ScienceCollege- Aerobic catabolism

Eukaryotic aerobic catabolism

This basically consists of three processes: glycolysis, Kreb's and oxidative phosphorylation.

Glycolysis

The central pathway of eukaryotic metabolism leads from glucose to pyruvate via the glycolytic pathway. Glucose is repeatedly phosphorylated, eventually yielding fructose-bis-phosphate, which cleaves to form dihydroxyacetone phosphate and glyceraldehyde phosphate. These are both then stripped of phosphate group (with a small profit), to yield pyruvate and NADH. The NADH is metabolically expensive, so it must be regenerated back to NAD⁺. Under anaerobic conditions, this is done by reacting it with pyruvate to yield reduced waste products, either two moles of lactate (animals, reversible), or two moles each of ethanol and carbon dioxide (plants and fungi, irreversible). Under aerobic conditions, NADH is fed to oxidative phosphorylation, and the pyruvate is further degraded to carbon dioxide by Kreb's cycle.

The yield of ATP from glycolysis is about 2 per glucose or 3 per starch-unit.

Kreb's cycle

This occurs exclusively in aerobic mitochondria, generating carbon dioxide, NADH, ATP and FADH₂. Pyruvate is fed into the cycle as acetyl-coenzyme A (acetyl-mercaptoethanol-pantothenate-ADP). The enzyme converting pyruvate to ACoA is pyruvate decarboxylase: it removes one of pyruvate's three carbons as carbon dioxide. ACoA combines with oxaloacetate produced by Kreb's cycle to produce citrate. By successive oxidation reactions (via ketoglutarate, succinate and malate), two moles of carbon dioxide are released from the remains of the ACoA, regenerating oxaloacetate, and producing reduced nucleotides (mostly NADH), which are fed to oxidative phosphorylation.

Oxidative phosphorylation

OxPhos works by using the energy of electrons from NADH to drive the pumping of protons from one side of the mitochondrial membrane (crista) to the other.

NADH is fed into OxPhos via NADH dehydrogenase, pumping three protons from the matrix side to the cytosolic side of the crista. Succinate feeds in via the FADH₂ in succinate dehydrogenase, without proton pumping. Both of these electron sources feed into the ubiquinone pool, which pumps four protons per reduced nucleotide via the Q-cycle, in interaction with cytochrome reductase (cyt-bc). The electrons are then fed to mobile cytochrome-c (on the cytosolic face), and thence to cytochrome oxidase (cyt-aa₃, cyanide inhibited). Cytochrome oxidase uses the electrons to reduce oxygen to water, with the removal of two protons from the matrix side of the crista.

All the protons pumped by the crista rections then flow back through the F₀F₁ATPase, which generates 1 mole of ATP on the matrix side per three protons.

Energy budget per glucose

Glycolysis: 2 ATP (and 2 NADH).
Kreb's cycle: 2 ATP (and 8 NADH, 2 FADH₂).
Oxidative phosphorylation:
10 NADH yields 90 H⁺ and 2 FADH₂ yields 12 H⁺.
102 H⁺ yields 34 ATP.

Overall 1 glucose yields 38 ATP, but the reactions are not 100% efficient, and some protons are used to pump biochemicals into the mitochondrion (NADH, etc.), so the exact number is meaningless, and variable. However, aerobic respiration is about 15 times more efficient than anaerobic.