But before the target cells can use the fatty acids for ATP production and β oxidation, the fatty acids with chain lengths of 14 or more carbons must be activated and subsequently transported into mitochondrial matrix of the cells in three enzymatic reactions of the carnitine shuttle.The first reaction of the carnitine shuttle is a two-step process catalyzed by a family of isozymes of acyl-CoA synthetase that are found in the outer mitochondrial membrane, where they promote the activation of fatty acids by forming a thioester bond between the fatty acid carboxyl group and the thiol group of coenzyme A to yield a fatty acyl–CoA.In the first step of the reaction, acyl-CoA synthetase catalyzes the transfer of adenosine monophosphate group (AMP) from an ATP molecule onto the fatty acid generating a fatty acyl–adenylate intermediate and a pyrophosphate group (PPi). The pyrophosphate, formed from the hydrolysis of the two high-energy bonds in ATP, is immediately hydrolyzed to two molecules of Pi by inorganic pyrophosphatase. This reaction is highly exergonic which drives the activation reaction forward and makes it more favorable. In the second step, the thiol group of a cytosolic coenzyme A attacks the acyl-adenylate, displacing AMP to form thioester fatty acyl-CoA.In the second reaction, acyl-CoA is transiently attached to the hydroxyl group of carnitine to form fatty acylcarnitine.
Activation
Carnitine shuttle activation occurs due to a need for fatty acid oxidation which is required for energy production. During vigorous muscle contraction or during fasting, ATP concentration decreases and AMP concentration increases leading to the activation of AMP-activated protein kinase (AMPK). AMPK phosphorylates acetyl-CoA carboxylase, which normally catalyzes malonyl-CoA synthesis. This phosphorylation inhibits acetyl-CoA carboxylase, which in turn lowers the concentration of malonyl-CoA. Lower levels of malonyl-CoA disinhibit carnitine acyltransferase 1, allowing fatty acid import to the mitochondria, ultimately replenishing the supply of ATP. Transcription factors
Peroxisome proliferator-activated receptor alpha (PPARα) is a nuclear receptor that functions as a transcription factor. It acts in muscle, adipose tissue, and liver to turn on a set of genes essential for fatty acid oxidation, including the fatty acid transporters carnitine acyltransferases 1 and 2, the fatty acyl–CoA dehydrogenases for short, medium, long, and very long acyl chains, and related enzymes.PPARα functions as a transcription factor in two cases; as mentioned before when there is an increased demand for energy from fat catabolism, such as during a fast between meals or long-term starvation. Besides that, the transition from fetal to neonatal metabolism in the heart. In the fetus, fuel sources in the heart muscle are glucose and lactate, but in the neonatal heart, fatty acids are the main fuel that require the PPARα to be activated so it is able in turn to activate the genes essential for fatty acid metabolism in this stage.