Light-Coupled Transport Processes

Light-driven (electron, cation) symport occurs when combining this system with the (nickel complex, macrocycle) process described above [6.62]. Photocontrol of ion extraction and transport has been realized with macrocyclic or acyclic ligands (containing, for instance, azo or spirobenzopyran groups) that undergo a reversible 

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The widespread presence of nicotinic receptors in the central nervous system coupled with the relative rarity of a classical synaptic role means that it is still difficult to describe a clear physiological role for these receptors. This is particularly true for receptors in presynaptic locations, for which the level and temporal pattern of exposure to the neurotransmitter are unknown. Approaches that are casting light on this problem include mouse knockout models and transmitter depletion by blockers of vesicular transport processes such as vesamicol the combination of these techniques has recently shown that normal evoked dopamine release in striatal slices is strongly dependent on endogenous cholinergic mechanisms that involve the activation of j32-containing nicotinic receptors (240). 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

As an attempt to connect the first discussion, which was concerned with diffusion-reaction coupling, with Dr. Williams presentation of enzymes as dynamic systems, I wanted to direct attention to a number of specific systems. These are the energy-transducing proteins that couple scalar chemical reactions to vectorial flow processes. For example, I am thinking of active transport (Na-K ATPase), muscular contraction (actomyosin ATPase), and the light-driven proton pump of the well-known purple... 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

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