Krebs cycle: a standalone construct

Hans Krebs and William Johnson first set out this pathway in 1937, in work on how animal tissues burn their fuel, showing that citric acid sits at the center of a repeating cycle of chemical reactions rather than at the end of a one-way chain; Krebs later gave a full account of its history and logic in 1970, and it now anchors every modern biochemistry text. The cycle takes acetyl-CoA, the two-carbon fragment left after the body breaks down sugars, fats, and many amino acids, and oxidizes it step by step to carbon dioxide, the gas we breathe out. Each turn regenerates the molecule it began with, oxaloacetate, so the pathway runs continuously, and along the way it charges the electron carriers NADH and FADH2, molecules that ferry high-energy electrons to the cell structures where most of its usable energy is made. It also draws off intermediate compounds for the building of amino acids, fats, and other cell parts, so it serves both to release energy and to supply raw material. Its position at the hub where the breakdown of carbohydrates, fats, and proteins converges is what makes it a landmark of metabolism, present in nearly all oxygen-using life.[1][2][3]

Unit

A closed sequence of enzyme-catalysed reactions in central metabolism, one complete turn oxidising one acetyl group to two molecules of carbon dioxide

Recovery means

Not applicable

Measured by

Traced by isotope labelling of individual carbon atoms, enzyme assays of its component steps, and measurement of substrate, intermediate, and product concentrations in tissue and mitochondria