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Glycolysis: Payoff Phase


MCAT Biochemistry

In the payoff phase of glycolysis, steps 6 through 10 of glycolysis generate energy in the form of ATP and NADH. Starting with two molecules of G3P, step 6 involves glyceraldehyde 3 phosphate dehydrogenase (GAPDH) converting G3P and NAD+ to 1,3-Bisphosphoglycerate (1,3BPG) and NADH. In step 7, phosphoglycerate kinase transfers a phosphate group from 1,3BPG to ADP, producing 3-phosphoglycerate (3PG) and ATP. Moving on to step 8, phosphoglycerate mutase transforms 3PG to 2-phosphoglycerate (2PG). In step 9, enolase turns 2PG into phosphoenolpyruvate (PEP). Lastly, in step 10 of glycolysis, pyruvate kinase transfers a phosphate group from PEP to ADP, forming pyruvate and ATP. Regulating step 10 are high ATP and Acetyl-CoA levels which inhibit pyruvate kinase and F1,6BP activation.

Lesson Outline

<ul> <li>Glycolysis Payoff Phase: Steps 6 through 10 of glycolysis</li> <ul> <li>Starting with two G3P molecules</li> </ul> <li>Step 6: Glyceraldehyde 3 phosphate dehydrogenase (GAPDH) transforms G3P into 1,3-Bisphosphoglycerate (1,3BPG)</li> <ul> <li>GAPDH transfers 2 electrons to NAD+ which makes NADH</li> <li>Produces 2 NADH for every 1 glucose molecule</li> </ul> <li>Step 7: Phosphoglycerate kinase transfers a phosphate group from 1, 3BPG to ADP to make ATP</li> <ul> <li>Produces 3-phosphoglycerate (3PG)</li> </ul> <li>Step 8: Phosphoglycerate mutase transforms 3PG to 2PG</li> <li>Step 9: Enolase removes a molecule of water from 2PG to form PEP</li> <li>Step 10: Pyruvate kinase irreversibly transfers the last phosphate group from PEP to ADP to make pyruvate and ATP</li> <ul> <li>Regulatory step in glycolysis</li> <li>ATP inhibits pyruvate kinase when ATP levels are high</li> <li>Acetyl-CoA, which enters the TCA cycle, also inhibits pyruvate kinase</li> <li>F1,6BP activates pyruvate kinase (feed-forward activation)</li> </ul> </ul>

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What is the role of G3P in the glycolysis payoff phase?

G3P, or glyceraldehyde 3-phosphate, is a crucial intermediate in glycolysis. After being generated during the first half of glycolysis (the investment phase), G3P undergoes a series of enzyme-catalyzed reactions to generate ATP and NADH, which are essential energy-providing molecules in cells. The conversion of G3P ultimately leads to the formation of pyruvate, which can then enter additional metabolic pathways like the citric acid cycle or fermentation depending on cellular conditions.

What are the main enzymes involved in the payoff phase of glycolysis?

There are several enzymes that play key roles in the payoff phase of glycolysis. These enzymes include glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which oxidizes G3P and generates NADH; phosphoglycerate kinase, which catalyzes the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate; phosphoglycerate mutase, which converts 3-phosphoglycerate into 2-phosphoglycerate; enolase, which eliminates water from 2-phosphoglycerate to produce phosphoenolpyruvate (PEP); and pyruvate kinase, which transfers a phosphate from PEP to ADP, forming ATP and pyruvate.

How does GAPDH contribute to the generation of ATP and NADH in glycolysis?

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a vital enzyme in the payoff phase of glycolysis. It catalyzes the conversion of glyceraldehyde 3-phosphate (G3P) into 1,3-bisphosphoglycerate by transferring a hydrogen from G3P to NAD+, reducing it to NADH. This reaction also involves the addition of an inorganic phosphate (Pi) to G3P, ultimately forming the high-energy intermediate 1,3-bisphosphoglycerate. This intermediate can then undergo further reactions to generate ATP.

What is the significance of converting 3-phosphoglycerate to 2-phosphoglycerate in the glycolysis pathway?

The conversion of 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase is an essential step in the glycolysis pathway. This isomerization reaction rearranges the position of the phosphate group on the molecule, favoring the subsequent reaction – the dehydration of 2-phosphoglycerate by the enzyme enolase. This step is crucial for the creation of the high-energy molecule phosphoenolpyruvate (PEP), which can then be used to generate ATP in the final reaction of glycolysis catalyzed by pyruvate kinase.

Why is pyruvate kinase an important enzyme in the payoff phase of glycolysis?

Pyruvate kinase is the final enzyme in the glycolysis pathway and plays a crucial role in the payoff phase. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP), generating adenosine triphosphate (ATP) and pyruvate as products. This reaction is highly exergonic, meaning it releases a significant amount of energy, which can be harnessed to meet the energy needs of the cell. In total, the payoff phase of glycolysis generates a net gain of 2 ATP molecules per glucose molecule, making pyruvate kinase a key enzyme in cellular energy production.