42. Arrange the metabolites in increasing order of phosphoryl transfer potential
p) ATP, q) Phosphoenolpyruvate, r) 1,3-Bisphosphoglycerate, s) pyrophosphate
a) s<q<r<p
b) q<r<s<p
c) s<p<r<q
d) p<s<r<q
The correct arrangement of the metabolites in increasing order of phosphoryl transfer potential is s, p, r, q (pyrophosphate, ATP, 1,3-bisphosphoglycerate, phosphoenolpyruvate). Phosphoryl transfer potential measures a compound’s tendency to donate its phosphoryl group to ADP, quantified by the standard free energy of hydrolysis (ΔG°’, more negative values indicate higher potential). This order reflects their ΔG°’ values: PPi (-19.2 kJ/mol), ATP (-30.5 kJ/mol), 1,3-BPG (-49.3 kJ/mol), and PEP (-61.9 kJ/mol).
Pyrophosphate (s)
Pyrophosphate (PPi), or s, exhibits the lowest phosphoryl transfer potential with ΔG°’ ≈ -19.2 kJ/mol for hydrolysis to two inorganic phosphates. This low energy arises from minimal resonance stabilization in products, making PPi a poor donor compared to others. In cells, PPi hydrolysis drives biosynthetic reactions forward but ranks lowest here.
ATP (p)
ATP, or p, holds moderate phosphoryl transfer potential with ΔG°’ ≈ -30.5 kJ/mol for ATP → ADP + Pi. Electrostatic repulsion between negatively charged phosphates and resonance in ADP/Pi contribute to this value, positioning ATP above PPi but below high-energy glycolysis intermediates. ATP serves as the universal energy currency, accepting phosphoryls from stronger donors like PEP.
1,3-Bisphosphoglycerate (r)
1,3-Bisphosphoglycerate (1,3-BPG), or r, shows higher potential with ΔG°’ ≈ -49.3 kJ/mol for hydrolysis at its acyl phosphate. The mixed anhydride structure provides instability, enabling transfer to ADP in glycolysis (step 7, via phosphoglycerate kinase). This places 1,3-BPG above ATP, reflecting its role in substrate-level phosphorylation.
Phosphoenolpyruvate (q)
Phosphoenolpyruvate (PEP), or q, has the highest phosphoryl transfer potential with ΔG°’ ≈ -61.9 kJ/mol, due to enol-keto tautomerization post-hydrolysis forming stable pyruvate. Dehydration from 2-phosphoglycerate elevates PEP’s energy, making it the top donor in bacterial phosphotransferase systems and glycolysis (step 10).
