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Many proteins perform their functions in the monomeric state, for example some hormones and signalling molecules. Others do not function in isolation. Haemoglobin, for example, forms a hetero-tetramer; its multimeric form is essential for the cooperative binding of oxygen, which enhances the efficiency of oxygen transport to the tissues. Haemoglobin contains four non-protein haem prosthetic groups which are non-covalently bound and actually bind oxygen. The subunits of haemoglobin are of similar sequence and structure, more heterogeneous complexes are also common (RNA polymerases, viral coats, electron transfer complexes). The importance of interactions such as these in protein function is discussed in more detail in Section 1.3.