Cific interaction

Gels usually consist of small amount of polymer as a network and a lai amount of solvent. Therefore when we discuss the dynamics erf polymer gels, we are tempted to deal with these Is from the stand point of the dynamics of polymer solutions. However, since the polymer chains in a gel are connected to each other via chemical bonds and/or some kinds of sj cific interaction, sudi as, hydrogen bonding or hydrophobic interaction, the gel has to be treated as a continuum. In addition, gels behave as an assembly of springs due to the entropy elasticity of polymer chains between the crosslink points. Therdbre, the dynamics of polymer gels is well described in terms of the theory of elasticity... 

Activation of the chemokine receptor leads to rapid activation of phosphoinositide-spe-cific phospholipases, which leads to inositol-1,4,5-triphosphate formation and a transient rise in intracellular calcium (18). Phospholipase C (PLC) isoforms that are involved in chemokine receptor activation become activated by direct interaction with the J3y subunits. In addition to its interaction with PLCs, the j3y subunits also interact with the type phosphoinositol 3 kinase y (PISKy), and activation of this enzyme results in the formation of PtdIns(3,4,5)P3 (17). Mice that do not express PI3Ky have severely impaired chemo-kine-stimulated signal transduction, and PKB is not activated, suggesting an important role for this pathway in the chemotactic process (27-29). Although leukocytes isolated from these mice showed a decrease in cell chemo-taxis, the response is not completely lost, and under conditions of complete PI3Ky inhibition, neutrophils can still chemotax in response to chemokines (30). 

The structural difference between the active GTP-state and the inactive GDP-state of the Ras protein is primarily confined to the switch I and switch II regions. The conformational change can be described best by a loaded spring mechanism (see Fig. 5.22), where the two switch regions are fixed by the y-phosphate of GTP in a tense state. Upon GTP-hydrolysis, the two switch regions are allowed to relax into the GDP-spe-cific position. Thereby, the coordination of switch I to the y-phosphate and to Mg2 is lost as well as the interaction of the conserved Gly60 in switch II with the y-phosphate. 

One particularly effective way nature uses to reduce activation energies and other molecular requirements is by the intensive use of intramolecular reactions. In this case the interacting species are held together on the same molecular template at a very short distance, thus paving the way to regio- and stereospe-cific reactions. 

The shocks observed after intravenous administration of plasma expanders such as dextran (Furhoff 1977) also appear to rest upon the presence of IgG-spe-cific antibodies (see Chap. 20). Whether the effect of anaphylatoxins such as C3a or C5a on the blood basophils and tissue mast cells occur following interaction of the drug with specific IgGs in solution or only after such IgGs have fixed to the mast cell membrane is not yet certain. 

Intraspecific chemical signaling could be involved in a wide range of interactions where marine and aquatic animals search for mates, avoid competing conspe-cifics, need to synchronize release of gametes or larvae, or identify near versus... 

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