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Monodentate Carboxylate Complexes and the Carboxylate Shift: Implications for Polymetalloprotein Structure and Function

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Monodentate Carboxylate Complexes and the Carboxylate Shift: Implications for Polymetalloprotein Structure and Function New J. Chem., 1991, 15, 417-430 MONODENTATE CARBOXYLATE COMPLEXES AND THE CARBOXYLATE SHIFT: IMPLICATIONS FOR POLYMETALLOPROTEIN STRUCTURE AND FUNCTION * R. Lynn Rardin, William B. Tolman and Stephen J. Lippard Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 U.S.A. Received August 20, accepted December 17, 1990. ABSTRACT. - Carboxylates serve as an important class of ligands in inorganic chemistry and biology. Of the various types of metal-carboxylate binding modes, monodentate bridging between metal centers offers unique flexibility. More than 30 complexes containing this structural motif have been characterized, many of them recently. Through a detailed analysis of their stereochemistry, it has been possible to arrange these complexes into three classes, depending upon the strength of the interaction of the "dangling" oxygen atom of the carboxylate group with one of the bridged metal atoms. The monodentate bridging mode is postulated to be an important intermediate between the other, more common, carboxylate attachment modes, based on observed variations of the geometry for monodentate bridges in structurally characterized complexes. The movement from monodentate bridging to these other binding modes, termed the "carboxylate shift", is analyzed for the three classes of carboxylate bridged complexes. Such a shift could be kinetically important in carboxylate containing metalloproteins. Introduction three transition metal ions have been shown to bind to a 2 single carboxylate ligand; 3 more common is the coordina­ Simple carboxylate anions are ubiquitous and versatile tion of one or two metals in structurally distinct ways to one ligands in coordination chemistry. 1-4 Numerous complexes or more of the four available electron lone pairs of the with a variety of elements have been prepared, many of which carboxylate anion (Figure 3). Examples of every type of have played key roles in the conceptual development of bonding situation shown in Figure 2 can be found in any modern inorganic chemistry. Perhaps most striking in terms of several reviews published on the inorganic chemistry of of their impact are the so called dinuclear "paddlewheel" carboxylates. 1-3 and trinuclear "basic" car boxy late complexes (Figure 1 ). Considering the myriad of biological functions performed R by metalloproteins, nature has evolved very few ligands to cI coordinate metal ions. Of these few types of biological o/'\ ligands, carboxylates provided by aspartate and glutamate 2 0 ......-R side chains form an important class. A survey 4-47 of rep­ L .. ,""M''',. .. 0- -c, 0- I ......._M1 '""""O resentative metalloprotein crystal structure determinations , reveals the presence of carboxylates bound to the mono- and ........-c--,-0- '-L R 0 1 '\ ......-o polymetallic active sites of numerous functionally significant c systems (Table I). Among these biomolecules are proteins that perform electron-transfer (azurin, photosystem I), RI dioxygen metabolism (superoxide dismutases, hemerythrin, Figure 1. - Paddlewheel (left) and trinuclear basic carboxylate ribonucleotide reductase), and hydrolytic chemistry (holoeno­ (right) complexes. Curved lines represent bridging carboxylate lase, carboxypeptidase A), to cite just a few examples. Despite groups. their potential importance, however, the roles of carboxylate coordination to metalloprotein active sites in redox potential Extensive structural and physicochemical studies of these attenuation and in mechanisms of catalysis, metalloregula­ species have been crucial for increasing our understanding of tion, and metal ion reconstitution have not been systematical­ bonding and magnetic interactions between proximate metal ly investigated. centers, subjects with implications ranging from industrial 22 catalysis to metalloprotein structure and function. 1- We and others have begun to study the structures, physical properties, and basis for functional activity of several carbox­ The versatility of the RC02 ligand is reflected by the wide ylate bound polymetallic proteins by preparing and fully range of metal binding modes it can adopt (Figure 2). Up to 22 characterizing models of their active sites. 11- By using * Correspondence and reprints. simple carboxylates and the appropriate metal starting ma- NEW JOURNAL OF CHEMISTRY, VOL. 15, N° 6-1991. - 1144-0546/91/6 417 14/$ 3.40/ <Cl CNRS- Gauthier-Villars 418 R. L. RARDIN, W. B. TOLMAN AND S. J. LIPPARD R R \ I c IA M......