Search | Send us your comments

Catabolism of Fats (Lipids)

©2000 Timothy Paustian, University of Wisconsin-Madison

Up until this point you might think sugars are the only substrates that bacteria can grow on. Nothing could be farther from the truth. Bacteria are able to grow on all sorts of substrates and in this section we cover how fats are catabolized.

Bacteria are capable of growth on fatty acids and lipids. Lipids are part of the membranes of living organisms and if available (usually because the organism that was using them dies) can be used as a food source. Lipids are large molecules and cannot be transported across the membrane. A class of extracellular enzymes called lipases are responsible for the breakdown of lipids. Lipases attack the bond between the glycerol molecule oxygen and the fatty acid. Phospholipids are attacked by phospholipases. There are four classes of phospholipases given different names depending upon the bond they cleave. Phospholipases are not particular about their substrate and will attack a glycerol ester linkage containing any length fatty acid attached to it. The result of this digestion is a hydrophillic head molecule, glycerol and fatty acids of various chain lengths. The head can be one of several small organic molecules that are funneled into the TCA cycle by one or two reactions that we won't cover here. Glycerol is converted into 3-Phosphoglycerate (depending upon the action of phospholipase C or phospholipase D) and eventually pyruvate via glycolysis. This leaves the fatty acids to deal with.

Figure 1 - Catabolism of a lipid.

Fatty acids are degraded by a four step process called b-oxidation. The fatty acid is first activated by the addition of Coenzyme-A to the end. This activation requires energy in the form of ATP, but is only performed once per fatty acid degraded. The b carbon (see figure) is then oxidized from CH₂ to C=O (a ketone) by three reactions. (This is where the pathway gets its name.) The oxidized b group is now susceptible to attack. An enzyme called b-ketothiolase splits the fatty acid into acetyl-CoA and adds another Coenzyme-A to the previously oxidized b group on the fatty acid.

Figure 2 - Pathway of b-oxidation. The blue box indicated the beta carbon and the red box the acetyl group that eventually gets split off by b-ketothiolase. The purple boxes indicate high energy compounds.

The fatty acid is now two carbons shorter and an Acetyl-CoA has been generated which can be fed into the TCA cycle. The smaller fatty acid moves through the b-oxidation pathway again, producing another Acetyl-CoA and shrinking by 2 carbons. By performing successive rounds of beta oxidation on a fatty acid, it is possible to convert it completely to Acetyl-CoA. The perceptive reader might notice that for fatty acids with odd numbers of carbons, the final reaction will yield acetyl-CoA and Coenzyme-A hooked to a three carbon fatty acid (propionyl-CoA). Propionyl-CoA is handled differently by different bacteria. In E. coli it is converted into pyruvate.

When comparing catabolism of fats and sugars two points jump out.

1. Reuse of components - Whenever possible, the cell will reuse a carrier, cofactor or enzyme. For example in fatty acid breakdown, Coenzyme-A plays a major role and the electron carriers FAD and NAD are used.

2. Funneling - Cells also try to reuse common pathways. A given substrate will be converted into a common metabolite and then funneled into an already existing pathway. Fatty acids are broken down into Acetyl-Coenzyme-A and this is fed into the TCA cycle.

Keep these points in mind as we cover other metabolic pathways.

[Previous] | [Next]