Transmembrane control elements

Fig. 3.6. Principles of signal transduction by transmembrane receptors and nuclear receptors, a) transmembrane receptors receive the signal on the cell surface and convert it into an intracellular signal that can be passed on until it reaches the nucleus, b) In signal transduction via nuclear receptors the hormone enters the cell and binds the receptor either in the cytosol (R) or nucleus (R ). Nuclear receptors act as nuclear transcription factors that bind specific DNA elements (HRE hormone responsive element) found in the promotor region of regulated genes to control their transcription rate.

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

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