The Krebs Cycle
The oxidation of glucose yields a comparatively small amount of energy and gives off pyruvic acid. Pyruvic acid is still energy-rich, containing a number of extractable hydrogens and electrons to power ATP synthesis, but this can be achieved only through the work of the second and third phases of respiration, in which pyruvic acid’s hydrogens are transferred to oxygen, producing CO2 and H2O. In the following section, we examine the next phase of this process, which takes place in the cytoplasm of bacteria and in the mitochondrial matrix in eukaryotes. To connect the glycolysis pathway to the Krebs cycle, for either aerobic or anaerobic respiration, the pyruvic acid is first converted to a starting compound for that cycle. This step involves the first oxidation-reduction reaction of this phase of respiration, and it also releases the first carbon dioxide molecule. It involves a cluster of enzymes and coenzyme A that participate in the dehydrogenation (oxidation) of pyruvic acid, the reduction of NAD to NADH, and the decarboxylation of pyruvic acid to a 2-carbon acetyl group. The acetyl group remains attached to coenzyme A, forming acetyl coenzyme A (acetyl CoA) that feeds into the Krebs cycle. The NADH formed during this reaction will be shuttled into electron transport and used to generate ATP via oxidative phosphorylation.
- Keep in mind that all reactions described actually happen twice for each glucose because of the two pyruvates that are released during glycolysis.
Steps in the Krebs Cycle
The Krebs cycle has eight steps, beginning with citric acid formation and ending with oxaloacetic acid. As we take a single spin around the Krebs cycle, it will be helpful to keep track of
- the numbers of carbons (#C) of each substrate and product,
- reactions where CO2 is generated,
- the involvement of the electron carriers NAD and FAD, and
- the site of ATP synthesis.
The reactions in the Krebs cycle follow:
- Oxaloacetic acid (oxaloacetate; 4C) reacts with the acetyl group (2C) on acetyl CoA, thereby forming citric acid (citrate; 6C) and releasing coenzyme A so it can join with another acetyl group.
- Citric acid is converted to its isomer, isocitric acid (iso citrate; 6C), to prepare this substrate for the decarboxylation and dehydrogenation of the next step.
- Isocitric acid 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-ketoglutaric acid (α-ketoglutarate; 5C).
- Alpha-ketoglutaric acid serves as a substrate for the last 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 the original 3-carbon pyruvic acid that began the Krebs. The remaining steps are needed not only to regenerate the oxaloacetic acid to start the cycle again but also to extract more energy from the intermediate compounds leading to oxaloacetic acid.
- Succinyl CoA is the source of the one substrate-level phosphorylation in the Krebs cycle. In most microbes, it proceeds with the formation of ATP. The product of this reaction is succinic acid (succinate; 4C).
- Succinic acid next becomes dehydrogenated, 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 cristae of eukaryotic cells. FADH2 then directly enters the electron transport system. Fumaric acid (fumarate; 4C) is the product of this reaction.
- The addition of water to fumaric acid (called hydration) results in malic acid (malate; 4C). This is one of the few reactions in respiration that directly incorporates water.
- Malic acid is dehydrogenated (with formation of a final NADH), and oxaloacetic acid is formed. This step brings the cycle back to its original starting position, where oxaloacetic acid can react with acetyl coenzyme A.
The Krebs cycle serves to transfer the energy stored in acetyl CoA to NAD+ and FAD by reducing them (transferring hydrogen ions to them). Thus, the main products of the Krebs cycle are these reduced molecules (as well as 2 ATPs for each glucose molecule). The reduced coenzymes NADH and FADH2 are vital to the energy production that will occur in electron transport. Along the way the 2-carbon acetyl CoA joins with a 4-carbon compound, oxaloacetic acid, and then participates in seven additional chemical transformations while “spinning off” the NADH and FADH2. That’s why we called the Krebs cycle the “carbon and energy wheel” in a preceding heading.