Proteins that contain linked binding sites are called allosteric proteins and can be of various types, such as enzymes, receptors, structural proteins, and motor proteins. For example, aspartate transcarbamoylase, also known as ATCase, is a large enzyme containing six catalytic subunits, each with a substrate binding site. It also has six additional regulatory subunits, each with a site that can bind both pyrimidines and purines, the building blocks of DNA and RNA. The enzyme catalyzes an essential step in pyrimidine synthesis, the condensation of aspartate and carbamoyl phosphate to produce carbamoyl aspartate. The rate of this reaction increases in a concentration-dependent manner. Binding of ATP, a purine, to the enzyme activates the enzyme. Whereas, the simultaneous binding of UTP and CTP, pyrimidines, results in a 95% inhibition of enzyme activity. This regulatory mechanism is essential to maintain the right amount of pyrimidines relative to purines in the cell.