The crystal structure of the ferric form of myoglobin from the mollusc Aplysia limacina has been refined at 1.6 Å resolution, by restrained crystallographic refinement methods. The crystallographic R-factor is 0.19. The tertiary structure of the molecule conforms to the common globin fold, consisting of eight α-helices. The N-terminal helix A and helix G deviate significantly from linearity. The distal residue is recognized as Val63 (E7), which, however, does not contact the heme directly. Moreover the sixth (distal) co-ordination position of heme iron is not occupied by a water molecule at neutrality, i.e. below the acid-alkaline transition point of A. limacina myoglobin. The heme group sits in its crevice in the conventional orientation and no signs of heme isomerism are evident. The iron atom is 0.26 Å out of the porphyrin plane, with a mean FeN (porphyrin) distance of 2.01 Å. The co-ordination bond to the proximal histidine has a length of 2.05 Å, and forms an angle of 4 ° with the heme normal. A plane containing the imidazole ring of the proximal His intersects the heme at an angle of 29 ° with the (porphyrin) 4N2N direction. Inspection of the structure of pH 9.0 indicates that a hydroxyl ion is bound to the Fe sixth co-ordination position.
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