Protein Binding through Side Chains

In many of the haemoproteins we shall be discussing, the protoporphyrin IX group is held to the polypeptide chain only by hydrogen bonding, Van der Wools forces and iron-protein bonds. In several other cases, notably in cytochrome-c and its related compounds, the haem is covalently linked to the protein via substituents at the pyrrole carbon atoms. Cyto-chrome-c can be regarded as an iron protoporphyrin IX group with the addition of a protein cysteine side-chain across the vinyl double bonds giving two thio-ether links (Fig. 3). 

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In the latter case an extraneous messenger has to dock at the receptor s extracellular binding site on the cell surface. The information about the occupancy of the corresponding receptor is transmitted through the transmembrane part of the protein into its cytosolic domains by conformational changes. This structural response can be induced by an additional dimerization and results in a covalent modification of intracellular side chains. The new conformation is then recognized by cytosolic partner molecules. In this connection GTP binding pro-... 

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