Propionyl-CoA carboxylase is an enzyme that plays a crucial role in the metabolism of certain fatty acids and amino acids. It catalyzes the conversion of propionyl-CoA into methylmalonyl-CoA by adding a carbon dioxide molecule. This reaction is essential for the proper breakdown of odd-chain fatty acids and some branched-chain amino acids. The enzyme requires biotin as a cofactor and is found in various tissues, including the liver and kidneys. Deficiencies in propionyl-CoA carboxylase can lead to metabolic disorders, resulting in the accumulation of propionic acid in the body, which can cause serious health issues.