Quiz 9: The Krebs Cycle


The cyclic pathway (Kreb's cycle) is fundamentally different from the glycolysis (linear pathway) because in cyclic pathway, the starting substrate (oxaloacetate) is again produced at the end. This provides a benefit that the cycle can be under tight regulation. The interruption occurs only when any of the intermediate diverts its pathway from Kreb's cycle to some other pathway which may occur due to any possible reason. On the other hand, a linear pathway, unlike cyclic pathway, does not recycle the starting metabolite back, at the end. Thus, the cyclic pathway is catalytic because its intermediates are regenerated after each cycle.

There are two basic strategies of Krebs cycle to oxidize and extract electron from the molecules. These are as follows: 1. Dehydrogenases: These are the enzymes that remove electrons, and thus, oxidize the substrate. The electrons that are removed are accepted by the coenzyme NAD + (nicotinamide adenine dinucleotide). The NAD + then get reduced to NADH. 2. The GDP (guanosine diphosphate) also helps to extract the electrons as phosphate ion from the molecule and removes forming GTP (guanosine triphosphate).

The fluoroacetate can be a poison for the body as it converts to fluorocitrate that irreversibly binds to the aconitase enzyme of Krebs cycle. It thus blocks the Krebs cycle and may prove fatal if not treated. This mechanism is defined as competitive inhibition. Both the substrate and inhibitor molecules are similar in structure, and can compete for the same binding site of the enzyme. It happens as follows: img