Two kids are brought to the clinic by their mothers. The first one’s Dalia, a 2 year old girl. Her mother is concerned because Dalia always seems to be tired and weak, and in general doesn’t eat much. On physical examination of the abdomen, you palpate an enlarged liver. You decide to run a blood test, which reveals that her blood glucose and ketone bodies are decreased, but what really stands out to you is that her carnitine levels are also really low.
After Dalia, you see Luca, a 3 year old boy who had a brief seizure earlier that day. Luca’s mother tells you that he’s had gastroenteritis for the past few days, so he’s been vomiting and not eating much. You decide to run a blood test, which also reveals low blood glucose and ketone bodies, but unlike Dalia, he has high levels of fatty acyl-carnitine.
Based on the initial presentation, both Dalia and Luca seem to have some fatty acid metabolism disorder. Now, let’s review fatty acid metabolism real quick. Normally, the body's main source of energy is the glucose we get from food. When glucose is running low, like with prolonged fasting or exercise, the body is able to obtain energy from stored fats.
The simplest form of fats are fatty acids, which are grouped by length into short, medium, long, and very long chain fatty acids. Short and medium chain fatty acids are primarily obtained from the diet, while long and very long chain fatty acids can be synthesized from acetyl-CoA by the liver and fat cells.
Now, keep in mind that acetyl-CoA is usually found in the mitochondrial matrix, whereas the enzymes required for fatty acid synthesis are all in the cytoplasm. For acetyl-CoA to cross the mitochondrial membranes and get to the cytoplasm, it first needs to combine with oxaloacetate to form citrate. Once in the cytoplasm, an enzyme called citrate lyase leaves citrate back into acetyl-CoA and oxaloacetate. This whole process is called the citrate shuttle.