(a) (b) (c) Models of designed D2-symmetric α-helical coiled-coil proteins containing two (a) and four (b), (c) iron porphyrin cofactors. Blue helices represent capping sequences. Green helices represent the metalloporphyrin-binding region, (d) Sequence of the two-porphyrin array (2PA) (e) Sequence of the four-porphyrin array (4PA). (f) Sequence of the four-porphyrin array with capping sequences removed (4PAwithout cap). Sequences (d), (e), and (f) correspond to models (a), (b), and (c), respectively. (g) Chemical structure of the iron porphyrin cofactor.