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This tutorial presents the pyruvate dehydrogenase complex and Kreb’s cycle at a level appropriate for most undergraduate biology classes and the MCAT exam.

Click to view the video tutorial: The Pyruvate Dehydrogenase Complex and Kreb’s Cycle.

During aerobic respiration the pyruvate produced by glycolysis moves from the cytoplasm to the mitochondrial matrix. Within the matrix, the pyruvate dehydrogenase enzyme complex converts pyruvate into an acetyl group with the release of energy. The acetyl group enters the Kreb’s cycle, a catabolic cycle of enzyme mediated reactions which transfer the remaining chemical energy from glucose to energy carrying molecules.

QUIZ ANSWERS
1. The first enzyme function of the pryruvate dehydrogenase complex removes a carbon and two oxygen from pyruvate. These come from pyruvate’s carboxyl group (-COO-) which is already fully oxidized and contains little chemical energy. The waste is given off as a molecule of CO2. This is the first step at which CO2 is released during aerobic respiration.

2. The Kreb’s cycle is also called the citric acid cycle and the tricarboxylic acid (TCA) cycle . The first name for this cycle honours Hans Kreb, a British-German biochemist who won a Nobel Prize for his work uncovering the pathway. Citrate is the first intermediate produced in the cycle, this is where the name critic acid cycle comes from. Citrate is produced when Acetyl CoA transfers it’s acetyl group to oxaloacetate. Citrate has three carboxylic acid functional groups, so it is also called it tricarboxylic acid, and this is where the name tricarboxylic acid cycle comes from.

3. OOA is an abbreviation for oxaloacetate, the molecule which reacts with Acetyl CoA producing citrate in the first step of the Kreb’s Cycle. MCAT students should know this molecule and it’s abbreviation for their exam.

4. Recall that the net production of energy carrying molecules from the Kreb’s cycle per pyruvate molecule is 3 NADH, 1 FADH2, 1 ATP and some GTP because it’s not quite one per cycle. Since glycolysis splits a single glucose molecule into two pyruvate molecules the net production of energy carrying molecules from the Kreb’s cycle per glucose molecule is 6 NADH, 2 FADH2, 2 ATP and some GTP.

5. During the preparatory conversion of pyruvate to Acetyl-CoA one molecule of NADH is produced per pyruvate molecule. Since glycolsis splits a single glucose molecule into two pyruvate molecules the net production of energy carrying molecules from the pyruvate dehydrogenase complex and Kreb’s Cycle is 8 NADH, 2 FADH2, 2 ATP and some GTP.

TERMS TO KNOW
aerobic respiration
acetyl-CoA
acetyl group (-COCH3)
adenosine triphosphate (ATP)
carboxyl group (-COO)
citric acid cycle
flavin adenine dinucleotide (FAD+ / FADH2)
glucose
glycolysis
guanosine triphosphate (GTP)
Kreb’s cycle
nicotinamide adenine dinucleotide (NAD+ / NADH)
oxaloacetate (OOA)
pyruvate
pyruvate dehydrogenase complex (PDC)
tricarboxylic acid cycle / TCA cycle

RELATED TOPICS (Note: These links will go active as videos appear online.)
fermentation
glycolysis
electron transport and oxidative phosphorylation

CONTENT REFERENCES
1. AAMC. (2008). Topics for Biological Science Section of the MCAT. [PDF Brochure].
2. Lodish, H., Berk, A., Kaiser, C., Kreiger, M., Scott, M. P., Bretscher. A., & Ploegh, H. (2008). Molecular Cell Biology (6th ed.). New York: W. H. Freeman and Company.
3. Campell, N.A., & Reece, J.B. (2005). Biology (7th ed.) San Francisco, Benjamin Cummings.