(Unit:12) KREBS CYCLE

Question 1

What is the Citric Cycle?
Where does it occur?

Tricarboxylic Acid Cycle (TCA) or Krebs cycle, for Hans Adolph Krebs who identified it in 1937

Answer 1

• Series of biochemical reactions used by all aerobic organisms to produce energy from carbohydrates, protein, and fat.
• Occurs through the oxidation of acetyl-CoA, leading to ATP production.
• Occurs in mitochondria.

Question 2

What is the main source of ATP in metabolism?

Answer 2

Processes in the citric acid cycle (in addition with oxidative phosphorylation) is the main source of ATP generated in metabolism, accounting for about 95% of energy in aerobic cells.

Question 3

What are two main sources of Acetyl CoA?

Answer 3

Fatty Acids
Glucose

Question 4

What do we accomplish in one cycle (1 pyruvate)?

Answer 4

In a single turn of the citric acid cycle:
So, for 1 glucose molecule, the energy output for the citric acid cycle is 2 ATP, 6 NADH, and 2 FADH2.

Question 5

Name the 8 Reactions.

Answer 5

• A 2-carbon unit of acetyl-CoA condenses with a 4-carbon unit of oxaloacetate to form a 6-carbon unit of citrate. ◦ Citrate synthase catalyzes this reaction.
• Aconitase catalyzes the isomerization of citrate to isocitrate. ◦ Isocitrate is much more readily oxidized than citrate. * 
• Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate. This removes a unit of CO2 , resulting in a 5-carbon unit of α-ketoglutarate. ◦ This reaction requires the input of NAD+ and produces NADH.
• The α-ketoglutarate dehydrogenase complex catalyzes the oxidative decarboxylation of α-ketoglutarate. This removes another unit of CO2 , resulting in a 4-carbon unit of succinyl-CoA, which is an energy-rich compound. ◦ This reaction also requires the input of NAD+ and produces NADH.
• Succinyl-CoA synthetase cleaves succinyl-CoA to produce succinate. ◦ This reaction is coupled with phosphorylation of guanosine diphosphate (GDP) to produce guanosine triphosphate (GTP). (Note that it is possible to convert GTP to ATP. The enzyme nucleoside diphosphokinase catalyzes the reaction of GTP + ADP → GDP + ATP.)
• Succinate dehydrogenase oxidizes succinate to fumarate. ◦ This is a difficult oxidation reaction, so it utilizes FAD, which is a more powerful oxidant than NAD+ . The FAD is reduced to FADH2.
• Fumarase hydrates (adds H2 O) fumarate to produce malate.
• Malate dehydrogenase oxidizes malate to produce oxaloacetate. ◦ This reaction requires the input of NAD+ and produces NADH.