Kreb's Cycle

Summary

This video explains the Krebs cycle, focusing on key processes like pyruvate oxidation and the enzymatic actions involved in converting acetyl-CoA to carbon dioxide. The cycle involves crucial steps like the formation of citrate, decarboxylation, and electron transfers to NAD+ and FAD, generating energy carriers NADH, FADH2, and ATP. Through two turns of the cycle per glucose molecule, the Krebs cycle contributes to cellular respiration by breaking down glucose into carbon dioxide and storing energy in high-energy molecules, which are later used in the electron transport chain to produce more ATP.

Takeaways

• 😀 Glycolysis splits glucose into two pyruvate molecules, which are then oxidized to form acetyl coenzyme A, entering the Krebs cycle.
• 😀 Acetyl coenzyme A reacts with oxaloacetate to form citrate in the first step of the Krebs cycle, catalyzed by citrate synthase.
• 😀 The dehydrogenase enzyme removes hydrogen from molecules, transferring them to electron carriers like NAD+ and FAD, which get reduced to NADH and FADH2.
• 😀 In step 5 of the Krebs cycle, isocitrate is oxidized to alpha-ketoglutarate with the release of CO2 and the reduction of NAD+ to NADH.
• 😀 Oxidation involves a loss of electrons, while reduction involves a gain of electrons, demonstrated by NAD+ reduction and isocitrate oxidation in the Krebs cycle.
• 😀 Step 6 sees alpha-ketoglutarate oxidized into succinyl-CoA, with the removal of another CO2 and the formation of NADH.
• 😀 The Krebs cycle produces two carbon dioxide molecules for every turn, with glucose contributing to two turns, resulting in four CO2 molecules lost.
• 😀 In step 7, succinyl-CoA is converted into succinate, producing one molecule of ATP via the transfer of a phosphate group from GTP to ADP.
• 😀 Step 8 involves the oxidation of succinate to fumarate, with the reduction of FAD to FADH2.
• 😀 One complete turn of the Krebs cycle for one glucose molecule generates three NADH, one FADH2, and one ATP.
• 😀 For each glucose molecule, two pyruvate molecules undergo two turns of the Krebs cycle, yielding a total of six NADH, two FADH2, and two ATP.

Q & A

What is the role of glycolysis in cellular respiration?

Glycolysis is the first step in cellular respiration where glucose is split into two pyruvate molecules, providing the starting material for further energy production processes.

What happens during pyruvate oxidation?

In pyruvate oxidation, each pyruvate molecule is oxidized to form acetyl-CoA, a crucial intermediate that enters the Krebs cycle. During this process, carbon dioxide is released.

How does acetyl-CoA enter the Krebs cycle?

Acetyl-CoA enters the Krebs cycle by combining with oxaloacetate, a four-carbon molecule, to form citrate, a six-carbon compound. This reaction is catalyzed by citrate synthase.

What is the function of dehydrogenase enzymes in the Krebs cycle?

Dehydrogenase enzymes in the Krebs cycle remove hydrogen atoms from molecules, transferring them to electron carriers like NAD+ or FAD, which are reduced to NADH and FADH2, respectively.

What is a decarboxylation reaction, and where does it occur in the Krebs cycle?

A decarboxylation reaction involves the loss of a carbon atom in the form of CO2. This occurs twice in the Krebs cycle—first when isocitrate is converted to alpha-ketoglutarate, and then when alpha-ketoglutarate is converted to succinyl-CoA.

How is ATP produced in the Krebs cycle?

ATP is produced in step 7 of the Krebs cycle when GTP transfers a phosphate group to ADP, forming ATP. This process involves the conversion of succinyl-CoA to succinate.

What happens to NAD+ and FAD during the Krebs cycle?

NAD+ and FAD act as electron acceptors in the Krebs cycle. They are reduced to NADH and FADH2, respectively, by accepting electrons and hydrogen ions from various intermediates during the cycle.

How many times does the Krebs cycle turn for each molecule of glucose?

The Krebs cycle turns twice for each molecule of glucose, since one glucose molecule produces two pyruvate molecules, each of which enters the cycle.

What is the net energy yield from one turn of the Krebs cycle?

One turn of the Krebs cycle produces 3 NADH molecules, 1 FADH2 molecule, 1 ATP (or GTP), and releases 2 CO2 molecules.

What happens to the products of the Krebs cycle after they are generated?

The products of the Krebs cycle—NADH, FADH2, and ATP—are used in subsequent stages of cellular respiration, particularly the electron transport chain, where the energy from NADH and FADH2 is used to produce additional ATP.