含义Meaning
不利反应（\(\Delta G > 0\)）通过共同中间体与 ATP 水解偶联，使总自由能变为负。细胞中 ATP 水解实际 \(\Delta G\) 约 −50 至 −60 kJ/mol。
An unfavourable step is driven by coupling to ATP hydrolysis through a shared intermediate, making the total ΔG negative. In cells ATP hydrolysis delivers about −50 to −60 kJ/mol.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(\Delta G_{\mathrm{ATP}}\) | ATP 水解自由能free energy of ATP hydrolysis | kJ·mol⁻¹ |
推导Derivation
自由能是状态函数，两反应相加时 ΔG 相加。
Free energy is a state function, so coupled reactions add their ΔG.
偶联要靠共同中间体：如谷氨酰胺合成 Glu + ATP → γ-谷氨酰磷酸 + ADP，再 + NH₃ → Gln + Pᵢ。
Coupling needs a shared intermediate: Glu + ATP → γ-glutamyl phosphate + ADP, then + NH₃ → Gln + Pᵢ.
实际 \(\Delta G = \Delta G^{\circ\prime} + RT\ln\frac{[\mathrm{ADP}][\mathrm{P_i}]}{[\mathrm{ATP}]}\)；细胞中 [ATP]/[ADP] 远高于平衡值，所以更负。
Actual \(\Delta G = \Delta G^{\circ\prime} + RT\ln([\mathrm{ADP}][\mathrm{P_i}]/[\mathrm{ATP}])\); cells keep [ATP]/[ADP] far from equilibrium.
适用条件与局限Assumptions & limitations
热力学可行不代表反应发生——需要酶提供偶联机理。
Thermodynamic feasibility needs an enzyme mechanism to realise coupling.
例题Worked example
葡萄糖 + Pᵢ → 葡萄糖-6-磷酸 ΔG°′ = +13.8 kJ/mol；与 ATP 水解偶联后 ΔG°′ = −16.7 kJ/mol（己糖激酶）。
Glucose + Pᵢ → G6P is +13.8 kJ/mol; coupled to ATP (hexokinase) it is −16.7 kJ/mol.