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Superimposed on this structure are red arrowheads to indicate sites where insertions of 5–100 amino acids are found in some members of the protein kinase family. These insertions are located in loops on the surface of the enzyme where other ligands interact with the protein. Thus, they distinguish different kinases and confer on them distinctive interactions with other proteins. The ATP (which donates a phosphate group) and the peptide to be phosphorylated are held in the active site, which extends between the phosphate-binding loop (yellow) and the catalytic loop (orange). See also Figure 3-12. (Adapted from D.R. Knighton et al., Science 253:407–414, 1991.)