere.[5c] A fourth metal ligand, an aspartic acid residue, is found in lanthanide-PQQ enzymes. The lower portion of the panel illustrates a 31 close-up view of the Ca-PQQ complex and a ChemDraw depiction of the cofactor. (B) Lactate racemase of Lactobacillus plantarum (PDB ID 6C1W) with the 32 protein shown as a yellow ribbon, Lys184 and His200 residues (yellow) as well as the NPN cofactor (with color as in panel A along with yellow sulfur, and orange phosphorus) as sticks, and nickel as a green sphere.[16] An expanded view of the Ni-NPN complex and a ChemDraw depiction are provided underneath. 33 34 35 substrate to glyceraldehyde; PedE using a calcium-PQQ enzyme ing enzymes in various microbes remains uncertain, but 36 and PedH using a lanthanide-PQQ version.[14] The physiological probably relates to the greater Lewis acidity of this metal when 37 significance of lanthanide incorporation into the PQQ-contain- compared to calcium or magnesium. In addition to catalyzing 38 39 40 Jorge L. Nevarez earned his B.S. degree in Jian Hu received his Ph.D. from Peking 41 Chemistry at Northern Illinois University in University Health Science Center, China, in 42 2018. He is now a graduate student co- 2004 and obtained postdoctoral training at mentored by Drs. Hausinger and Hu. His Florida State University (2005-2007) and Yale 43 current research focuses on elucidating the School of Medicine (2007-2013). He joined 44 broader role of the nickel-pincer nucleotide Michigan State University in 2013 with ap- 45 cofactor in nature. pointments in Biochemistry & Molecular Biol- 46 ogy and Chemistry. His research focuses on 47 structural biology of metal transporters and metalloenzymes. 48 Aiko Turmo received B.S. degrees in Molecular 49 Genetics and Genomics & Microbiology from Robert P. Hausinger received his Ph.D. from 50 Michigan State University in 2013. She was a the University of Minnesota (1982) and ob- 51 research technician for four years, and is now tained postdoctoral training at M.I.T. (1982- a Ph.D. candidate in the Hausinger laboratory. 1984). He is a University Distinguished Profes- 52 Her current research focuses on the biochem- sor of Microbiology & Molecular Genetics and 53 ical and structural characterization of the Biochemistry & Molecular Biology at Michigan 54 nickel insertase enzyme critical to the syn- State University. His research focuses on 55 thesis of the nickel-pincer nucleotide. metallocenter biosynthesis and the catalytic 56 mechanisms of metalloenzymes. 57 ChemCatChem 2020, 12, 4242–4254 www.chemcatchem.org 4244 © 2020 Wiley-VCH Verlag GmbH & Co.