Group transmission potential

In biochemistry, the group transfer potential describes the ability to store chemical energy in bonds , often expressed as the standard free enthalpy change ΔG ^{0 ′} related to the physiological pH 7 . In contrast, the free standard enthalpy change ΔG ⁰ is given at pH 0.

meaning

The term group transfer potential is mostly used to describe the direction of energy flow. The standard enthalpy of hydrolysis is measured with water as the reference compound. The cleavage of ATP to ADP and inorganic phosphate (P _i ) yields a ΔG ^{0 ′} value of −30.5 kJ / mol. The value for glycerol-3-phosphate is lower:

${\displaystyle \mathrm {{\text{Glycerin-3-phosphat}}+H_{2}O\longrightarrow Glycerin+P_{i}\qquad \Delta G^{0^{\prime }}=-9{,}2\,kJ/mol} }$

${\displaystyle \mathrm {ATP+H_{2}O\longrightarrow ADP+P_{i}\qquad \Delta G^{0^{\prime }}=-30{,}5\,kJ/mol} }$

In general, positive values ​​of ΔG ^{0 ′} mean a lack of group transfer potential. Energy would first have to be used to transfer the group ( endergonic reaction). Negative values ​​of ΔG ^{0 ′} mean that the reaction takes place willingly ( exergonically ) and that energy is released during group transfer. The more negative the value of ΔG ^{0 ′} , the more energetic the bond, the higher the group transfer potential and the more likely the transfer will take place. In the above example it follows that ATP is more likely to be hydrolyzed than is the case with glycerol-3-phosphate.

When a phosphate group is transferred, it is called the "phosphorylation potential". But there are also high-energy Thiosesterbindungen with coenzyme A .

Living things consume energy in the form of ATP . This energy has to be replaced as it is used up. For the synthesis of ATP from ADP , phosphorylated metabolites with a high group transfer potential are required. On the other hand, ATP can phosphorylate metabolites such as glucose , as it has a higher group transfer potential than the resulting glucose phosphates.

Overview table

Group transfer potentials ΔG ^{0 ′} (at pH 7 under standard conditions)

reaction	ΔG 0 ′ in [kJ · mol −1 ]
Adenylyl sulfate → adenosine monophosphate (AMP) + SO 4 2−	−88.0
PEP → pyruvate + P i	−62.2
1,3-bisphosphoglycerate → 3-phosphoglycerate + P i	−49.6
Acetyl phosphate → acetate + P i	−44.0
Succinyl-CoA → succinate + CoA	−43.3
Creatine phosphate → creatine + P i	−43.3
ATP → ADP + P i	−35.7
Aminoacyl-tRNA → amino acid + tRNA	−35.0
ATP → ADP + P i (excess of Mg 2+ )	−30.5
PP i → 2 P i (in 5 mM Mg 2+ )	−33.6
ATP → AMP + PP i (excess of Mg 2+ )	−32.3
UDP-glucose → UDP + glucose	−31.9
Acetyl-CoA → acetate + CoA	−31.5
SAM → L - methionine + adenosine	−25.6
Glucose-1-phosphate → glucose + P i	−21.0
PP i → 2 P i	−19.0
Glucose-6-phosphate → glucose + P i	−14.0
Glutamine → glutamic acid + NH 4 +	−14.0
Glycerine-3-phosphate → glycerine + P i	−9.2
AMP → adenosine + P i	−9.2

P _i , inorganic phosphate ; PP _i , inorganic pyrophosphate .

This results among other things:

• Glucose phosphates (G-1P and G-6P) are thermodynamically more stable than ATP;
  • formally they would be easier to produce than ATP by reversing the hydrolysis reaction;
• In the presence of a suitable enzyme (glucokinase, hexokinase ), the phosphate residue could readily be transferred from ATP to glucose (Glc);
• only metabolites whose group transfer potential is higher than that of ATP are suitable for producing ATP, for example
  • Arginine phosphate (Arg-P); a reaction used in Crustaceans to gain ATP;
  • Creatine phosphate , which is used as an energy reserve for skeletal muscles and can quickly provide energy (ATP);
  • Succinyl-CoA, which is used (via succinyl phosphate) in the citric acid cycle for GTP gain;
  • 1,3-bisphosphoglycerate (1,3-BPG) and phosphoenolpyruvate (PEP), energy-rich metabolites that are used to gain ATP in glycolysis. For PEP, the equilibrium position follows from the following consideration:

${\displaystyle {\begin{array}{lcll}{\text{PEP}}&{\xrightarrow {-{\text{H}}_{2}{\text{O}}}}&{\text{Pyr}}+{\text{P}}_{i}&-60\,{\text{kJ}}/{\text{mol}}\\{\text{ADP}}+{\text{P}}_{i}&{\xrightarrow {{\text{H}}_{2}{\text{O}}}}&{\text{ATP}}&+35\,{\text{kJ}}/{\text{mol}}\\&&&\\{\text{PEP}}+{\text{ADP}}&\longrightarrow &{\text{Pyr}}+{\text{ATP}}&-25\,{\text{kJ}}/{\text{mol}}\end{array}}}$

• The coupling of a strongly exergonic reaction (conversion of phosphoenolpyruvate (PEP) into pyruvate ) with an endergonic reaction (ATP synthesis from ADP) results in an exergonic reaction that can be used to provide energy.

Under physiological conditions, the current concentrations influence the ΔG ^{0 ′} value. This results in deviations, as shown in the example of the hydrolysis of ATP or Mg-ATP (see table).

Individual evidence

1. ↑ Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Biochemistry. 6 edition. Spectrum Academic Publishing House, Heidelberg 2007; ISBN 978-3-8274-1800-5 ; P. 464.
2. ^ Reginald Garrett and Charles M. Grisham: Biochemistry . (International Student Edition). Cengage learning services; 4th edition 2009; ISBN 978-0-495-11464-2 ; P. 57.
3. ^ Peter Karlson, Detlef Doenecke, Jan Koolman, Georg Fuchs and Wolfgang Gerok: Karlsons Biochemie und Pathobiochemie . Thieme, Stuttgart; 15th, revised. u. remodel. Edition 2005; ISBN 978-3133578158 ; P. 83.
4. ↑ ^{a b} Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 320f.

See also

• Glycolysis
• Gluconeogenesis
• Citric acid cycle