- Beta oxidation
Beta oxidation is the process by which
fatty acids, in the form of Acyl-CoAmolecules, are broken down in mitochondriaand/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Krebs Cycle.
Activation of fatty acids
Free fatty acids can penetrate the plasma membrane due to their poor water solubility and high fat solubility. Once in the cytosol, a fatty acid reacts with ATP to give a fatty acyl adenylate, plus inorganic pyrophosphate. This reactive acyl adenylate then reacts with free
coenzyme Ato give a fatty acyl-CoA ester plus AMP.
Four recurring steps
Once inside the mitochondria, the β-oxidation of fatty acids occurs via four recurring steps:
For sources that use the larger ATP production numbers described above, the total would be 129 ATP equivalents per palmitate.
Beta-oxidation of unsaturated fatty acids changes the ATP yield due to the requirement of two possible additional enzymes. If a cis-Δ3 double bond is encountered, the action of enoyl-CoA isomerase replaces the action of acyl-CoA dehydrogenase, meaning no FADH2 will be generated. Thus, for each cis-Δ3 double bond enocountered, ATP yield is altered by -1.5 ATP. However, if a cis-Δ4 double bond is encountered, the action of 2,4-dienoyl-CoA reductase is required after the acyl-CoA dehydrogenase step (before the cycle can continue), which uses a molecule of NADPH, equivalent to 3.5 ATP (some sources say 4 ATP). Thus, each cis-Δ4 double bond encountered in a fatty acid molecule alters the ATP yield by -3.5 ATP.
Regulation of Beta Oxidation
Malonyl-CoA can act to prevent fatty acyl-CoA derivatives from entering the mitochondria by inhibiting the carnitine acyltranferase that is responsible for this transport. Thus, the beta oxidation pathway is inhibited. When fatty acyl-CoA levels rise, beta oxidation is stimulated. However, increased citrate levels inhibit beta oxidation.
Fatty acid metabolism
* [http://www.ufp.pt/~pedros/bq/fatty.htm The chemical logic behind fatty acid metabolism] at ufp.pt
* [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=oxidation+yield+AND+stryer%5Bbook%5D+AND+216592%5Buid%5D&rid=stryer.section.3050#3060 JEREMY M. BERG,JOHN L. TYMOCZKO and LUBERT STRYER Biochemistry, 2002 ]
* [http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/carnitine/carnitine1.html Animations] at brookscole.com
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