Acetyl coenzyme A, or better known as acetyl-CoA, is an important molecule used in metabolic processes. It is primarily used by the body for energy production through the citric acid cycle, or Krebs cycle.
Acetyl-CoA is a product of the oxidation of several amino acids, pyruvate and fatty acids. It is formed after pyruvate enters the mitochondria via active transport. The oxidation transformation that converts pyruvate into acetyl-CoA is known as a pyruvate dehydrogenase reaction. In this process, each pyruvic acid molecule loses one carbon atom, and combines with oxygen to produce carbon dioxide. This carbon dioxide is then exhaled out of the cell. This process is catalyzed by a complex of three enzymes through a process known as pyruvate dehydrogenase complex (PDC). 
The main function of Acetyl-CoA is to carry acyl groups or thioesters. It is the precursor to HMG CoA, an important part of cholesterol and ketone synthesis. It can also be found as a vital reagent in the synthesis of fatty acids and sterols, as well as the oxidation of fatty acids as well as the breaking down of many amino acids. 
Acetyl-CoA is well known as the junction between Glycolysis and the Citric Acid Cycle as well as an essential component in balancing between carbohydrate and fat metabolism. Acetyl-CoA has also been a central metabolite that is involved in many metabolic transformations within the cell. The acetyl group of the acetyl-CoA is used to oxidize via the TCA cycle to reduce NAD+ and FAD to NADH and FADH2, respectively. These products are then used to fuel ATP production through the electron transport train.
In May 2011, Ling Cai et al. found that Acetyl-Coa functioned as a carbon-source rheostat that signals the initiation of the cellular growth program by promoting the acetylation of histones specifically at growth genes. By intimately coordinating with nutrient availability, the acetyl-CoA was concluded to play as a critical signal for metabolic growth and proliferation.
Activation of acetyl-CoA: Some acetyl groups within the cell are tagged with coenzyme A (CoA). Acetyl-CoA molecules are tagged at the back end by a process called condensation with carbon dioxide, catalyzed by acetyl-CoA carboxylase in order to redirect acetyl groups into fatty acid biosynthesis.