Caffeine binds to adenosine receptor

Caffeine contains structurally two ring systems which are also called purines. One of the purine ring systems is adenosine. Caffeine binds to receptors that normally bind adenosine, i.e. adenosine receptors. There are several types of adenosine receptors, and the many functions of these receptores is not yet fully understood. Activation of adenosine receptors is known to activate the conversion of ATP to cAMP. cAMP is an intracellular signaling molecule that, activates cAMP kinase. This kinase opens a K+ channel in the membrane, and this leads to K+ leaking out of the cell. Thereby, the cell becomes hyperpolarized so nervous activity is inhibited. Adenosine belongs therefore to a system that inhibits nerve signals.

Caffeine is a receptor antagonist for adenosine receptors. The substance inhibits all the signals of adenosine, and hence the conversion of ATP to cAMP. Therefore, it also counteracts cAMP's inhibition of nerve signals in the brain. Caffeine creates increased activity, and also affects other neurotransmitters, increasing dopamine and serotonin levels. These two neurotransmitters are linked to several of the effects we have seen from caffeine, including on stamina, fatigue, relaxation and dementia.

Figure: Caffeine located in the binding pocket of the adenosine A2A receptor.