Question

Within a cell, a GTP nucleotide is added onto the 3' end of an mRNA during transcription. In 1-2 sentences, briefly explain how the process of polynucleotide formation (i.e. making a polynucleotide) is an example of a coupled reaction:

a) Name/describe the two chemical reactions in the process above that are coupled

b) Explain what it means for these two reactions to be coupled.

Answer 1

Polynucleotide formation is an example of a coupled reaction because the process of phosphodiester bond formation and the the subsequent release of pyrophosphate (from the free nucleotides that had 3 phosphate group attached on them) after attaching each nucleotide onto the growing chain is not energetically highly favourable. In order to carry out this process, this polynucleotide formation must be coupled with another reaction which can provide enough energy for the ease of this reaction. Here the released pyrophosphate is hydrolysed to form two molecules of inorganic phosphate by the enzyme pyrophosphatase which helps in providing the extra energy for polynucleotide formation.

a) (dNMP)_n + dNTP ---------> (dNMP)_n+1 + PPi ΔG = - 3.5 kcal/mol

PPi + H2O -----------------> 2Pi   ΔG = -4 kcal/mol

Net reaction - (dNMP)_n + dNTP + H2O -------------> (dNMP)_n+1 + 2Pi   ΔG = -7.5 kcal/mol

b) In a coupled reaction energy required by one reaction to occur favourably is provided by another reaction. Here the hydrolysis of pyrophosphate providing energy for polynucleotide formation.