Krebs Cycle or Citric Acid Cycle or TCA Cycle

The two molecules of acetyl CoA generated by the link reaction now enter the Krebs cycle, named for its discoverer, the German biochemist Sir Hans Adolf Krebs, who worked it out in 1937 and later won the 1953 Nobel Prize in Physiology for the discovery. Structurally, the cycle takes place inside a mitochondrion whose inner membrane is folded into finger-like cristae carrying numerous F1 'oxysome' particles (the stalked ATP-synthase heads) that project into the matrix; nearly all the cycle's enzymes sit free in the mitochondrial matrix, with the single exception of succinate dehydrogenase, which is embedded in the inner membrane itself. The cycle begins with the condensation of acetyl CoA with a four-carbon acceptor molecule, oxaloacetate, in the presence of water, to form the six-carbon citrate (citric acid) -- which is why the pathway is also called the Citric Acid Cycle (CAC) or Tricarboxylic Acid (TCA) Cycle. From citrate, a fixed sequence of enzyme-catalysed steps regenerates oxaloacetate to complete the cycle: citrate is isomerised via cis-aconitate to isocitrate (aconitase); isocitrate is oxidised and decarboxylated to alpha-ketoglutarate, releasing CO2 and reducing NAD+ (isocitrate dehydrogenase); alpha-ketoglutarate is oxidatively decarboxylated to succinyl CoA, again releasing CO2 and reducing NAD+ (alpha-ketoglutarate dehydrogenase); succinyl CoA is converted to succinate by succinyl CoA synthetase (succinate thiokinase), which couples this step to substrate-level phosphorylation -- ATP synthesis directly from a substrate without involving the electron transport chain (in animal cells this step actually generates GTP, which is then converted to ATP); succinate is oxidised to fumarate by succinate dehydrogenase, reducing FAD to FADH2; fumarate is hydrated to malate by fumarase; and finally malate is oxidised back to oxaloacetate by malate dehydrogenase, reducing one more NAD+. Across three steps (4, 5 and 9 in the numbered pathway) NAD+ is reduced, and at step 7 FAD is reduced to FADH2. Because two pyruvate molecules (hence two acetyl CoA) are produced per glucose, the Krebs cycle turns twice per glucose molecule, and the combined summary for the link reaction plus one full turn is $\text{Pyruvic acid} + 4NAD^+ + FAD + 4H_2O + ADP + P_i \rightarrow 3CO_2 + 4NADH + 4H^+ + FADH_2 + H_2O + ATP$; doubled for two pyruvates, the cycle yields 6 CO2, 8 $NADH+H^+$, 2 $FADH_2$ and 2 ATP per glucose. Beyond energy production, the Krebs cycle has several other roles that make it central to plant metabolism: it supplies carbon skeletons for a wide range of anabolic (biosynthetic) reactions; several of its intermediates are further used to build amino acids, proteins and nucleic acids; succinyl CoA specifically is a precursor for chlorophylls, cytochromes, phytochrome and other pyrrole-based pigments; alpha-ketoglutarate and oxaloacetate can undergo reductive amination to form amino acids directly; and it functions as a metabolic 'sink' at the centre of the cell's intermediary metabolism. Because it is simultaneously a catabolic pathway (breaking down acetyl CoA) and an anabolic one (supplying biosynthetic precursors), the Krebs cycle is described as amphibolic. This dual role also lets the cycle process fats and proteins, not just carbohydrates: fats are …