Transient binding mode

The second water ligand leaves the Fe i coordination sphere, leaving it five-coordinate. We have now arrived at a crucial step in the reaction mechanism. The cA-aconitate has to flip from its isocitrate binding mode to its citrate binding mode, i.e., a 180° rotation is required to interconvert Ca-H and Cp-R. It is hypothesized (cf. [15]) that this flip is brought about by displacement of the cA-aconitate by another molecule of cA-aconitate. This implies release of the cA-aconitate into solution and formation, if only transient, of 6. 

Conjugated enynes possess several of the same transition metal binding modes exhibited by their diyne counterparts, including Jt-coordination, a-carbenoid formation, and a/jt metal complexation (Scheme 35). In some cases, these constructs are very stable and can be isolated, but they are frequently transient structures in which the metal ion acts as a catalyst for enyne cycloisomerisation and ligand transformation reactions. 

In the kiss-and-run mode exocytosis and endocytosis are directly coupled to each other, while in the case of classical complete vesicle fusion, exocytosis and slow clathrin-mediated endocytosis are timely and spatially separated. However, it appears that also in the latter case exocytosis and endocytosis occur coordinated, as both are stimulated by an increase of the cytoplasmic calcium concentration. It has been shown that after calcium entry the enzyme phospho-inositol-5 kinase Iy, which is enriched in the synapse, catalyzes the synthesis of phosphatidylinos-itol (4,5)-bisphosphate and that this mechanism is important for synaptic vesicle trafficking (Di Paolo et al. 2004). As many proteins involved in clathrin-mediated endocytosis are recruited to the plasma membrane by binding to phosphatidylinosi-tol (4,5)-bisphosphate (e.g., amphiphysin, dynamin, epsin, AP-180, and AP-2) it is attractive to speculate that elevated levels of calcium mediate the recruitment of en-docytic proteins to the plasma membrane by this mechanism. The increased level of phosphatidylinositol (4,5)-bisphosphate could be in part degraded by synaptojanin that thereby initiates the disassembly of the clathrin coat. Hence, calcium-induced transient increases in the level of phosphatidylinositol (4,5)-bisphosphate appear to play a central role for coupling exocytosis to clathrin-mediated endocytosis. In addition, it has been demonstrated that calcium also leads to the dephosphorylation of endocytic proteins as amphiphysin, dynamin, and synaptojanin, which in vitro is important for efficient coat assembly (Cousin and Robinson 2001). 

Ligands that selectively bind to G-quadruplex structures may modulate telom-erase activity, alter telomere structure or repress the transcription of key oncogenes (for a review, see this book s chapters by M. Searle and J.F. Riou). The way ligands interact with quadruplexes also give clues on the dynamics of the G-quartets with the possible exception of a porphyrin derivative, true intercalation (between two quartets) is considered unfavourable or impossible. This indicates that, contrary to DNA base pairs, transient unstacking/opening of two G-quartets is an extremely rare or short event, incompatible with the incorporation of a planar chromophore. End stacking, which does not require the separation of two quartets nor the release of cation is therefore the most frequently observed mode of interaction. ... 

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