Empowering Biological Insights: Unveiling 5 Key Roles of Copper(II) and Zinc(II) Dinuclear Enzymes Model Compounds

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Abstract

First series transition metals are used abundantly by nature to perform catalytic transformations of several substrates. Furthermore, the cooperative activity of two proximal metal ions is common and represents a highly efficient catalytic system in living organisms. In this work three dinuclear μ-phenolate bridged metal complexes were prepared with copper(II) and zinc(II), resulting in a ZnZn, CuCu and CuZn with the ligand 2-ethylaminodimethylamino phenol (saldman) as model compounds of superoxide dismutase (CuCu and CuZn) and metallo-β-lactamases (ZnZn). Metals are coordinated in a μ-phenolate bridged symmetric system.

Cu(II) presents a more distorted structure, while zinc is very symmetric. For this reason, [CuCu(saldman)] shows higher water solubility and also higher lability of the bridge. The antioxidant and hydrolytic beta-lactamase-like activity of the complexes were evaluated. The lability of the bridge seems to be important for the antioxidant activity and is suggested to because of [CuCu(saldman)] presents a lower antioxidant capacity than [CuZn(saldman)], which showed to present a more stable bridge in solution. The hydrolytic activity of the bimetallic complexes was assayed using nitrocefin as substrate and showed [ZnZn(saldman)] as a better catalyst than the Cu(II) analog. The series demonstrates the importance of the nature of the metal center for the biological function and how the reactivity of the model complex can be modulated by coordination chemistry.

Introduction

Since ancient times human inventions has been inspired by Nature [1], [2]. Biomimetic chemistry is a field that grows fast where the scientists create new substances and reactions that imitate those found in biological systems [1], [3], [4]. As an interface field between biology and chemistry, information flows in both directions. From the perspective of biology, reactions from living systems can be better contextualized and understood when simpler models are built to explain, for example, why certain chemical structures are chosen by nature and why they work efficiently [5], [6], [7], [8]. From the second point of view, chemistry can benefit from biology by gaining a wider horizon of bioinspired reactions and systems, leading to the creation of more efficient catalysts, energy generating systems or drugs [9], [10], [11], [12], [13], [14].

Protein secondary, tertiary and quaternary structures determine its function in living systems. Protein structure is mainly determined by the side chains of the twenty canonical amino acids, which cover a wide range of interactions that are ionic, hydrophobic, hydrophilic and covalent. However, there is a limit to what is possible to create using the canonical amino acids only. For that reason, Nature uses organic and inorganic cofactors to widen the range of attainable protein structures even more. Here, we are interested in the inorganic cofactors, particularly a metal ion. They can be added by nature for a number of reasons, from achieving a specific protein folding to composing active sites used in many catalytic transformations.

Nature predominately uses elements from the first transition series that are usually more available, when a catalytic transformation of a substrate is required. Such impressive reactions are a source of inspiration for coordination chemists to create bioinspired systems to further understand a more complex biological system of interest [11], [15], [16], [17]. The cooperative activity of two proximal metal ions is commonly found in nature and represents a highly efficient catalytic system in living organisms [18]. Hydrolases encompass a wide class of enzymes, including several metallohydrolases, which can have one or two metal ions as a cofactor, often zinc(II), and that exhibit a two metal centers cooperativity strategy. Metallo-beta-lactamases (MBL) are metallohydrolases that compose the bacterial resistance machinery. They cleave the beta-lactam ring present in the main class of antibacterial agents, leading to resistant strains and causing several problems in the bacterial infection management [19], [20], [21], [22], [23], [24]. Up to date, no metallo-beta-lactamase inhibitors are available and most of the mechanistic knowledge of these enzymes came from studying model complexes [6], [7], [25], [26]. A better understanding of metalloenzymes is crucial for the design of efficient inhibitors.

The two metal ions cooperativity strategy is also seen in the Cu/Zn superoxide dismutase (SOD). They catalyze superoxide dismutation as a mechanism of superoxide detoxification [27], [28]. Under normal circumstances, superoxide is kept under control by SOD. The overexpression of SOD is induced as a response to oxidative stress, representing an important protective mechanism [29], [30], [31]. Some diseases trigger oxidative processes and the administration of native SOD has been studied in pre-clinical and clinical trials [32], [33]. It has been shown to be beneficial for the treatment of many conditions such as inflammation [34], arthritis [35], Parkinson [36], cancer [37], AIDS [38], [39], among others. Mimetics of the manganese form have been developed as a more efficient alternative to the large natural-occurring protein and it is now undergoing Phase I of clinical trials [36]. A Cu/Zn complex mimicking SOD presented antineuroglycemic and neuroprotective effects in vitro and in vivo [40].

In this study, three dinuclear coordination compounds were prepared with the ligand 2-ethylaminodimethylamino phenol (saldman). Two of them are homonuclear with Cu(II) or Zn(II) and another one is the heteronuclear Cu(II)/Zn(II), inspired by the Cu/Zn SOD. The structural formulas of the complexes and the free ligand are represented in Fig. 1. The amino phenol ligand, named here as Saldman for simplicity, was chosen because it can form dinucluear system, holding the metal ions together by the phenolate bridge, as in the chemical environment of the bimetallic cooperative enzymes. X-ray diffraction and electron paramagnetic resonance (EPR) data confirm the structures of the complexes and the formation of the heterometallic complex. These system mimic metallo-beta-lactamases when the ions are zinc and it is also similar to other phenolate-bridged copper dimers designed as models of oxidized-hemocyanin, copper monooxigenase. Here, the three complexes [CuCu(saldman)], [CuZn(saldman)] and [ZnZn(saldman)] had their hydrolytic activity assayed using beta-lactam compounds as substrate and antioxidant activity evaluated using the Trolox equivalent antioxidant capacity protocol. The results allowed to compare the effect of the nature of metal ions in the usual biological functions for Zn(II) complexes (hydrolysis) and for Cu(II) complexes as antioxidant mimetics.