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About
the Myoglobin Molecule
Myoglobin
is an iron- and oxygen-binding protein found in the muscle tissue
of vertebrates in general and in almost all mammals. It is related
to hemoglobin, which is the iron- and oxygen-binding protein in
blood, specifically in the red blood cells. The only time myoglobin
is found in the bloodstream is when it is released following muscle
injury. It is an abnormal finding, and can be diagnostically relevant
when found in blood.
Myoglobin
(abbreviated Mb) is a single-chain globular protein of 153 or 154amino
acids, containing a heme (iron-containing porphyrin) prosthetic
group in the center around which the remaining apoprotein folds.
It has eight alpha helices and a hydrophobic core. It has a molecular
weight of 17,699 (with heme)daltons, and is the primary oxygen-carrying
pigment of muscle tissues. Unlike the blood-borne hemoglobin, to
which it is structurally related, this protein does not exhibit
cooperative binding of oxygen, since positive cooperativity is a
property of multimeric/oligomeric proteins only. Instead, the binding
of oxygen by myoglobin is unaffected by the oxygen pressure in the
surrounding tissue. Myoglobin is often cited as having an "instant
binding tenacity" to oxygen given its hyperbolic oxygen dissociation
curve. High concentrations of myoglobin in muscle cells allow organisms
to hold their breaths longer. Diving mammals such as whales and
seals have muscles with particularly high myoglobin abundance.
Myoglobin
was the first protein to have its three-dimensional structure revealed.
In 1958, John Kendrew and associates successfully determined the
structure of myoglobin by high-resolution X-ray crystallography.
For this discovery, John Kendrew shared the 1962 Nobel Prize in
chemistry with Max Perutz. Despite being one of the most studied
proteins in biology, its true physiological function is not yet
conclusively established: mice genetically engineered to lack myoglobin
are viable, but showed a 30% reduction in cardiac systolic output.
They adapted to this deficiency through hypoxic genetic mechanisms
and increased vasodilation. In humans myoglobin is encoded by the
MB gene.
About
the Alpha Helix
A
common motif in the secondary structure of proteins, the alpha helix
(a-helix) is a right-handed coiled or spiral conformation, in which
every backbone N-H group donates a hydrogen bond to the backbone
C=O group of the amino acid four residues earlier ( hydrogen bonding).
This secondary structure is also sometimes called a classic Pauling–Corey–Branson
alpha helix. Among types of local structure in proteins, the a-helix
is the most regular and the most predictable from sequence, as well
as the most prevalent.
Structure
and Hydrogen Bonding
The amino
acids in an alpha- helix are arranged in a right-handed helical
structure where each amino acid residue corresponds to a 100°
turn in the helix (i.e., the helix has 3.6 residues per turn),
and a translation of 1.5 Å (0.15 nm) along the helical axis.
The pitch of the alpha-helix (the vertical distance between one
consecutive turn of the helix) is 5.4 Å (0.54 nm) which is the
product of 1.5 and 3.6. What is most important is that the N-H
group of an amino acid forms a hydrogen bond with the C=O group
of the amino acid four residues earlier; this repeated hydrogen
bonding is the most prominent characteristic of an a-helix.
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