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Figure 4.3

Basic structure of a receptor-excitation assembly, used to capture, bind and transport specific target molecules into and within the phytoplankton cell. Based on a figure in Simon (1995) and reproduced with permission from Reynolds (1997a).

Figure 4.3

Basic structure of a receptor-excitation assembly, used to capture, bind and transport specific target molecules into and within the phytoplankton cell. Based on a figure in Simon (1995) and reproduced with permission from Reynolds (1997a).

In the specific case of nutrient uptake, the linkages involve sequences of protein-protein interactions in which the binding of a specific target ligand at a peripheral receptor stimulates an excitation of the transfer response. The basic structure of the transmembrane assembly is sketched in Fig. 4.3. The receptor region is periplasmic and constitutes the ligand-specific protein. Reaction with the target molecule stimulates a molecular transformation, which, in turn, becomes the excitant substrate to the proteins of the transmembrane region. The central reaction within this complex is to catalyse the phosphorylation of the substrate. This is, of course, the principal reaction through which cells regulate the transfer of redox power. The high-energy pyrophosphate bond between the second and third radicals of ATP is broken by a kinase, the conversion to the diphosphate releasing some 33 kJ (mol)-1 of chemical energy.

The further proteins in the series react analogously and sequentially, the excitation of a recep tor by the reaction with the target becoming the excitation of the next. The sequenced reactions of the transporter proteins provide a redox-gradient 'channel' along which the target molecule is passed. The whole functions rather like a line of people helping to douse a fire. The first lifts the filled bucket and passes it to the next, who, in turn passes it to a third. Only after the second has accepted a bucket from the first can the first turn to pick up another bucket. The second cannot accept another bucket until he has passed on the last and is once more receptive to the next.

In much the same way as the fire-fighters might be supplied with filled buckets more rapidly than they can be dispatched down the line, so the molecular sequence can become saturated and the fastest rate of uptake then fails to deplete the supply of target nutrients. At the other extreme, exhaustion of the immediate source of buckets or targets leaves the entire sequence idle but the full transport capacity remains 'open' and primed to react to the stimulation of the next arriving molecule.

The activity state of the transport system is communicated to the controlling genes. It is extremely important that the cell can react to the symptoms of shortages of supply (of, say, phosphorus) by regulating and closing down the assembly processes before the supply of components (or a particular component) is exhausted. What happens is that a second group of regulatory proteins associated with the transporters activate the transcription of particular genes called operons. While the uptake and transport mechanism is functioning normally, the operons repress the expression of further genes which regulate the reactions to nutrient starvation (Mann, 1995). For instance, an external shortage of a given nutrient (say, orthophosphate ions) will result in a diminishing frequency of receptor reactions and a weakening suppression of the genes that will activate the appropriate cellular response. The response may be to produce more phosphatases or to promote the metabolic closedown of the cell, including entry into a resting stage, before the cell starves to death (see also Sections 4.3.3, 5.2.1).

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