Tuna ferrocytochrome c and ferricytochrome c have been refined at 1.5Å and 1.8Å resolution, respectively. Significant conformational differences were detected in the haem crevice. The relationship between motions of a buried water molecule, Asn 52, Tyr 67 and the haem, and alkaline transition and redox-potential control is discussed.
The chemiosmotic hypothesis has brought new aspects in the plant membrane research. The H+-pump of the plasma membrane generates the electrochemical potential gradient of H+ which is available for transport of ions, sugars and amino acids. Further, the H+ pump works electrogenic. This article summarizes the results on the electrogenic mechanism and its regulation in Characeae cells which were revealed by the intracellular perfusion study. 1) The electrogenic pump is fueled by Mg·ATP. 2) The H+-efflux is dependent on ATP, evidence that the ion species carried by the pump is H+. 3) The electrogenic activity of the pump almost saturated at 100μM ATP. 4) The pump is inhibited by vanadate applied internally and half inhibition concentration is about 5μM. 5) ADP inhibits the pump in the presence of ATP. 6) The pump is activated by light via photosynthesis. Activiation is caused by the factor (s) other than the substrates, H+ and ATP. 7) Anoxia inhibits the pump. However, the extent of ATP decrease caused by anoxia is not enough to account for the inhibition of the membrane potential. Some part of the inhibition is caused by increased ADP. 8) In excitation of the plasma membrane, the pump component of the membrane potential also changes together with the diffusion potential. Advantages of the intracellular perfusion for studying electrogenesis were discussed.
Neighbouring cells in gland tissues communicate with each other through special channels (gas junctions). These junctions allow electrical communication and the transfer of molecules with mol. wt. up to about 1000 from cell to cell. In the exocrine pancreas, electrical communication is restricted to units consisting of a small number of neighbouring cells. Cell to cell communication is generally thought to be controlled by the cytosolic ionized calcium ([Ca]i) in such a way that an increased [Ca]i tends to decrease the conductance of the junctional channels. Junctional conductance can be reduced markedly by neurotransmitters and hormons in exocrine pancreas and this is due to a neurotransmitter or hormon-induced increased in cytosolic [Ca]i. It has, however, recently been shown that a very large decrease in intracellular pH can also evoke electrical uncoupling of neighbouring cells. The physiological significance of the control of the junctional conductance is still obscure.