As you learned earlier, a cyclic pathway is one in which the starting compound is regenerated at the end of the cycle so it will be ready to go around again. The Krebs cycle has eight steps, beginning with citrate formation and ending with oxaloacetate (figure 1). The 2-carbon acetyl groups from coenzyme A combine with a 4-carbon oxaloacetate molecule to form citrate, a 6-carbon molecule. Although this seems a cumbersome way to dismantle such a small molecule, it is necessary in biological systems for extracting the remaining energy from the acetyl fragment.
Process Fig1. The reactions of a single turn of the Krebs cycle. Each glucose will produce two turns of this pathway. The top portion (a) depicts the conversion of pyruvic acid to acetyl coenzyme A, a necessary reaction that sets up the first step of Krebs. Note that this is an enlarged.
As we take a single spin around the Krebs cycle, it will be helpful to keep track of 1. the numbers of carbons (#C) of each substrate and product,
2. reactions where CO2 is generated,
3. reductions of the electron carriers NAD+ and FAD, and
4. the site of ATP synthesis.
Be aware that the terms and structures used for organic acids can be shown as either the acid form (oxaloacetic acid) or its salt (oxaloacetate). We present the salt form in this text for the most part.
The eight reactions in the Krebs cycle are:
1. Oxaloacetate (4C) reacts with the acetyl group (2C) on acetyl CoA, forming citrate (6C) and releasing coenzyme, making it immediately available for another acetyl group.
2. Citrate is converted to its isomer, isocitrate (6C), to prepare this substrate for the decarboxylation and redox reaction of the next step.
3. Isocitrate is acted upon by an enzyme complex including NAD+ or NADP (depending on the organism) in a reaction that generates NADH or NADPH, splits off a carbon dioxide, and leaves α-ketoglutarate (5C).
4. Alpha-ketoglutarate serves as a substrate for the final decarboxylation reaction and yet another redox reaction, involving coenzyme A and yielding NADH. The product is the high- energy compound succinyl CoA (4C).
At this point, the cycle has completed the formation of 3 CO2 molecules that balance out each original 3-carbon pyruvic acid released by glycolysis. The remaining steps serve not only to regenerate the oxaloacetate to start the cycle again but also to extract more energy from the intermediate compounds leading to oxaloacetate.
5. Succinyl CoA is the source of the one substrate-level phosphorylation in the Krebs cycle. In most bacteria, it proceeds with the formation of ATP, although eukaryotes produce guanosine triphosphate (GTP), an equivalent source of energy. The other product at this step is succinate (4C).
6. Succinate undergoes a redox reaction, but in this case, the electron and H+ acceptor is flavin adenine dinucleotide (FAD). The enzyme that catalyzes this reaction, succinyl dehydrogenase, is found in the bacterial cell membrane and mitochondrial crista of eukaryotic cells. The FADH2 generated directly enters the electron transport system but at a different carrier site than NADH. Fumarate (4C) is the product of this reaction.
7. The addition of H2O to fumarate results in malate (4C). This is one of the few reactions in respiration that directly incorporates a water molecule. 8. Malate is dehydrogenated (with formation of a final NADH), and oxaloacetate is formed. This step brings the cycle back to its original starting position, where the oxaloacetate molecule is available to react with acetyl coenzyme A.
An important feature of this pathway is that acetyl groups from the breakdown of certain fats can enter the pathway at step one and be used as an energy source.
The full Krebs cycle is not present in all cells, and it may not function under all metabolic conditions. But even in cells with al ternate metabolic schemes, it is essential for generating small organic molecules that microbes require for synthesis.
