Fatty acids

What are fatty acids

a reduced carbon source that produces large amounts of energy for bacteria. when sugars are limited fatty acids are used as a main carbon and energy source.

Key pathway for fatty acid breakdown

b-oxidation which is the break down of fatty acids so they can be used for energy this process removes two carbon units at a time and converts them into acetyl-CoA which can then enter the TCA cycle or glyoxulate shunt

Activation of Fatty acids

before the fatty acid is oxidised it must be activated. this is done by acyl CoA synthetase which attracts coenzyme A to the fatty acid making fatty acyl CoA. this traps the fatty acid inside the cell and commits it ti degradation. once the fatty acid is activated it can enter b-oxidation cycle

first step of b-oxidation

dehydrogenation catalysed by acyl-CoA dehydrogenase which introduced a double bond between the a and b carbons and produces FADH2.

Step 2 of b-oxidation cycle

hydration of the double bond by enoyl-CoA hydratase which forms a hydroxyl group

3 step

dehydrogenation carried out by hydroxacyl-CoA dehydrogenase which convertys the hydroxyl group to a keto group while producing NADH

4 step

thiolysis where b-ketothiolase cleaves the molecule and releases one acetyl-CoA while leaving behind a shortened acyl-CoA. the shortened acyl-CoA re-enters the cycle and the process continues until the entire fatty acid chain has been converted to acetyl CoA

energy yield

Each cycle produces one NADH and one FADH2 which feed electrons to the electron transport chain which feed oxidative phosphorlylation which produces ATP. in addition acetyl CoA will enter the TCA cycle and produce NADH and FADH2 and ATP or GTP. they are more energy rich than glucose. in anaerobic conditions they are not as NADH and FADH2 cannot be fully oxidised

Connection to the TCA cycle and glyoxylate stunt

acetyl CoA enters the TCA cycle and is oxidised to carbon dioxide. TCA cycle also produced reducing power. however when fatty acids or acetate are the only carbon source each turn of the TCA cycle loses two molecules of CO2 to avoid this bacteria use the glyoxylate shunt which bypasses the decorboxylation steps.

fatty acid catabolism and hydrocarbon degradation

B-oxidation is also important in the breakdown of hydrocarbons such as alkanes which are first oxidised by monooxygenases to form alcohols which are then converted into aldehydes and fatty acids. Once in fatty acid form these molecules enter B-oxidation.

special cases

old-chain fatty acids go through the cycle until the final step produces propionyl-CoA. this cannot enter TCA cycle and had to be converted to succinyl CoA before it can be further oxidised. Unsaturated fatty acids require additional reactions to reduce double bonds before they can go thorugh th cycle

regulations

When fatty acids or hydrocarbons are available bacteria induce genes encoding b-oxidation enzymes and when glucose is abundant they repress fatty acid catabolism and rely on glycolysis. The glyoxylate shunt is activated when cells need to cons erve carbon