Tricks that life uses to carry out specific Metabolic pathways

• Speed up reactions (Chemical kinetics): use catalyzers: Enzyme
• Make reactions happens. Sometimes, a reaction's energetics is unfavorable ($\Delta G > 0$). However, if the reaction is coupled with a very favourable reaction, the overall reaction can also be made favorable (happen spontaneously).
  • Direct coupling. Couple two reactions so that they occur simultaneously, by making the enzyme that makes one happen, also make the other happen. See here This is another way of saying that the favourable reaction runs the the other reaction. The fuel of the very favorable reaction is often ATP.
  • Indirect coupling. See video, we deplete the products of a reaction, and so keep the reaction from reaching equilibrium, and force it to keep running forward.

Energy sources

• ATP –> Phosphorylation –> ADP
• GTP –> GTP binding. Hydrolysis gives GDP