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The three-dimensional structure of horse heart metmyoglobin has been refined to
a final R-factor of 15.5% for all observed data in the 6.0 to 1.9 A resolution
range. The final model consists of 1242 non-hydrogen protein atoms, 154 water
molecules and one sulfate ion. This structure has nearly ideal bonding and bond
angle geometry. A Luzzati plot of the variation in R-factor with resolution
yields an estimated mean co-ordinate error of 0.18 A. An extensive analysis of
the pattern of hydrogen bonds formed in horse heart metmyoglobin has been
completed. Over 80% of the polypeptide chain is involved in eight helical
segments, of which seven are composed mainly of alpha-helical (3.6(13))-type
hydrogen bonds; the remaining helix is composed entirely of 3(10) hydrogen
bonds. Altogether, of 102 hydrogen bonds between main-chain atoms only six are
not involved in helical structures, and four of these six occur within
beta-turns. The majority of water molecules in horse heart metmyoglobin are
found in solvent networks that range in size from two to 35 members. The size of
water molecule networks can be rationalized on the basis of three factors: the
number of hydrogen bonds to the protein surface, the presence of charged
side-chain atoms, and the ability to bridge to neighboring molecules in the
crystal lattice. Bridging water networks form the dominant intermolecular
interactions. The backbone conformation of horse heart metmyoglobin is very
similar to sperm whale metmyoglobin, with significant differences in secondary
structure occurring only near residues 119 and 120, where residues 120 to 123 in
sperm whale form a distorted type I reverse turn and the horse heart protein has
a type II turn at residues 119 to 122. Nearly all of the hydrogen bonds between
main-chain atoms (occurring mainly in helical regions) are common to both
proteins, and more than half of the hydrogen bonds involving side-chain atoms
observed in horse heart are also found in sperm whale metmyoglobin. Unlike sperm
whale metmyoglobin, the heme iron atom in horse heart metmyoglobin is not
significantly displaced from the plane of the heme group.
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