Energy-linked processes

Uncouplers and Energy-linked Processes 1025 Table 18-5 Some Artificial Electron Acceptors... 

Synthesis of ATP by mitochondria is inhibited by oligomycin, which binds to the OSCP subunit of ATP synthase. On the other hand, there are processes that require energy from electron transport and that are not inhibited by oligomycin. These energy-linked processes include the transport of many ions across the mitochondrial membrane (Section E) and reverse electron flow from succinate to NAD+ (Section C,2). Dinitrophenol and many other uncouplers block the reactions, but oligomycin has no effect. This fact can be rationalized by the Mitchell hypothesis if we assume that Ap can drive these processes. 

Another energy-linked process is the transhydro-genase reaction by which NADH reduces NADP+ to form NADPH. In the cytoplasm various other reactions are used to generate NADPH (Chapter 17, Section I), but within mitochondria a membrane-bound transhydrogenase has this function.283-2863 It couples the transhydrogenation reaction to the transport of one (or possibly more than one) proton back into the mitochondria (Eq. 18-14). A value of Ap of -180 mV could increase the ratio of [NADPH] / [NADP+] within mitochondria by a factor of as much as 1000. 

However, It must be energetically more advantageous for a cell to be able to make use of the LQ liberated during PPi hydrolysis in the form of a proton motive force driving other energy-linked processes than it Is to waste the energy as heat. 

Emulsifiers (see also Surfactants) 27, 46 End-blockers 12, 18, 19, 76 End-capping 157 End-linking processes 163 End-stoppers (see also End-blockers) 10 End-to-end cyclization 159,160 Energy, cavitation 188,189 —, electrostatic interactions 188, 189... 

The thyroid is able to concentrate T against a strong electrochemical gradient. This is an energy-dependent process and is linked to the Na -K ATPase-dependent thyroidal T transporter. The ratio of iodide in thyroid to iodide in serum (T S ratio) is a reflection of the activity of this transporter. This activity is primarily controlled by TSH and ranges from 500 1 in animals chronically stimulated with TSH to 5 1 or less in hy-pophysectomized animals (no TSH). The T S ratio in humans on a normal iodine diet is about 25 1. 

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

A3. The end-to-end distance of the free strands is practically unchanged by the cross-linking process, i.e., the extent of chemical modification of the polymer chains produced by high energy radiation should be small. 

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