1. The Citric Acid Cycle Chapter 9-3

2. The Second Phase The Citric Acid cycle is just the second step towards harvesting energy as glycolysis has already generated 2 ATP. -Also referred to as the Krebs cycle & TCA cycle -Energy remains harnessed in 2 pyruvate molecules

3. In the presence of O 2, pyruvate enters the mitochondrion to be converted to a compound called acetyl coenzyme A, or acetyl CoA.

4. Step One  Pyruvate’s carboxyl group, which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO 2.

5. Step Two  The remaining two-carbon fragment is oxidized to form a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD+, storing energy in the form of NADH.

6. Step Three  Finally, coenzyme A, a sulfur-containing compound derived from a B vitamin, is attached to the acetate by an unstable bond that makes the acetyl group (the attached acetate) very reactive.  The product of this chemical grooming, acetyl CoA, is now ready to feed its acetate into the Krebs cycle for further oxidation.

7. Citric Acid Cycle In More Detail  Pyruvate is broken down to 3 CO 2 molecules  The cycle has eight steps, each catalyzed by a specific enzyme.  All the citric acid cycle enzymes are located in the mitochondrial matrix except for the enzyme that catalyzes step 6, which resides in the inner mitochondrial membrane.  These steps decompose citrate back to oxaloacetate.  Regeneration of oxaloacetate makes process a cycle

8. Figure 9.11 A closer look at the Krebs cycle (Layer 1) 1.Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing citrate. 2. Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another.

9. Figure 9.11 A closer look at the Krebs cycle (Layer 2) 3. Citrate loses a CO 2 molecule, and the resulting compound is oxidized, reducing NAD+ to NADH. 4. Another CO 2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond.

10. Figure 9.11 A closer look at the Krebs cycle (Layer 3) 5. CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, and then ADP, forming ATP (substrate-level phosphorylation.) 6. Two hydrogens are transferred to FAD, forming FADH 2 and oxidizing succinate.

11. Figure 9.11 A closer look at the Krebs cycle (Layer 4) 7. Addition of a water molecule rearranges bonds in the substrate. 8. The substrate is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate.

12. Why it’s important.  NAD+ is reduced to NADH & FAD+ to FADH 2 to transfer more electrons.  The whole cycle generates 1 ATP molecule per turn  3 NADH molecules and 1 FADH molecule, which are important for an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps instead of one explosive reaction.  Net = 2 cycles through; 2 ATP molecules