Sequence and structural comparison between APD and 4,5-PCD. (a) Structure-based sequence alignment of the β and α chains of APD (APD-B and APD-A, respectively) and the β chain of 4,5-PCD. The secondary-structure elements of APD and 4,5-PCD are labelled on the top and bottom lines, respectively. (b) Structure superimposition on the β subunit of both enzymes: APD is in green and 4,5-PCD is in gold. The Fe atom is represented by a magenta sphere. (c) Superimposition of the αβ dimeric structures, with the α subunit of APD in red and that of 4,5-PCD in light blue. (d) Overlay of active sites. The model of APD is colour coded by atom (C in grey, N in blue, O in red and Fe in magenta) and that of 4,5-PCD in gold. The labels for APD and 4,5-PCD are in black and gold, respectively.