Highlights Lecture #27 Spring 2017

1. Animals, plants, and microorganisms use the same pathway from pyruvate when oxygen is available. This involves converting pyrvate into acetyl-CoA for oxidation in the citric acid cycle. When oxygen is present, NADH donates its electrons to the electron transport system, creating NAD+. This means there is plenty of NAD+ when oxygen is abundant.

2. In animals, lactate is made from pyruvate when oxygen is missing (anaerobic – such as in muscles during heavy exertion). This is done to regenerate NAD+, which is low when oxygen is low. NAD+ is needed to keep glycolysis going under these conditions.

3. In microorganisms, pyruvate is converted to ethanol during anaerobic conditions for the same reasons lactate is made in animals – because it creates NAD+ needed to keep glycolysis going when oxygen concentration is low.

4. Pyruvate is the ending point for glycolysis. Which pathway is taken from that point forward depends on the needs of the cell. Since cells have a VERY strong interest in keeping glycolysis going, the primary consideration is keeping NAD+ levels high. Under aerobic conditions (plenty of oxygen), NAD+ is readily made from NADH without problems. Thus under aerobic conditions, cells (animal and microbial cells) convert pyruvate to acetyl-CoA, CO2, and NADH, since the NADH can readily be converted back to NAD+.

5. Metabolism of glucose by anaerobic pathways does not release nearly as much energy as when glucose is metabolized by the aerobic pathway. Note that conversion of pyruvate to ethanol by microorganisms is a two step process. The last step in the process is catalyzed by alcohol dehydrogenase. In microorganisms, the direction of the reaction is towards producing ethanol. Animals also have an alcohol dehydrogenase, but they use it for the reverse direction to break down ethanol. The product of the reverse reaction is acetaldehyde and may be responsible for hangovers.

6. Glycolysis is regulated by three enzymes – hexokinase (inhibited by G6P), phosphofructokinase (inhibited by ATP), and pyruvate kinase (inhibited by ATP).

7. Hexokinase’s regulation is not simple and you are not responsible for it. PFK’s regulation is allosteric and involves several possible molecules. They include AMP (indicates low cellular energy and stimulates enzyme to be active), F2,6BP (activates PFK strongly in very low concentrations), and ATP (indicates high energy and inhibits the enzymes under high concentrations).

8. Pyruvate kinase is actually regulated by covalent modification (phosphorylation/dephosphorylation) and allosterically. Allosteric activation of pyruvate kinase occurs with either F1,6BP (feedforward activation) or AMP (indicates low cellular energy). Allosteric inhibition occurs with ATP. Inhibition of pyruvate kinase is very important to keep gluconeogenesis going (synthesis of glucose) when the cell is needing to make glucose.

Highlights Gluconeogenesis

1. Gluconeogenesis is the pathway that is involved in the synthesis of glucose. It operates mostly in the liver and kidney. It uses 7 of the same enzymes as glycolysis by running the reactions in the opposite direction from glycolysis. The other three enzymes are replaced by four new enzymes in gluconeogenesis. The two of these are pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK).

2. Pyruvate carboxylase is a biotin-containing enzyme that is found in mitochondria and it catalyzes the addition of a carboxyl group to pyruvate to make oxaloacetate. The reaction requires ATP

3. In the cytoplasm, PEPCK catalyzes the conversion of oxaloacetate to PEP and the reaction requires GTP. The gluconeogenesis reactions that follow the PEPCK reaction are the same enzymes as in glycolysis up to PFK, but with reversal of the reactions of glycolysis.

4. The PFK enzymatic reaction of glycolysis is replaced by the enzyme fructose-1,6bisphosphatase (F1,6BPase). It acts to remove a phosphate from F1,6BP, yielding F6P. This reaction is energetically favorable, since ATP is NOT regenerated (as it would need to be if the glycolysis reaction were reversed).

5. The hexokinase reaction of glycolysis is replaced by the enzyme glucose-6-phosphatase (G6Pase), which acts to remove a phosphate from G6P to yield free glucose. This reaction is energetically favorable, since ATP is NOT regenerated (as it would need to be if the glycolysis reaction were reversed).

6. Reciprocal regulation of glycolysis and gluconeogenesis is accomplished mainly by the molecule fructose-2,6-bisphosphate (F2,6BP). It acts to stimulate glycolysis by turning on PFK (sometimes also called PFK-1) while at the same time acting to inhibit gluconeogenesis by turning off the corresponding enzyme, F1,6BPase. Thus, when F2,6BP is present, glycolysis is running and gluconeogenesis is inhibited. When F2,6BP is absent, gluconeogenesis is running and glycolysis is inhibited.

Exam 3 material ends here.

Print Friendly, PDF & Email