Other Metal-containing Biomolecules

Although iron zinc and copper proteins are most common, there are a range of well-known examples that involve other metal ions. One of the best known is cobalamin, a cobalt containing molecule that is not an enzyme but is a tightly bound prosthetic group for various enzymes which plays a central role in the catalytic reaction. It is known as a co-enzyme. A co-enzyme’s role is to support an enzyme by operating in conjunction with it; in fact, the enzyme is inactive without it. In the case of cobalamin, the coenzyme is actually the site where the substrate binds and is changed to the product. The enzyme/coenzyme assembly has a primary task of promoting one-carbon transfer reactions exemplified in Figure 8.11. Cobalamin(orVitaminB12) is unusual–not only is it is one of Nature’s rare organometallic compounds (forming Co C bonds), but also it is the only vitamin known to contain a metal ion. It is essential for all higher animals, but is not found in plants. It appears to be synthesized exclusively by bacteria. Vitamin B12 has a similar (but not identical) ring to porphyrins. Formally, it is a cobalt (III) compound; reversible reduction to cobalt (II) [called B12r] and cobalt(I) [called B12s] is possible. In its active form, it contains an adenosine group as the sixth ligand of the cobalt octahedron. This can be replaced by a substrate (R), involving Co-C bonding. The chemistry in the enzyme/coenzyme system occurs on the cobalt centre, where the substrate binds and reacts in a reaction as a coordinated ligand.

Figure 8.11

The one-carbon transfer reaction (left) facilitated by the octahedral cobalt(III) complex Vitamin B12 (right) as coenzyme. Carbon–cobalt bonding of the substrate is involved in the mechanism, at the site designated by the R-group in the complex.

Figure 8.12 The octahedral environment of manganese in catechol dioxygenase, featuring three fac-arranged exchangeable water molecules(left), and the structure at the active site of a Ni-Fe hydrogenase (right), another example of a biomolecule with a metal-carbon bond. The positions of S-donor cysteine amino acid residues on the biopolymer are identified by their peptide location numbers; these can be well apart, indicating that folding of the polymer is important to bring them into appropriate positions.

The human form of superoxide dismutase containing manganese is a tetrameric enzyme, with four identical subunits each containing a Mn³⁺ atom. Groups adjacent to the man- ganese(III) centre that act as ligands are four imidazole and two carboxylate groups. Thus the Mn(III) centre is in a distorted octahedral environment of four N- and two O-donors. The cat- alytic mechanism for this enzyme involves cycling between the Mn(III) and Mn(II) forms. Catechol dioxygenase is a manganese-dependent extradiol-cleaving catechol dioxygenase. The manganese at the active site is in an octahedral MnN₂O₄ environment bound to the protein backbone by two imidazoles and a carboxylate group, with three 'free' sites carrying ex- changeable water molecules (Figure 8.12). Coordinated water groups at the active site of an enzyme are often the sites for substrate substitution and subsequent activation and reaction.

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