Complexation Enhances Solubility & Controls Reactivity of Transition Metal Ions

The levels of free transition metals in the body are, under normal circumstances, extremely low. The vast majority are found either associated directly with proteins via the oxy gen, nitrogen, and sulfur atoms found on the side chains of amino acids such as aspartate, glutamate, histidine, or cysteine (Figure 1); or with other organic moieties such as porphyrin, corrin, or pterins (Figure 2). The sequestration of transition metals into organometallic complexes confers multiple advantages that include protection against oxidation, suppression of ROS pro duction, enhancement of solubility, control of reactivity, and assembly of multimetal units (Figure 3).

Fig1. Ribbon diagram of a consensus C₂H₂ zinc finger domain. Shown are the bound Zn2+ (purple) and the R groups of the conserved phenylalanyl (F), leucyl (L) cysteinyl (C), and histidyl (H) residues with their carbon atoms in green. The polypeptide backbone is shown as a ribbon, with alpha helical portions highlighted in red. The sulfur and nitrogen atoms of the R groups of the cysteinyl and histidyl residues are shown in yellow and blue, respectively.

Fig2. Molybdopterin. Shown are the oxidized(left) and reduced(right)forms of molybdopterin.

Fig3. Hydrolysis of urea requires the cooperative influence of two active site Ni atoms. The figure depicts the formation of the transition state intermediate for the hydrolysis of the first C-N bond in urea (red) by the enzyme urease. Note how the Ni atoms chelate a water molecule to form a nucleophilic hydroxide and weaken the C-N bond through Lewis acid interactions with lone pairs of electrons on the O and one of the N atoms of urea.

Significance of Multivalent Capability

 Physiologic function of transition metal–containing cofactors and prosthetic groups is dependent on maintaining a multivalent ion’s appropriate oxidation state. For example, the porphyrin ring and proximal and distal histidyl residues of the globin polypeptide chain that complex the Fe2+ atoms in hemoglobin protects them from oxidation to Fe3+-containing methemoglobin, which is incapable of binding to and transporting oxygen. Free transition metal ions are vulnerable to oxidation by O2 and agents inside the cell. Not only are free transition metal ions vulnerable to nonspecific oxidation, their interaction with oxidizing agents such as O2 , NO, and H₂O₂ generally results in the generation of even more potent ROS. Incorporation into organometallic complexes thus protects the functionally relevant oxidation state of the transition metal and against the potential for generating harmful ROS.

Adjacent Ligands Can Modify Redox Potential

In organometallic complexes, the position and identity of the surrounding ligands can modify or tune the redox potential and Lewis acid potency of transition metal ions, thereby optimizing them for specific tasks (Table 1). For example, both cytochrome c and myoglobin are small, 12 to 17 kDa, monomeric proteins that contain a single heme iron. While the iron in myoglobin is optimized by its surroundings to maintain a constant, Fe2+, valence state, for oxygen binding; the iron atom in cytochrome c is optimized to cycle between the +2 and +3 valence states so the protein can carry electrons between complexes III and IV of the electron transport chain. The superoxide dismutases (SODs) illustrate how complexation can adapt different transition metals as catalysts for a common chemical reaction, the disproportionation of H₂O₂ into H₂O and O2. Each of the four distinct, nonhomologous SODs contain different transition metals whose atomic symbols are used to designate each family: Fe-SODs, Mn-SODs, Ni-SODs, and Cu, Zn-SODs.

Table1. Some Biologically Important Metalloproteins

Complexation Can Organize Multiple Metal Ions in a Single Functional Unit

 The formation of organometallic complexes also allows multiple metal ions to be assembled together in a single functional unit with capabilities that lie beyond those obtainable with a single transition metal ion. In the enzyme urease, which catalyzes the hydrolysis of urea in plants, the presence of two Ni atoms within the active site enables the enzyme to simultaneously polarize electrons in the C-N bond targeted for hydrolysis and to activate the attacking water molecule (see Figure 3). The presence of two Fe and two Cu atoms in cytochrome oxidase enables the complex IV of the electron transport chain to accumulate the four electrons needed to carry out the reduction of oxygen to water. Similarly, the bacterial enzyme nitrogenase employs an 8Fe-7S prosthetic group called the P-cluster and a unique Fe, Mo-cofactor to carry out the eight-electron reduction of atmospheric nitrogen to ammonia.

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