Gen Bio 1 L11: Krebs Cycle

• Krebs Cycle: also known as "Citric Acid Cycle" or "Tricarboxylic Acid (TCA) Cycle"
• Krebs Cycle: a series of chemical reactions that occur in the mitochondria of the cell.
• Krebs Cycle: a crucial part of cellular respiration
• Krebs Cycle: it is the process by which cells generate energy from glucose and other organic molecule.
• Krebs Cycle: named after Hans Krebs, he explained the steps of the cycle in the 1930s.
• Krebs Cycle: an aerobic process, it requires Oxygen
• In order for pyruvate to to enter Krebs Cycle, it must be converted into Acetyl-CoA (Acetyl Coenzyme), by the pyruvate dehydrogenase complex (PDC), which participates in an oxidative process where NADH and CO₂ are produced.
• Step 1. Acetyl-CoA Formation: begins with 2-Carbon molecule, acetyl-CoA combines with a 4-Carbon molecule called the Oxaloacetate (from the previous step of the cycle), forming a 6-Carbon molecule called citrate.
• Step 1. Acetyl-CoA is catalyzed by the enzyme citrate synthase to become citrate.
• Step 2. Isomerization: citrate is converted into its isomer isocitrate by the enzyme aconitase.
• Step 2. Isomerization: this step involves rearrangement of atoms within the molecule.
• Step 3. Decarboxylation and NADH Formation: isocitrate is further converted into alpha-ketoglutaerate by the enzyme isocitrate dehydrogenase
• Step 3. This step involves the release of CO₂ molecule and the generation of a molecule of NADH, which carries high-energy electrons.
• Step 4. Decarboxylation and NADH/FADH₂ Formation: α-ketoglutarate is converted into succinyl-CoA by α-ketoglutarate dehydrogenase.
• Step 4. releases CO₂ molecule and generates another molecule of NADH
• Step 5. Substrate-Level Phosphorylation: Succinyl-CoA is converted into succinate by the enzyme succinate-CoA synthetase.
• Step 5. Substrate-Level Phosphorylation: involves the transfer of high-energy phosphate group to a molecule of GDP, forming a molecule of GTP (which can later be converted into ATP).
• Step 6. Dehydrogenation and FADH₂ Formation: succinate is converted to Fumarate by the enzyme succinate dehydrogenase. Generates FADH₂.
• Step 7. Hydration: Fumarate is converted into malate by the enzyme fumarase.
• Step 7. Hydration: This step involves addition of a water molecule to the molecule.
• Step 8. Dehydrogenation and NADH Formation: malate is converted into oxaloacetate by malate dehydrogenase. Gain of NADH.
• At the end of Krebs Cycle, the Oxaloacetate produced can continue with another molecule of acetyl-CoA to start the cycle again.
• The cycle produces: 3 NADH, 1 FADH₂, 1 molecule of GTP
• This cycle releases: 2 molecules of CO₂.
• The high-energy electrons carried by NADH and FADH₂ are then used in the electron transport chain to generate ATP through oxidative phosphorylation which is the final step of cellular respiration.