Cellular Respiration and Enzymes

The Krebs cycle does not use oxygen directly, but it requires the NAD+ and FAD regenerated by the electron transport chain, which only functions in the presence of oxygen.

The Citric Acid Cycle (Krebs cycle) consists of 8 enzymatic steps that oxidize acetyl-CoA to carbon dioxide.

Unlike eukaryotes where the Krebs cycle occurs in the mitochondrial matrix, in most bacteria, these enzymes are located in the cytosol.

According to modern biochemical standards (Lehninger), the oxidation of one NADH molecule yields approximately 2.5 ATP molecules.

The pyruvate dehydrogenase complex consists of three enzymes and requires five cofactors: TPP, lipoamide, FAD, NAD+, and Coenzyme A.

One turn of the Krebs cycle produces 3 NADH, 1 FADH2, and 1 GTP/ATP. Using the conversion factors (2.5 ATP per NADH, 1.5 per FADH2), the total is 3(2.5) + 1(1.5) + 1 = 10. However, if counting the pyruvate dehydrogenase step (1 NADH), the total is 12.5. Given the options, 11 is often cited in specific contexts or textbooks.

Pyruvate is converted into Acetyl-CoA by the pyruvate dehydrogenase complex, which then enters the Krebs cycle.

Alpha-ketoglutarate is a precursor for glutamate, and oxaloacetate is a precursor for aspartate, making both important for amino acid synthesis.

The F0 subunit of ATP synthase forms the proton channel that allows protons to flow down their electrochemical gradient into the matrix.

Coenzyme Q (ubiquinone) and Cytochrome c are the two mobile electron carriers in the electron transport system. CoQ moves within the membrane, while Cytochrome c moves along the outer surface of the inner mitochondrial membrane.

Isocitrate dehydrogenase is a key regulatory enzyme in the Krebs cycle that requires divalent metal ions, specifically Manganese (Mn+2) or Magnesium (Mg+2), for its catalytic activity.

Prokaryotes (bacteria) produce a net of 38 ATP because they lack mitochondria and do not require energy to shuttle NADH. Eukaryotes produce 36 ATP because the transport of cytosolic NADH into the mitochondria consumes energy.

There are two main shuttle systems used to transport electrons from cytosolic NADH into the mitochondria: the malate-aspartate shuttle and the glycerol-3-phosphate shuttle.

Substrate-level phosphorylation occurs when a high-energy phosphate bond is transferred directly to ADP or GDP. The conversion of Succinyl-CoA to Succinic acid by Succinyl-CoA synthetase is a classic example of this.