The body requires glucose for much of its energy supply. It is an essential nutrient during rapid burst, short duration energy as well as slower, sustained activities. So how is it utilized?
If you’ll recall (See Insulin: Friend or Foe?), muscle requires insulin to express the GLUT-4 transporter which is how glucose gets into the cell (myocyte). In order for it to stay in the cell, it must be converted to glucose-6-phosphate. Glycolysis is the process of breaking down glucose into energy (see ATP is the Energy Currency for the Cell). Glucose goes through glycolysis as energy demands necessitate. Glucose is not immediately broken all the way down to ATP as soon as it gets into the cell. The enzymes for glycolysis are free in the cytoplasm. Therefore, glycolysis occurs in the cytoplasm of the cell.
The end result of glycolysis is:
- 2 pyruvate
- 2 ATP (net gain)
- 2 NADH + H+
- 2 H2O (however, this is insignificant in understanding glucose metabolism)
Obviously, the 2 ATP can be used immediately. But what about the rest of this stuff?
Let’s start with the NADH + H+. It can follow one of 2 fates: the first occurs when there is adequate oxygen, such as during aerobic exercise or at rest. In these cases, NADH + H+ is transported to the mitochondria (so to speak) where O2 will serve as the electron acceptor. However, when O2 is deficient, such as during hard exercise (anaerobic), there is not enough oxygen to accept all of the electrons being carried by NADH due to glycolysis and the Kreb’s cycle. In this case (this is the second possible fate of NADH), pyruvate can serve as a temporary reservoir for these electrons and is reduced to lactate (lactic acid). This is how lactic acid is produced during anaerobic exercise.
This is what I call my Dump Truck Analogy to help explain this process:
- Think of NAD+ as being a dump-truck that functions in taking trash from your front yard to the dump.
- The trash, in this analogy, is the hydrogens that are removed from the glucose molecule during glycolysis.
- Each molecule of glucose produces 4 hydrogens but only 2 electrons (I think).
- The city has a limited supply of dump-trucks so it is possible that they get backed up from time to time. This occurs anytime you increase production of trash or decrease the number of trucks allowed into the dump.
- In this case, one of two things must happen. One, you have to shut down production of trash (glycolysis stops or slows down and you can’t get enough energy) or, two, the NAD+ finds a way to dump its trash somewhere else.
- In the first case, you don’t have the energy production to meet the demand and performance suffers.
- The second case is possible using pyruvate as a temporary “dump” for the trash (hydrogens).
- The formation of lactic acid frees up NAD+ and enables energy production to continue.
- The lactic acid must be dealt with at some later point or in another location, such as relatively inactive muscle or the liver.
- The formation of lactic acid contributes to what is termed “oxygen debt.”
Now that we have NADH out of the way, we can discuss pyruvate which has a couple of possible fates. The first is that it is converted to acetyl CoA and enters the Kreb’s cycle. The second fate has already been discussed above.