_ /� 0 ,0 0 0 \ ,' M anti monodentate syn monodentate symmetric asymmetric terminal terminal chelating chelating R R I I R c R C M 'c�9 o�o I o�o/ I I \ M C M /o,: M------M 'o�o/ J M 1\1 syn-syn bidentate anti-anti bidentate syn-anti bidentate monodentate bridging bridging bridging bridging Figure 2. - Carboxylate binding modes in metal complexes. R terial, bidentate carboxylate bridged polynuclear complexes have been assembled, some of which are good structural and 11-22• 8-55 spectroscopic metalloprotein core analogs. 4 For an� l anti the most part, these efforts have focused on complexes of iron and manganese in their higher (greater than+ 2) oxida­ tion states, prepared in order to model the more accessible o�) forms of polyiron and polymanganese oxo proteins. Recent l:o�. emphasis on the synthesis of divalent iron and manganese compounds to mimic the less well understood but functionally \_syn_} important reduced forms of these metalloproteins has led to a novel class of molecules that contain a relatively rare Figure 3. - Schematic of a carboxylate depicting the oxygen syn 16• 8• 56-5 monodentate bridging carboxylate. 4 7 This binding and anti lone pairs. mode is unique in that two metal ions coordinate to a single carboxylate oxygen atom, utilizing both of its available lone Table I. - Representative structurally characterized metalloproteins pairs. Also distinguishing monodentate bridging from other with carboxylate coordination at their active sites binding geometries are widely varying interactions of the Active Site second, or "dangling", oxygen atom either with one of the Metalloprotein Function Metal(s) Ref. bridged me.tal ions or with a third metal center. The flexibility of this additional metal-oxygen interaction results in signifi­ 24 Azurin electron transfer Cu cant structural variations within the monodentate bridging 25-26 Su peroxide 02 detoxification Cu, Zn carboxylate class of molecules. This phenomenon, which we 5 Dismutase have termed the "carboxylate shift", 7 identifies the mono­ 27 Carboxypeptidase A peptide hydrolysis Zn dentate bridging mode as an important intermediate between 28-32 Thermolysin thermostable peptide Zn other carboxylate orientations. hydrolysis 33 In this article we present a structural base from which the Holoenolase 2-phosphoglycerate Zn consequences of the monodentate bridging mode and the dehydration 34 carboxylate shift can be assessed. Although, for proteins, Lactoferrin iron binding Fe 35-36 monodentate bridging has thus far been definitively identified Photosystem I electron transfer Fe 2- only in concanavalin A, 4 43 the isolation of a series of Reaction Center 37 synthetic complexes containing such a bridge presages its Hemerythrin 02 transport Fe 38-39 potential presence in additional metalloproteins and suggests Ribonucleotide ribonucleotide Fe a possible functional role in biology for the carboxylate shift. Reductase reduction 40-41 Superoxide 02 detoxification Fe, Mn Dismutase 42-43 Concanavalin A saccharide binding Mn, Ca Classification of compounds containing monodentate bridging 44 Trypsin polypeptide Ca carboxylates hydrolysis 45 Phospholipase A2 phospholipid Ca The versatility of carboxylates as metal ligands arises from hydrolysis the presence of four lone pairs of electrons available for 46 Staphylococcal DNA/RNA Ca metal binding to the carboxylate oxygen atoms. Each lone Nuclease hydrolysis pair lies in the plane of the carboxylate group and subtends 47 Intestinal Ca BP calcium binding Ca an angle of approximately 120° with respect to the O-C-0 NEW JOURNAL OF CHEMISTRY, VOL. 15, N° 6-1991 THE CARBOXYLA TE SHIFT 419 interbond angle. It has been suggested on the basis of stereo­ R electronic arguments that syn lone pairs (Fig. 3) are more basic than those in the anti positions, 58 and theoretical � �?dd(angling)(angling) - 0 calculations have supported this idea. 59 6 One consequence of the different relative basicities of the lone pairs is a prefer­ red orientation of carboxylates in proteolytic enzymes to i__.--0 favor syn protonation. Protonation of the syn electrons is I : suggested to be > I 04 times faster than anti protonation in A 5 "" ob(ridging)b(ridging) these proteins. 8 This phenomenon has led to the design aa·V /Cd:;.---� B and synthesis of polycarboxylates structurally constrained to y permit interactions of proton donors to syn rather than anti / ��: lone pairs (e.g. 1 in Figure 4) in order to take advantage of Mnon(interacting) 9 Mint(eracting) I- M-M I 1 't=a - �=tilt Figure 5. - Structural parameters defining the geometry of mono­ dentate carboxylate-bridged polymetallic centers. Figure 4. - Schematic of a ligand, 1, constrained to bind carboxylate syn lone pairs to a metal. CLASS I: NO OR WEAK INTERACTION 1 the basicity difference and facilitate molecular recognition. 6 The monodentate bridging carboxylate geometry can be The greater basicity of the syn lone pairs also has been distinguished by several characteristic parameters shown in used to rationalize preferential syn binding of metal ions, a Figure 5, where
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