Throughout the day, we are normally consuming foods that have lots and lots of carbohydrates. Normally, it is extremely important for our body to maintain a constant blood glucose concentration of around 100 mg/dL. Low blood sugar can cause autonomic disturbances and high blood sugar can cause damages to our nerves, retina, kidney, and other vital organs in our body. Therefore, in order to keep blood sugar at a constant level, our body has to now find ways on what to do with all of the glucose that is in our system!
Carbohydrates are organic molecules that are composed of carbon, oxygen, and hydrogen atoms. They are made up of simple sugars like glucose and fructose, as well as complex sugars such as starch and glycogen. During digestion, carbohydrates are broken down into simple sugars that can then be transported into the circulatory system to be transported to tissues throughout the body. Once we eat food, an enzyme from our saliva called salivary amylase starts breaking up complex sugars into monosaccharides so that it can be absorbed effectively into our system. Once the absorbed monosaccharides are transported by glucose transporters (GLUT 1-4), glucose is oxidized to release the energy stored in its bonds to produce ATP (see figure 1).
This entire process is called glycolysis. First, glucose enters our cells by either facilitated diffusion or active transport and is converted into glucose-6-phosphate with the help of either hexokinase, which can be found in the peripheral tissues or glucokinase, which is found only in liver and pancreatic B-islet cells. Glucose is now effectively trapped inside the cell for two specific reasons: 1) Glucose transporters are only specific for glucose. Therefore, because the sugar molecule has already been phosphorylated, it now has a negative charge and cannot diffuse through the membrane; 2) the phosporylation of glucose destabilizes the structure and further facilitates its metabolism.
Once isomerase converts glucose-6-phosphate to fructose-6-phosphate, it is then phosphorylated to fructose 2,6-biphosphate with the help of phosphofructokinase-1 (PFK-2). PFK-1 then converts the substrate to fructose 1, 6-biphosphate. PFK-1 is an enzyme of interest because it acts as a rate-limiting enzyme for glycolysis. When there sufficient energy (high ATP) or citrate (an intermediate of the citric acid cycle), PFK-1 is inhibited so that the cell can turn off glycolysis. On the other hand, when there's a high level of AMP, this means that glycolysis needs to be turned on because there's not enough sufficient energy.
Aldolase then split fructose-1,6-bisphosphate into two 3-carbon molecules: dihyroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). Since both DHAP and GAP are isomers of each other, triose-phosphate isomerase can readily inter-convert these two substrates. Glyceraldehyde-3-phosphate dehydrogenase then catalyze the oxidation to convert GAP to 1,3-biphosphoglycerate. Phosphoglycerate kinase then help convert 1,3-biphosphoglycerate to 3-phosphoglycerate. Then after mutase and enloase convert the substrates to phosphoenolpyruvate (PEP), pyruvate kinase use ATP to convert it to 2 molecules of pyruvate.
I'm Linh - a science geek who loves experimenting and tinkering with recipes! I hope that this blog brings more ideas into your kitchens! Happy eating folks! XOXO