Question

1. What makes “high-energy bonds” “high-energy”? Give some examples of phosphate groups with a high group-transfer potential and explain how they differ from phosphate groups with a low group-transfer potential. Why do thioester bonds have a high group-transfer potential?

Answer 1

High energy bonds : Usually inorganic phosphates makes high energy bonds (usually have high group transfer potential).

Phosphoryl group transfer potential : It is the measure of tendency of a compound to transfer phosphate to water.

They are many phosphate groups :

a) Phosphate anhydride bonds : It is the bond between 2 phosphate groups

ex : ATP, NTP where N is nucleoside

NTP + H₂O --------> NDP + P_i   G^o = -31 kJ/mol

NDP + H₂O --------> NMP + P_i   G^o = -31 kJ/mol

NMP + H₂O --------> N + P_i   G^o = -14 kJ/mol

b) Mixed anhydrides or carboxylate-phosphate anhydrides : Phosphate group is attached to carboxylic acid group.

ex : R-COO-PO₃-R'

ex : 1,3 biphosphoglycerate (1,3 BPG) + H₂O --------> 3-Phosphoglycerate + P_i   G^o = -49 kJ/mol

c) Enoyl phosphate bond : The keto group undergo taut-isomerism form is called enol form.

It is formed between enol form with the phosphate. Hydrolysis of this bond have high negative energy in free energy.

ex : Phosphoenolpyruvate (PEP) + H₂O --------> pyruvate + + P_i   G^o = -60 kJ/mol

d) Thioester bond : The bond is between carboxylic acid and sulfhydryl group (R-C=O-S-R').

ex : CH₃ - C=O-S-CoA (Ac-SCoA) + H₂O -------->CH₃COOH + CoA-SH   G^o = -30 kJ/mol

  Acetyl coenzyme Acetic acid

Thioesters are less stable than oxygen esters which is due to unshared electrons of sulphur (S) is not effectively delocalized.

ATP is an intermediate phosphor group potential and it is kinetically stable that mediates most energy transfer.

From the above high energy group potential or high energy bond includes Phosphoenolpyruvate (PEP), phosphocreatine etc. These are used to prepare ATP molecules.

low energy group potential or low energy bond includes glucose-6-phosphate, glycerol-3-phosphate etc

Usually high group potential (high energy bond groups) can accept phosphate which are above them and donate its phosphate group to the below compounds i.e low group potential (low energy bond groups)

Difference between phosphoryl low and high group potential is that high energy bonds have delta G free energy of less than

-25 kJ/mol i.e. equal to or greater than that of ATP molecules

Low energy group potential is that low energy bonds have delta G free energy of more than - 25 kJ/mol i.e less than that of ATP molecules.