Amino acids are the basic units from which proteins are made. Over 170 amino acids are currently known to occur in cells and tissues.of these, only 20 are commonly found in proteins. Plants are able to make all the amino acids they require from simpler substances. However, animals are unable to synthesize all that they need, and therefore must obtain the other amino acids directly from their diet. These are termed essential amino acids.
ABC transporters play important roles in all types of organisms by participating in physiological and pathological processes. In order to modulate the function of ABC transporters, detailed knowledge regarding their structure and dynamics is necessary. Available structures of ABC proteins indicate three major conformations, a nucleotide-bound “bottom-closed” state with the two nucleotide binding domains (NBDs) tightly closed, and two nucleotide-free conformations, the “bottom-closed” and the “bottom-open”, which differ in the extent of separation of the NBDs. However, it remains a question how the widely open conformation should be interpreted, and whether hydrolysis at one of the sites can drive conformational transitions while the NBDs remain in contact