= ATP-driven sugar phosphorylation
{c}
<Hexokinase> provides a concrete connection between <sugar> chemistry and <energy coupling>: it transfers a terminal <phosphate> group from <ATP> to <glucose>, forming a phosphorylated <sugar> and <adenosine diphosphate>. The <enzyme> accelerates the transfer, while consuming <ATP> changes the net <chemical reaction> being carried out. The donated group reaches the <substrate> through the catalytic route.<Biology 2e by Mary Ann Clark and others>[§7.2, First Half of Glycolysis, step 1: hexokinase and ATP-dependent glucose phosphorylation]{ref}
For a symbolic teaching model, write $S$ for an unphosphorylated <sugar>, $S\text{-}P$ for its phosphorylated form, and $P_i$ for <inorganic phosphate>. These are labels for chemical states, not complete elemental formulas. Using one consistent reference convention at fixed <pH> and <temperature>, stipulate the following molar changes in <Gibbs free energy>:
$$
\begin{aligned}
S+P_i&\longrightarrow S\text{-}P+\mathrm{H_2O},
&\Delta G_1^{\circ\prime}&=+12\,\mathrm{kJ\,mol^{-1}},\\
\mathrm{ATP}+\mathrm{H_2O}&\longrightarrow\mathrm{ADP}+P_i,
&\Delta G_2^{\circ\prime}&=-32\,\mathrm{kJ\,mol^{-1}},\\
S+\mathrm{ATP}&\longrightarrow S\text{-}P+\mathrm{ADP},
&\Delta G_{\mathrm{net}}^{\circ\prime}&=-20\,\mathrm{kJ\,mol^{-1}}.
\end{aligned}
$$
All three numbers are chosen illustrative values, not measured properties of <glucose> or a particular <enzyme>. The prime marks a biochemical reference convention with <pH> held fixed; explicit proton and metal-binding bookkeeping is suppressed consistently. The first two lines are a thermodynamic decomposition: cancelling <water> and <inorganic phosphate> gives the net transfer. They do not assert that the <reaction mechanism> first releases free <inorganic phosphate> and then recaptures it.
\Image[/biochemical-atp-phosphate-transfer.png]
{title=<ATP>-driven <phosphorylation> connects group transfer to a favourable net change in <Gibbs free energy>}
{description=Original symbolic reaction diagram, with a schematic catalytic cycle and a separate thermodynamic sum. $S$ and $S\text{-}P$ label hypothetical <sugar> states. The cycle groups many possible microscopic steps and draws no molecular structure. The chosen $+12-32=-20\,\mathrm{kJ\,mol^{-1}}$ refers to one consistent reference convention; it is not a measured energy profile or an <activation energy> barrier.}
Because <Gibbs free energy> is a <state function>, consistently defined reaction changes add. A working <reaction mechanism> must also connect the desired conversion to <ATP> consumption; independently allowing <ATP> <hydrolysis> beside the <sugar> does not establish <energy coupling>. The general distinction is developed under <energy coupling>. <ATP>-dependent <phosphorylation> of a membrane <protein> can likewise connect a chemical change to <active transport>.<Biology 2e by Mary Ann Clark and others>[§6.4, Energy Coupling, Figure 6.14: transfer to the sodium-potassium pump and its conformational cycle]{ref}
Actual <Gibbs free energy> changes depend on <thermodynamic activities>. Even a favourable reference sum can be overcome by accumulation of <reaction products> or depletion of <ATP>; it supplies no prediction of the <reaction rate>. A living <biological cell> also regenerates <ATP>. Sustained operation therefore requires a connected <metabolism>, not an inexhaustible stock of a special <molecule>.
Back to article page
