SIKLUS KREBS

Summary

This video provides a clear and engaging explanation of the Krebs cycle, the third stage of aerobic respiration, occurring in the mitochondrial matrix. It breaks down all eight stages—from the formation of citrate from acetyl-CoA and oxaloacetate to the regeneration of oxaloacetate—highlighting key enzymes, reactions, and byproducts like NADH, FADH2, ATP, and CO2. The video also introduces helpful mnemonics and a song to aid memorization. By the end, viewers gain a comprehensive understanding of how the Krebs cycle contributes to energy production, making complex biochemical processes easy to follow and remember.

Takeaways

• 😀 The Krebs cycle, also known as the citric acid cycle, is the third stage of aerobic respiration and occurs in the mitochondrial matrix.
• 😀 It was discovered by Hans Krebs, and the first compound formed in the cycle is citric acid.
• 😀 The cycle consists of 8 main stages: citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
• 😀 Acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons) in the first stage, catalyzed by citrate synthase.
• 😀 Isocitrate is formed from citrate through isomerization, involving dehydration and hydration, assisted by aconitase.
• 😀 Isocitrate is converted into alpha-ketoglutarate (5 carbons) with the release of CO2 and electrons captured by NAD+, catalyzed by isocitrate dehydrogenase.
• 😀 Alpha-ketoglutarate is transformed into succinyl-CoA (4 carbons), releasing CO2 and electrons, with coenzyme A addition, catalyzed by alpha-ketoglutarate dehydrogenase.
• 😀 Succinyl-CoA is converted into succinate, producing GTP (or ATP) and releasing coenzyme A, catalyzed by succinyl-CoA synthetase.
• 😀 Succinate is converted to fumarate with electrons captured by FAD to form FADH2, catalyzed by succinate dehydrogenase.
• 😀 Fumarate is hydrated to form malate (enzyme: fumarase), which is then converted to oxaloacetate with NAD+ forming NADH (enzyme: malate dehydrogenase).
• 😀 Each acetyl-CoA produces 3 NADH, 1 FADH2, 1 ATP, and 2 CO2; thus, one glucose molecule yields 6 NADH, 2 FADH2, 2 ATP, and 4 CO2 through two cycles.
• 😀 A mnemonic song can be used to remember the sequence of compounds and products in the Krebs cycle.

Q & A

What is the Krebs cycle and why is it important?

The Krebs cycle, also known as the citric acid cycle, is the third stage of aerobic respiration. It is important because it produces energy carriers (NADH, FADH2) and ATP, which are essential for cellular energy production.

Who discovered the Krebs cycle and how did it get its name?

The Krebs cycle was discovered by German scientist Hans Krebs. It is named in his honor.

Where in the cell does the Krebs cycle take place?

The Krebs cycle occurs in the mitochondrial matrix of the cell.

How many main stages are there in the Krebs cycle and what are they?

There are eight main stages in the Krebs cycle: Citrate, Isocitrate, Alpha-ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, and Oxaloacetate.

What happens during the first stage of the Krebs cycle?

In the first stage, Acetyl-CoA (2C) condenses with Oxaloacetate (4C) to form Citrate (6C), catalyzed by the enzyme citrate synthase.

Which enzyme catalyzes the conversion of isocitrate to alpha-ketoglutarate, and what key reactions occur?

The enzyme is isocitrate dehydrogenase. During this step, NAD+ is reduced to NADH and one carbon is released as CO2.

What is produced when succinyl-CoA is converted into succinate?

When succinyl-CoA is converted into succinate, GTP (or ATP) is produced, and coenzyme A is released. This reaction is catalyzed by succinyl-CoA synthase.

Which steps of the Krebs cycle involve electron carrier formation and what are they?

Electron carriers are formed in several steps: NADH is produced during conversion of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate. FADH2 is produced when succinate is converted to fumarate.

How many NADH, FADH2, and ATP molecules are produced per glucose molecule through the Krebs cycle?

Per glucose molecule, which produces two acetyl-CoA, the Krebs cycle produces 6 NADH, 2 FADH2, and 2 ATP.

What byproducts are released during the Krebs cycle?

Four molecules of carbon dioxide (CO2) are released as byproducts during the Krebs cycle per glucose molecule.

How can the sequence of the Krebs cycle stages be memorized?

The sequence can be memorized using a mnemonic or song. For example: 'Acetyl-CoA, Citrate, Isocitrate, NADH2, Ketoglutarate, NADH, Succinyl, ATP, Succinate, FADH2, Fumarate, NADH2, Oxaloacetate.'

What type of reactions occur when citrate is converted into isocitrate?

During this stage, citrate undergoes an isomerization reaction involving dehydration (removal of H2O) and hydration (addition of H2O), catalyzed by the enzyme aconitase.