Six standard-state reactions, measured at 298 K, are listed below. Asterisks denote steps that are known to be enzyme-catalyzed and therefore proceed rapidly in vivo.
Label
Reaction
ΔG° (kJ mol⁻¹)
| (1) P ⇌ Q | +12 |
| (2)* Q + R ⇌ S | –45 |
| (3) S ⇌ T | +18 |
| (4) R ⇌ P | –5 |
| (5)* T ⇌ U | –22 |
| (6) Q ⇌ R | +9 |
A researcher wishes to convert the starting mixture P + R into product U such that
(a) Devise a multistep reaction pathway that meets both criteria. List explicitly each selected reaction and, if necessary, indicate any required multiplication of a reaction by an integer coefficient.
(b) Add together the reactions chosen in part (a), canceling all intermediates to obtain the overall stoichiometric equation from P + R to U.
(c) Compute the overall ΔG° for your pathway and verify that it satisfies the researcher’s thermodynamic requirement. Show all numerical steps.
(d) Suppose an alternative pathway with no enzyme-catalyzed steps could achieve the same overall ΔG°. Provide two kinetic arguments for why including at least one enzyme-catalyzed step is still advantageous in a biological system.