Final Answer:
ATP-binding cassette (ABC) transporters have two ATP-binding sites.
Step-by-step explanation:
ATP-binding cassette (ABC) transporters are a superfamily of membrane proteins involved in the active transport of various substrates across cellular membranes.
These transporters utilize energy derived from the hydrolysis of ATP to facilitate the movement of substrates against their concentration gradient. Each ABC transporter typically consists of four domains, with two nucleotide-binding domains (NBDs) responsible for binding and hydrolyzing ATP. The two ATP-binding sites, one in each NBD, play a crucial role in the energy-coupling mechanism of ABC transporters.
The ATP-binding process induces conformational changes in the transporter, enabling it to alternately expose its substrate-binding sites to both sides of the membrane. This process allows the transporter to pick up the substrate on one side of the membrane, undergo a conformational change powered by ATP hydrolysis, and release the substrate on the other side.
The presence of two ATP-binding sites is a characteristic feature of ABC transporters and is essential for their function in active transport. Understanding the structural and functional aspects of ABC transporters is crucial for comprehending various physiological processes, including drug resistance, nutrient uptake, and the regulation of ion concentrations across cellular membranes.