Online Biology Tutor


The Pyruvate Dehydrogenase Complex and Kreb’s Cycle

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.

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Glycolysis

This tutorial presents glycolysis at a level appropriate for most undergraduate biology classes and the MCAT exam.

Click to view the video tutorial: Glycolysis.

Glycolysis is a metabolic pathway in which glucose is degraded anaerobically by cytosolic enzymes to produce two smaller pyruvate molecules and ATP.

The net product of glycolysis for the catabolism of a single glucose molecule is 2 ATP, 2 NADH and 2 pyruvate molecules.

QUIZ ANSWERS
1. Two three carbon pyruvate molecules are produced by splitting one six carbon glucose molecule.

2. Hexokinase uses ATP to phosphorylate glucose, making glucose 6-phosphate. Phosphofructokinase uses ATP to phosphorylate fructose 6-phosphate, making fructose 1,6-bisphosphate. Thus two molecules of ATP are consumed early in the glycolytic pathway. Because early glycolytic reactions use ATP, early glycolysis is sometimes called the energy investment phase of glycolysis. Later, during the energy production phase four molecules of ATP are produced. Since two molecules of ATP are invested and four molecules of ATP are produced, the net ATP production for glycolysis is two ATP.

-2 ATP + 4 ATP = 2 ATP

3. When the cytosolic concentration of ATP is high ATP binds to an allosteric site on phosphofructokinase inhibiting it’s ability to phoshorylate fructose 6-phosphate to fructose 1,6-bisphosphate. Without fructose 1,6-bisphosphate reactions that occur later in the glycolytic pathway can not occur and the rate of glycolysis slows.

4. Glycolysis is an anaerobic process, meaning that it does not require oxygen. Obligate anaerobes (organisms that die in the presence of oxygen) may use glycolysis and fermentation to produce ATP.

5. Catabolic pathways break down large molecules into smaller molecules. The energy released by breaking bonds may be harvested by the cell to do work or it may dissipate as heat. Anabolic pathways construct large molecules from smaller molecules. Anabolic pathways require energy and are usually powered by ATP. During glycolysis enzymes in the cytoplasm convert a single molecule of glucose into two smaller pyruvate molecules. The energy released by breaking bonds is used to make ATP. Thus the glycolytic pathway is also a catabolic pathway.

TERMS TO KNOW
adenosine triphosphate (ATP)
adenosine diphosphate (ADP)
adenosine monophosphate (AMP)
fructose 6-phosphate
fructose 1,6-bisphosphate
glucose
glucose 6-phosphate
glycolysis
hexokinase
phosphofructokinase
pyruvate
nicotinamide adenine dinucleotide (NAD+ / NADH)

RELATED TOPICS (Note: These links will go active as videos appear online.)
fermentation
the citric acid cycle
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., Reece, J. B., Taylor, M., & Simon, E. (2008). Biology: Concepts and Connections (5th ed.). San Francisco: Benjamin Cummings.
4. Fox, S. I. (2002). Human Physiology. (7th ed.). New York: McGraw-Hill.