Since adenine and guanine are also purines, methylxanthines such as, caffeine, can act as inhibitors of enzymes that use compounds containing adenine or guanine as substrates. A good example is an enzyme, which converts cyclic Adenosine monophosphate into non-cyclic Adenosine Monophosphate (AMP). When caffeine is present it inhibits the action of the enzyme by binding to its active site which is where the adenosine part of cyclic AMP. This is an example of competitive inhibition.
This effect of caffeine is important
because the cells of the human body produce cyclic AMP when activated
by Adrenaline (epinephrine) which indirectly activates an enzyme
called adenylate cyclase which converts ATP into cyclic AMP this
cyclic AMP then goes onto activate Protein Kinase A which activates
enzymes involved in metabolism and other signaling pathways.
The latter effect of PKA causes an increased responsiveness of the heart muscle cells to the calcium currents that control beating. So as can be seen from before when caffeine is present cyclic AMP remains active in the cell for longer, this is not a problem in normal cells but in cells such as the heart where cyclic AMP production kept at a high level as well as its deactivation a small change in the activity of one of the enzymes can cause a significant effect.
However there is no need for the reader
to stop drinking coffee as there are many other enzymes involved
that are not mentioned here that can help to compensate the effect
of caffeine over time the cells become tolerant to caffeine and
it takes more to produce the same effect.