Highenergy Phosphates Play A Central Role In Energy Capture And Transfer

In order to maintain living processes, all organisms must obtain supplies of free energy from their environment. Autotrophic organisms utilize simple exergonic processes; eg, the energy of sunlight (green plants), the reaction Fe2+ ^ Fe3+ (some bacteria). On the other hand, heterotrophic organisms obtain free energy by coupling their metabolism to the breakdown of complex organic molecules in their environment. In all these organisms, ATP plays a central role in the transference of free energy from the exergonic to the ender-gonic processes (Figure 10-3). ATP is a nucleoside triphosphate containing adenine, ribose, and three phosphate groups. In its reactions in the cell, it functions as the Mg2+ complex (Figure 10-4).

The importance of phosphates in intermediary metabolism became evident with the discovery of the role of ATP, adenosine diphosphate (ADP), and inorganic phosphate (P;) in glycolysis (Chapter 17).

CHI CHI

Figure 10-4. Adenosine triphosphate (ATP) shown as the magnesium complex. ADP forms a similar complex with Mg2+.

CHI CHI

Figure 10-4. Adenosine triphosphate (ATP) shown as the magnesium complex. ADP forms a similar complex with Mg2+.

The Intermediate Value for the Free Energy of Hydrolysis of ATP Has Important Bioenergetic Significance

The standard free energy of hydrolysis of a number of biochemically important phosphates is shown in Table 10-1. An estimate of the comparative tendency of each of the phosphate groups to transfer to a suitable acceptor may be obtained from the AG0 of hydrolysis at 37 °C. The value for the hydrolysis of the terminal

Table 10-1. Standard free energy of hydrolysis of some organophosphates of biochemical importance.1,2

AG0'

Table 10-1. Standard free energy of hydrolysis of some organophosphates of biochemical importance.1,2

Compound

kJ/mol

kcal/mol

Phosphoenolpyruvate

-61.9

-14.8

Carbamoyl phosphate

-51.4

-12.3

1,3-Bisphosphoglycerate

-49.3

-11.8

(to 3-phosphoglycerate)

Creatine phosphate

-43.1

-10.3

ATP ^ ADP + Pi

-30.5

-7.3

ADP ^ AMP + Pi

-27.6

-6.6

Pyrophosphate

-27.6

-6.6

Glucose 1-phosphate

-20.9

-5.0

Fructose 6-phosphate

-15.9

-3.8

AMP

-14.2

-3.4

Glucose 6-phosphate

-13.8

-3.3

Glycerol 3-phosphate

-9.2

-2.2

1Pi, inorganic orthophosphate.

2Values for ATP and most others taken from Krebs and Kornberg (1957). They differ between investigators depending on the precise conditions under which the measurements are made.

1Pi, inorganic orthophosphate.

2Values for ATP and most others taken from Krebs and Kornberg (1957). They differ between investigators depending on the precise conditions under which the measurements are made.

phosphate of ATP divides the list into two groups. Low-energy phosphates, exemplified by the ester phosphates found in the intermediates of glycolysis, have AG0 values smaller than that of ATP, while in high-energy phosphates the value is higher than that of ATP. The components of this latter group, including ATP, are usually anhydrides (eg, the 1-phosphate of 1,3-bisphosphoglycerate), enolphosphates (eg, phos-phoenolpyruvate), and phosphoguanidines (eg, creatine phosphate, arginine phosphate). The intermediate position of ATP allows it to play an important role in energy transfer. The high free energy change on hydrolysis of ATP is due to relief of charge repulsion of adjacent negatively charged oxygen atoms and to stabilization of the reaction products, especially phosphate, as resonance hybrids. Other "high-energy compounds" are thiol esters involving coenzyme A (eg, acetyl-CoA), acyl carrier protein, amino acid esters involved in protein synthesis, S-adenosylmethionine (active methionine), UDPGlc (uridine diphosphate glucose), and PRPP (5-phosphoribosyl-1-pyrophosphate).

High-Energy Phosphates Are Designated by ~ ©

The symbol ~© indicates that the group attached to the bond, on transfer to an appropriate acceptor, results in transfer of the larger quantity of free energy. For this reason, the term group transfer potential is preferred by some to "high-energy bond." Thus, ATP contains two high-energy phosphate groups and ADP contains one, whereas the phosphate in AMP (adenosine monophosphate) is of the low-energy type, since it is a normal ester link (Figure 10-5).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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