Once you've finished gorging yourself on turkey and pie and have decided to burn off that extra "fuel" by shopping non-stop (as an aerobic workout) on Black Friday morning, remember that you have your cells' mitochondria to thank [1].
The mitochondria house the enzymes and conditions necessary for aerobic respiration via the citric acid cycle [1p761-2]. Just a couple of turns of the cycle and the food you eat can give you double the energy than what the cell can produce under anaerobic conditions of glycolysis and the pyruvate dehydrogenase complex [1p761-2].
But the citric acid cycle also does so much more. In the same oxygen-requiring breath of turns, the citric acid cycle makes up molecular precursors or "building blocks" needed by the body [2]. Like glycolysis this role gives the citric acid cycle amphibolic status that includues both catabolism of food and anabolism, or biosynthesis [1p770].
In fact, we owe the citric acid cycle thanks for biosynthesizing non-essential amino acids [1p770]. When the body runs low on one of these, the cell uses the carbon skeletons of citric acid cycle intermediates to produce them again [1p770]. Two intermediates used are:
- Oxaloacetate, in conjunction with glutamate, for the production of aspartate via a transamination reaction—where an enzyme catalyzes the transfer of an alpha-amino group to an alpha-ketoic acid [1p770].
- Alpha-ketoglutarate, in conjunction with ammonium and reduced by NADPH, produces glutamate, which is used to produce glutamine, proline and arginine [1p770].
These amino acids are then used for production of various essential proteins including enzymes and for building body tissues [1p596-7]. Other anabolic building blocks of the citric acid cycle include citrate for creating fatty acids and cholesterol, succinyl-CoA for heme, malate for producing pyruvate, and oxaloacetate for producing glucose [2].
Although the citric acid cycle is also found in bacteria [2], the mitochondria's efficient amphibolic role made its inclusion as part of eukaryotic cells an essential step in the evolution of multi-celled organisms [1p746]. Because mitochondria have their own mtDNA and grow and multiply like bacteria, they are thought to have descended of bacteria captured in eukaryotic cells millions of years ago [1p746].
Genetic research reveals the capture happened only once because all mitochondria can be traced back to one alpha-proteobacterial ancestor [3] and humans one Mitochondrial Eve [4]. As more is learned about that day of capture of our Mitochondrial Eve, we might also find it to be a true day for celebration and to express gratitude.
References
1. Denniston KJ, Topping JJ, Caret RL. General, Organic, And Biochemistry, 5th ed. New York: McGraw Hill; 2007.
2. Wiley. The citric acid cycle [animation]. Available at http://www.wiley.com/college/pratt/0471393878/student/animations/citric_acid_cycle/index.html. Accessed November 28, 2008.
3. Gray MW, Burger G, Lang BF. The origin and early evolution of mitochondria. Genome Biol. 2001; 2(6): reviews1018.1–reviews1018.5. Available at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=138944. Accessed on November 28, 2008.
4. Cann RL, Stoneking M, Wilson AC. Nature. 1987 January;325:31-36. Available at http://www.nature.com/nature/ancestor/pdf/325031.pdf. Accessed on November 28, 2008.
