Krebs Cycle

Question 1

Where does Krebs Cycle occur?

Answer 1

In the mitochondrial matrix

Question 2

What is happening during the Citric Acid Cycle?

Answer 2

Pyruvate is further broken down, releasing CO₂ and transferring high-energy electrons to NADH and FADH₂.

Question 3

What does 1 turn of the TCA generate?

Answer 3

• 3 NADH
  - 1 FADH₂
  - 2 CO₂ (waste products)
  - 1 ATP (or GTP, depending on the cell)

Question 4

Is Citric Acid Cycle aerobic or anaerobic?

Answer 4

The Citric Acid Cycle is aerobic (requires oxygen indirectly, as it is coupled with the ETC).

Question 5

What is Acetyl-CoA?

Answer 5

The enzyme is formed in mitochondria from pyruvate by the enzyme pyruvate dehydrogenase.

Acetyl-CoA serves as the entry point for carbon atoms derived from nutrients.

In TCA, Acetyl-CoA is oxidized, and its carbon atoms are eventually released as carbon dioxide (CO₂).

Acetyl-CoA (derived from pyruvate) combines with oxaloacetate to form citric acid (6 carbons).

Question 6

3 names for the process

Answer 6

Citric Acid Cycle
Krebs Cycle
Tricarboxylic Acid Cycle

Question 7

Why is TCA important?

Answer 7

The TCA cycle (also known as the Krebs cycle or citric acid cycle) is central to cellular metabolism and is essential for several critical reasons:

1. Energy Production:
   The TCA cycle plays a key role in generating energy for the cell. Through a series of oxidation reactions, it produces high-energy electron carriers:

NADH (Nicotinamide adenine dinucleotide)
FADH₂ (Flavin adenine dinucleotide)

These molecules are critical for the next step in cellular respiration, the electron transport chain, where they donate electrons to generate ATP via oxidative phosphorylation.

2. Carbon Dioxide Production:
   During the TCA cycle, carbon atoms from acetyl-CoA are fully oxidized and released as carbon dioxide (CO₂).
3. Regulation of Metabolic Flux:
   The TCA cycle is tightly regulated to match the cell’s energy needs. Key enzymes, such as citrate synthase and isocitrate dehydrogenase, are regulated by the levels of ATP, NADH, and other metabolites. When the cell needs energy, the cycle is activated to produce more ATP. When energy is abundant, the cycle slows down to avoid wasteful overproduction of ATP.
4. Oxygen-Dependent Energy Production:
   Unlike glycolysis, which can occur in both aerobic and anaerobic conditions, the TCA cycle requires oxygen to proceed efficiently (since NADH and FADH₂ produced in the cycle donate electrons to the electron transport chain, which depends on oxygen). In aerobic organisms, this makes the TCA cycle crucial for aerobic respiration, where large amounts of ATP are generated.

Question 8

What is the starting molecule of the Krebs cycle?

Answer 8

Acetyl-CoA
The Krebs cycle begins with Acetyl-CoA, which is derived from pyruvate after glycolysis.

Question 9

Which enzyme catalyzes the first step of the Krebs cycle, where Acetyl-CoA combines with oxaloacetate?

Answer 9

Citrate synthase
Citrate synthase catalyzes the combination of Acetyl-CoA and oxaloacetate to form citrate.

Question 10

What is the first product formed after the combination of Acetyl-CoA and oxaloacetate?

Answer 10

Citrate
The first product is citrate, not succinate, formed after Acetyl-CoA and oxaloacetate combine.

Question 11

Which molecule is reduced during the Krebs cycle to form NADH?

Answer 11

NAD+ is reduced to NADH during several steps of the Krebs cycle.

Question 12

How many NADH molecules are produced per turn of the Krebs cycle?

Answer 12

3 NADH molecules are produced per cycle.

Question 13

What is the role of FAD in the Krebs cycle?

Answer 13

FAD accepts electrons from succinate during its oxidation to fumarate, forming FADH2 in the process.

Question 14

Which step of the Krebs cycle involves the production of ATP (or GTP)?

Answer 14

Step 5 involves substrate-level phosphorylation, where ATP (or GTP in some cells) is produced.

Question 15

Which of the following is the final product of the Krebs cycle?

Answer 15

Oxaloacetate
Oxaloacetate is regenerated at the end of the cycle and is ready to combine with another Acetyl-CoA molecule to begin the cycle again.

Question 16

How many carbon atoms are lost as CO2 during one full turn of the Krebs cycle?

Answer 16

2 carbon atoms are lost as CO2 during the cycle.

Question 17

Which of the following is a key regulatory enzyme in the Krebs cycle?

Answer 17

Citrate synthase is a key regulatory enzyme, especially in the first step of the cycle.

Question 18

What is substrate-level phosphorylation?

Answer 18

Substrate-level phosphorylation is a type of ATP synthesis that occurs when a phosphate group is directly transferred from a high-energy substrate molecule to ADP, forming ATP. This process doesn’t require oxygen and happens in the cytoplasm during glycolysis and in the mitochondria during the Krebs cycle.

In substrate-level phosphorylation, the energy needed to add the phosphate to ADP comes directly from the breakdown of a high-energy molecule (the “substrate”).
It contrasts with oxidative phosphorylation, where ATP is produced using energy from electrons transferred through the electron transport chain.

Steps of Substrate-Level Phosphorylation:

During Glycolysis:
In step 7 of glycolysis, 1,3-bisphosphoglycerate (a high-energy substrate) donates a phosphate group to ADP, forming ATP and 3-phosphoglycerate.
In step 10, phosphoenolpyruvate (PEP) donates a phosphate group to ADP, forming another ATP molecule and pyruvate.

During the Krebs Cycle:
In Step 5 of the Krebs cycle, succinyl-CoA transfers a phosphate group to GDP (or ADP in some tissues), forming GTP (or ATP) and succinate.