At times, excessive acetyl CoA is produced after lipolysis. This excess acetyl CoA does not enter the citric acid cycle for energy production, instead gets converted into compounds known as ketone bodies. Acetyl CoA transforms into a compound known as hydroxymethyl glutaryl CoA (HMG CoA). HMG CoA then gets converted into acetoacetate, which eventually gets converted into a ketone body known as beta-hydroxybutyrate. The process of production of ketone bodies is referred to as ketogenesis. Using this process, fats are metabolized and this serves as an alternate pathway for the generation of energy. When blood glucose levels become low in conditions of starvation, energy is generated via these ketone bodies. It can also be utilized as a source of energy in diabetic patients. Major organs like the brain derive energy from ketone bodies when there is a depletion of glucose levels in the body. The process by which the ketone bodies get converted into acetyl CoA for energy production is known as ketone body oxidation. When energy is needed, beta-hydroxybutyrate converts into acetoacetate, which combines with a molecule of CoA-HS to produce acetoacetyl CoA. This acetoacetyl CoA gets split into two acetyl CoA molecules, which are channelized into the citric acid cycle for the generation of energy. When the production rate of ketone bodies is higher than their utilization rate, they dissociate to form acetone and carbon dioxide. Acetone is excreted from the body through the exhaled breath. Blood can become acidic with a high amount of carbon dioxide, leading to a life-threatening condition known as ketoacidosis.
Lipogenesis
High levels of glucose in the body can produce excessive amounts of acetyl CoA. Excess glucose arises either due to high carbohydrate intake or improper carbohydrate metabolism. The extra acetyl CoA gets converted into different lipids like fatty acids, cholesterol, triglycerides, and steroids. Lipogenesis is the process by which acetyl CoA gets converted into fat molecules. This is therefore a fat metabolizing process. This process takes place in the cytoplasm of fat cells and liver cells.
Acetyl CoA generated within the mitochondria cannot cross over to enter the cytoplasm.
For this process, pyruvate is utilized. In the presence of pyruvate carboxylase, it is converted into oxaloacetate. Simultaneously, the catalytic action of pyruvate dehydrogenase converts pyruvate into acetyl CoA.
Acetyl CoA and oxaloacetate react to produce the compound citrate.
Citrate is capable of crossing the membrane and entering the cytoplasm. Here, it forms oxaloacetate and acetyl CoA.
The acetyl CoA becomes malonyl CoA, which is used for fatty acid synthesis.
Energy is utilized for the production of fatty acids from acetyl CoA.
