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Cytochromes binding old molecules in new ways

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    Cytochromes are a widely studied class of molecules used in biological systems to transport electrons.  Because a cytochrome’s function and destination are dependent upon the molecules to which it binds, it is important to know the biochemistry involved in binding of ligands.

    According to the current ‘distal dogma’ theory, small gases bind to the heme on the opposite side of it's attachment to the protein, that side is called the distal side(Figure 1).  However, in a recent paper David Lawson (Lawson et all. 2000, 5561) suggests that nitric oxide (NO) binds to the same side as the heme's attachment to the protein, the proximal side.  Their results show that a conformational shift occurs, rotating a histidine residue, breaking the coordinate bond between the cytochrome and the heme.  This movement provides enough space for the NO to bind to the proximal side.  This is a very important discovery because it contradicts the current theory.  Also, there are implications for other molecules with similar properties to cytochrome c’, for instance, soluble guanylate cyclase.

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