Structure of the titin kinase domain (PDB 1tki chain A). Functional residues are represented as ball and stick, D24874 is the catalytic aspartate. The ATP binding site includes residue K24783 as well as the nearby yellow loop. The calcium/calmodulin binding helix is colored blue, the helix in orange blocks the ATP binding site in this autoinhibited conformation. Residue Q24857 is solvent exposed on the side opposite to the functional residues