Exergonic, reaction

Reactions are classified as exergonic or endergonic according to the sign of AG°. An exergonic reaction is one in which AG° is negative, an endergonic reaction has a positive value of AG°. 

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate (Eigure 23.7), like all phosphate ester hydrolyses, is a thermodynamically favorable (exergonic) reaction under standard-state conditions (AG° = —16.7 kj/mol). Under physiological conditions in the liver, the reaction is also exergonic (AG = —8.6 kJ/mol). Fructose-1,6-bisphosphatase is an allosterically regulated enzyme. Citrate stimulates bisphosphatase activity, hut fructose-2,6-bisphosphate is a potent allosteric inhibitor. / MP also inhibits the bisphosphatase the inhibition by / MP is enhanced by fructose-2,6-bisphosphate. 

Active Figure 5.6 Some hypothetical energy diagrams (a) a fast exergonic reaction (small AG, negative AG°) ... 

Draw an energy diagram for a two-step exergonic reaction whose second step is faster than its first step. 

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Hammond postulate (Section 6.10) A postulate stating that we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species. Exergonic reactions have transition states that resemble reactant endergonic reactions have transition states that resemble product. 

Ethynylestradiol, structure and function of, 1083 von Euler, Ulf Svante, 1068 Exergonic reaction, 153 Hammond postulate and,... 

To take place, the reaction must be coupled with another—more exergonic—reaction such as the hydrolysis of the terminal phosphate of ATP. 

When (1) and (2) are coupled in a reaction catalyzed by hexokinase, phosphorylation of glucose readily proceeds in a highly exergonic reaction that under physiologic conditions is irreversible. Many activation reactions follow this pattern. 

The initial reaction between acetyl-CoA and oxaloacetate to form citrate is catalyzed by citrate synthase which forms a carbon-carbon bond between the methyl carbon of acetyl-CoA and the carbonyl carbon of oxaloacetate. The thioester bond of the resultant citryl-CoA is hydrolyzed, releasing citrate and CoASH—an exergonic reaction. 

For the emission of visible light, the necessary energy ranges from 40 kcal/mole (red light) to 70 kcal (violet light). Thus, only rather exergonic reactions can be chemiluminescent in the visible range of the spectrum. 

If the emitting species is not a reaction-product molecule directly formed by the exergonic reaction (as is the case in the luminol reaction, for example), chemiluminescence can occur via energy transfer processes ... 

CL is the emission of light by molecules that are excited by participation in a highly exergonic reaction, usually an oxidation. It can also be defined as the fluorescence of the electronically excited product of a chemical reaction however, unlike fluorescence, no external light source is used. 

BL has independently evolved many times some 30 different independent systems are still existent [7], Thus the responsible genes are unrelated in the various organisms, the enzymes show no homology to each other, and the substrates are chemically unrelated. There is, however, one common thread tying different systems at molecular level. All involve exergonic reactions of molecular... 

Fast spectroscopy was also used to probe the reactivity of PBN +. The 266 nm laser excitation of peroxydisulfate ion in aqueous solution at room temperature gives the powerful oxidant SOr, which oxidizes PBN in an exergonic reaction (by about 0.8 eV, see Tables 1 and 5) with k = 3 X 109 dm3 mol-1 s 1. The pseudo-first-order rate constant for the decay of PBN + by reaction with water to give HO-PBN" was 2 x 106 s 1, a relatively slow reaction (k = 3.6 x 104 dm3 mol-1 s-1 at ambient temperature). 

Figure 2.1 Reaction progress graph exergonic reaction...

Not all endergonic reactions will be so conveniently placed adjacent to an exergonic reaction. In such a situation, the required energy is usually supplied by ATP ... 

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