ReminderFrom glycolysis, 2 ATP

net were produced, along with 2 NADH and 2 pyruvate molecules.

If oxygen is present, pyruvate will move on with aerobic cellular respiration.

If oxygen is not present, pyruvate will undergo anaerobic cellular respiration (a.k.a. fermentation).

Pyruvate OxidationThe two pyruvate molecules from glycolysis

go from the cytoplasm to the matrix of the mitochondrion.

Pyruvate OxidationThere a multi-

enzyme catalyzes the following 3 reactions:

1)A carboxyl group that has lost its hydrogen atom is taken away as CO2. (pyruvate decarboxylase)

Pyruvate Oxidation2) A redox reaction

occurs. NAD+ is reduced to NADH plus H and pyruvate is oxidized to acetate (an acetic acid group). This transfers potential energy to NAD+.

Pyruvate Oxidation3) A sulfur containing

enzyme called Coenzyme-A attaches to the acetate forming acetyl-CoA. The C-S bond is unstable which prepares acetyl-CoA to be easily oxidized in the Krebs Cycle that follows.

Acetyl-CoA enters the Krebs cycle while the two NADH (one per pyruvate) go to the electron transport chain.

Carbon dioxide leaves as a waste product.

Krebs CycleDiscovered by Hans Krebs (Nobel Prize in 1953)It is an 8-step process with each step catalyzed by a specific

enzyme.Occurs in the matrix of the mitochondrion.Starts and ends with oxaloacetate, showing that this is indeed a

cyclic process.

The Reactions of the Krebs Cycle

Reaction #1Acetyl-CoA joins with oxaloacetate to form citrate. The coenzyme complex leaves the acetyl group and

can go back and oxidize another pyruvate molecule. Enzyme is citrate synthase.

Reaction #2Citrate isomerizes into isocitrate. The enzyme is aconitase.

Reaction #3Isocitrate loses a carbon dioxide molecule and

two hydrogen atoms.The hydrogen atoms reduce NAD+ to NADH. The new 5C molecule is called -ketoglutarate.

Notice the ketone group! The enzyme is isocitrate dehydrogenase.

Reaction #4-ketoglutarate is converted to succinyl-CoA as carbon

dioxide is given off and the Coenzyme returns. 2 hydrogen atoms reduce NAD+ to NADH. The enzyme is -ketoglutarate dehydrogenase.

Reaction #5The coenzyme leaves again creating succinate

and ATP is formed by substrate level phosphorylation.

The enzyme is succinyl CoA synthetase.

Reaction #6Succinate is converted into fumerate as two hydrogen

atoms leave to reduce FAD to FADH2. FADH2 is like NADH, but holds lower energy electrons.The enzyme is succinic dehydrogenase.

Reaction #7Water is added to fumerate to create malate by breaking

the double bond between the second and third carbon. The enzyme is fumerase.

Reaction #8Malate converts into oxaloacetate as two hydrogen

atoms are used to reduce NAD+ to NADH. The enzyme is malate dehydrogenase.

The end…The cycle then restarts with

the other acetyl-CoA molecule.

At the end of the Krebs cycle, all of the original carbons from the glucose molecule have been removed as carbon dioxide.

All that is left is the 2 ATP that is formed and the reduced high energy electron carriers, 6 NADH (3 per acetyl-CoA) and 2 FADH2 (1 per acetyl-CoA) which go onto the Electron Transport Chain.