7.10 Other Uses for Nucleobases
Besides coding for genes, the five nucleobases and their ribose complexes also have functions in the body related to energy. The nucleosides are combinations of nucleobases with either ribose or deoxyribose. There are ten of them, so I will just mention adenosine as it is an important one. Nucleotides are like nucleosides but with a phosphate group added. There are also ten of those, including adenosine monophosphate. Polymers of nucleotides are what make up the individual strands of DNA and RNA.
Finally, there are also diphosphate and triphosphate versions of the nucleosides, including adenosine triphosphate or ATP.
|
adenosine |
adenosine monophosphate (AMP) |
adenosine triphosphate (ATP) |
cyclic adenosine monophosphate (cAMP) |
cAMP is a molecule involved in signalling within the cell. ATP has a great many functions in the body, and is heavily involved with metabolism. It supplies energy to cells and assists in propagating signals through nerves. ATP causes muscles to contract by means of the way it interacts with the muscle proteins actin and myosin; the protein fibers are arranged side by side throughout the muscle cell, which itself is very very long and slender. ATP causes the two protein strands to slide past each other, making the cell contract; when all the cells in a muscle do this, the entire muscle contracts. As I type this, ATP signals are being sent through my nerves to the muscles that move my fingers just the right way to spell out all the words (typos and all), and as you read this, ATP signals are being sent to the muscles around your eyes to move your gaze across the words. Pretty cool!
ATP is also the signal that initiates luminescence in fireflies, though the mechanism isn't fully understood.
The energy that ATP carries is in its phosphate-phosphate bonds. When this energy is used, the phosphate groups are hydrolyzed away, one by one, until only adenosine remains. Adenosine levels build up in the body during waking hours, correlating with an increased desire for sleep; while sleeping, the opposite happens, and the body's adenosine is used for making fresh ATP.
There are receptors in the body for adenosine; some of them, including one called ADORA2A, are inhibited by caffeine. As you can see from the table below, caffeine and adenosine have similarities in their molecular structures. That enables caffeine to bind to the receptor but not to activate it, and the inhibition of specifically ADORA2A appears to be the mechanism by which caffeine decreases sleepiness.
|
adenosine |
caffeine |