Copper-containing Biomolecules

Copper proteins display an array of functions including: in electron transfer involving the Cu(I)/Cu (II) couple through either an outer sphere mechanism or acting as an inner-sphere reductase; as mono-terminal oxidases forming either water or hydrogen peroxide from dioxygen; as oxygenases whose task is to incorporate an oxygen atom into a substrate; in superoxide degradation to form dioxygen and peroxide; and in oxygen transport. From both a structural and spectroscopic point of view, three main types of biologically active copper centres have been found in the copper proteins (Figure 8.9). These may be distinguished according to their spectroscopic behaviour. Type 1 has what is called a ‘blue’ copper centre, with the copper normally coordinated to two nitrogen and two sulfur atoms in a distorted tetrahedral shape. Blue copper proteins form perhaps the best-known examples of copper proteins. They have a far more intense blue

Figure 8.9

The three types of four-coordinate copper (II) centres found in most copper proteins. An example of a rarer five-coordinate square-based pyramidal geometry of copper (II) found in the prion protein appears at right.

colour than Cu2+aq ion, consistent with the usual observation that tetrahedral complexes exhibit more intense absorbance bands than octahedral or square planar complexes, but are similar in hue.

Type 2 has a "non-blue' copper centre, with the copper coordinated to two or three nitrogen donors in addition to oxygen donor(s) in a square planar geometry with the different donor set and shape together responsible for a markedly different colour compared with Type 1. Type 3 has two copper centres closely adjacent, forming a hydroxide-bridged dimer with each copper ion in an approximately square planar geometry.

In this family of compounds, the N-donors come from unsaturated nitrogens in histidine amino acid residues, the S-donors come from methionine and cysteine residues, and the O-donors come from a carboxylic acid in the protein chain. Water, hydroxide and alkoxide oxygens are also employed as O-donor ligands.

Electron transfer copper proteins usually belong to the blue copper proteins (Type 1): azurin is a simple example. This family of proteins are also called cupredoxins and they participate in many redox reactions involved in processes fundamental to biology, such as respiration or photosynthesis. The striking electron transfer capabilities of blue copper proteins have been studied extensively. Plastocyanin, with a tetrahedral CuN2S2 core, acts as the electron donor to Photo System I in photosynthesis in higher plants and some algae. Non-blue copper centres are actually most common in copper proteins and the copper centre in these adopts, as mentioned above essentially square-planar geometry. The copper ion is bound to the imidazole nitrogen of two or three histidine residues and to O-donor ligands; there is weak additional O-donor coordination in axial sites with the typical Jahn-Teller distortion expected of do Cu(II) complexes. The Type 2 sites are more ionic than Type I sites, having mainly neutral donor groups rather than the thiolate anions of the latter. Working cooperatively with organic coenzymes Type 2 copper centres direct a wide range of biological oxidation reactions, which include alcohol oxidation and amine degradation. In caeruloplasmin, a Type 2 monomer joins with a Type 3 dimer to form a larger copper trimer; similar clusters appear in laccase and ascorbate oxidase.

Compounds with another stereochemistry are also observed. The copper binding site in the prion protein, an approximately 200-amino-acid residue glycosylated protein that carries one copper ion is of a square-based pyramidal form (Figure 8.9) having one imidazole nitrogen two amide nitrogens and an amide oxygen bound around the copper with a water    molecule in the axial site. The normal function of the prion protein seems to involve copper regulation in the central nervous system. It exists in two different conformational forms the infectious form of which causes neurodegenerative diseases such as ‘mad cow’ disease (bovine spongiform encephalopathy, BSE), and related diseases such as Creutzfeldt–Jakob disease, kuru and Gerstmann–Straussler syndrome.

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