Pyruvic acid occupies an important position in several pathways, and different organisms handle it in different ways (figure 1). In strictly aerobic organisms and some anaerobes, pyruvic acid is channeled into the Krebs cycle for processing and energy release. Facultative anaerobes can adopt a fermentative metabolism, in which pyruvic acid is further reduced into acids or other products.
Fig1. The fates of pyruvic acid (pyruvate). This metabolite is an important hub in the processing of nutrients by microbes. It may be fermented anaerobically to several end products or oxidized completely to CO2 and H2O through the Krebs cycle and the electron transport system. It can also serve as a source of raw material for synthesizing amino acids and carbohydrates.
The Preparation of Pyruvic Acid for the Krebs Cycle
The oxidation of glucose by glycolysis yields a comparatively small amount of energy. Pyruvic acid is still energy-rich, containing a number of extractable electrons to power ATP synthesis, and this will be achieved through the work of the second and third phases of respiration, during which pyruvic acid is converted to CO2 and H2O. In the following section, we examine the second phase— the Krebs cycle—which takes place in the mitochondrial matrix in eukaryotes and in the cytoplasm of bacteria.
To connect the glycolysis path way to the Krebs cycle, the pyruvic acid is first converted to the starting compound for that cycle (process figure 2, tan oval at top). This step brings in an enzyme complex and coenzyme A to participate in the oxidation of pyruvic acid, the reduction of NAD+ to NADH, and the decarboxylation of pyruvic acid to a 2-carbon acetyl group accompanied by release of carbon dioxide. The acetyl group remains attached to coenzyme A, forming acetyl coenzyme A (acetyl CoA) that feeds into the Krebs cycle. Pyruvate dehydrogenase, the enzyme complex that makes this reaction possible, is huge. In E. coli, it contains four vitamins and has a molecular weight of 6 million—larger than a ribosome!
Process Fig2. 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.
The NADH produced in the formation of acetyl CoA will be shuttled into electron transport and used to generate ATP via oxidative phosphorylation in the third phase of aerobic respiration.
Keep in mind that all reactions during these subsequent phases happen twice for each glucose because of the two pyruvates that are released during glycolysis.
