Other oxygen transport systems

Key point: Arthropods and molluscs use haemocyanin and certain marine worms use haemerythrin. In many organisms, such as arthropods and molluscs, O₂ is transported by the Cu protein haemocyanin, which, unlike haemoglobin, is extracellular, as is common for Cu proteins. Haemocyanin is oligomeric, with each monomer containing a pair of Cu atoms in close proximity. Deoxyhaemocyanin (Cu(I)) is colourless but it becomes bright blue when O₂ binds. The active site is shown in Fig. 27.19. In the deoxy state, each Cu atom is three-coordinate and bound in a pyramidal array by three histidine residues. The two Cu atoms are so far apart (460 pm) that there is no direct interaction between them. The low coordination number is typical of Cu(I), which is normally two- to four-coordinate. Rapid and reversible coordination of O₂ occurs between the two Cu atoms in a bridging dihapto manner (μ-2 2) and the low vibrational wavenumber of the coordinated O₂ molecule (750 cm1) shows it has been reduced to peroxide O₂^-2, with an accompanying lowering of the bond order from 2 to 1. To accommodate the binding of O₂, the protein adjusts its conformation to bring the two Cu atoms closer together. The Cu sites become five-coordinate, which is typical of Cu (II). Haemerythrin is an example of a special class of dinuclear Fe centres that are found in a number of proteins with diverse functions, such as methane monooxygenase, and some ribonucleotide reductases and acid phosphatases. The two Fe atoms in the active site of haemerythrin (21) are each coordinated by amino acid side chains but are also linked by two bridging carboxylate groups and a small ligand. In the reduced form, which binds O₂ reversibly, this small ligand is an OH^- ion. Coordination of O₂ occurs at only one of the Fe atoms and the distal O atom forms a hydrogen bond to the H atom of the bridging hydroxide.

Figure 27.19 Binding of O₂ at the active site of haemocyanin causes the two Cu atoms to be brought closer together. The O₂ complex is regarded as a binuclear Cu(II) centre in which the two Cu atoms are bridged by an _n², _n²-peroxide.

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Heating

Expansion against constant pressure

The general expression for work

Work, heat, and energy

Mixtures of gases

Future directions

The contributions of individual elements

Imaging agents

Anti-arthritis drugs

Cancer treatment

Chelation therapy

Biomineralization