Some of the most important functions of mitochondria are as follows:
1. The Glyoxylate Cycle 2. Fatty Acid Oxidation 3. Fatty Acid Chain Elongation 4. Superoxide Dismutase and Catalase 5. Amphibolic and Anaplerotic Reactions.
Mitochondria are generally described as the powerhouses of the cell, and as such most attention is directed to the processes that evolve energy, namely, the TCA cycle, electron transport, and oxidative phosphorylation.
However, a number of other processes also take place in mitochondria.
1. The Glyoxylate Cycle:
The enzymes are localized in the matrix and some of them are the same as those of the TCA cycle. In effect, the glyoxylate cycle is a modified form of the Krebs cycle, but its function appears to be primarily associated with conversion of acetate produced by the catabolism of fatty acids into oxaloacetate.
Oxaloacetate is also an important intermediate in the conversion of fatty acids to carbohydrates. Whereas animals lack certain enzymes of the glyoxylate cycle and are incapable of converting fatty acids to carbohydrates using this pathway, plants and microorganisms have functional glyoxylate systems. In higher plants, some of the enzymes of the system (e.g., isocitrate lyase and malate synthetase) are localized in specific organelle called glyoxysomes as well as in mitochondria.
2. Fatty Acid Oxidation:
Free fatty acids are rarely found in more than trace quantities in cells because they are highly toxic. The fatty acids associated with mono-, di-, and triglycerides and in phospholipids are generally hydrolyzed from the glycerol in the cytosol and immediately activated for transport into the matrix of the mitochondrion, where they are then oxidized.
3. Fatty Acid Chain Elongation:
Fatty acids are generally synthesized by the smooth endoplasmic reticulum. However, there are a number of enzymes in mitochondria that catalyze the elongation of palmitic and other saturated fatty acids by successive additions of acetyl-CoA to the carboxyl end. In smooth ER, both unsaturated and saturated fatty acids are elongated but by the addition of malonyl-CoA rather than acetyl-CoA.
4. Superoxide Dismutase and Catalase:
During electron transport, a number of toxic reductive products of oxygen are formed. The most common are the superoxide radical (O2–) and hydrogen peroxide (H2O2). A protective enzyme has been identified in mitochondria called superoxide dismutase, which converts O2– and H+ into hydrogen peroxide. The hydrogen peroxide, in turn, is decomposed by catalase, that is,
5. Amphibolic and Anaplerotic Reactions:
The intermediates of the TCA cycle can act as precursors of a variety of products of anabolic pathways (Table 16-3). The TCA cycle thus is amphibolic in that it can act in both a catabolic and anabolic manner. These anabolic reactions can drain intermediates away from the TCA cycle and thus deplete the oxaloacetate supply necessary to keep the cycle functioning. Reactions that supply the TCA cycle with intermediates that replace those lost are called anaplerotic reactions. Several anaplerotic reactions of the Krebs cycle are shown in Table 16-4.
