All organisms, from us to bacteria, need to adapt to survive. As I start to write this post, I have just had lunch and so I guess my blood sugar is probably peaking nicely. Some of this sugar will need to stored for later consumption, and doing this will involve flipping switches in my biochemistry to go from my pre-lunch state of burning food reserves to storing excess calories from my lunch.
Bacteria are the same. E. coli loves to burn the sugar glucose, but if there is no glucose and it is hungry, it will switch to burning another sugar called lactose, if it is available. So, how does an E. coli cell know it is hungry? An important sign is that the concentration of a molecule cAMP builds up, concentration of this molecule in well fed E. coli is low.
E. coli measures the concentration of cAMP via a protein called CRP. CRP molecules bind to cAMP molecules, a pair of CRP molecules are shown above, one in blue and one in purple. The much much smaller cAMP molecules are shown side on in orange. Despite the cAMP molecules being dwarfed by the much larger CRP molecules, binding to cAMP causes the flexible CRP molecules to change shape.
This has profound consequences, as the shape-shifted CAP molecules can now bind to particular E. coli genes (which they couldn’t before). The green double helix above is the DNA at the start of a gene. Two molecules of CRP molecule have bound to the DNA molecule.
This binding switches on genes, making the protein molecules E. coli needs to metabolise lactose (and other molecules) and so become less hungry. We have thousands of such switches made of proteins shifting shape in response to sugars, hormones, proteins and pretty much everything else floating around in our cells. These are constantly adjusting the biochemistry that runs your cells, and with all the food and drink we’ll be consuming over Christmas, it will probably need adjusting.