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First published online August 31, 2004
Journal of Experimental Biology 207, 3441-3446 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01172

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Myoglobin: an essential hemoprotein in striated muscle

George A. Ordway^1,* and Daniel J. Garry^2,3

¹ Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
² Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
³ Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA

View larger version (31K): [in a new window]   Fig. 1. Myoglobin is expressed in oxidative skeletal myofibers. (A) Schematic of fiber-type diversity associated with mammalian skeletal muscle. Skeletal myofibers are characterized based on the myosin heavy chain (MHC) isoforms, oxidative capacity (i.e. mitochondrial content), contractility (slow-twitch vs fast-twitch) and myoglobin content. Myoglobin is expressed in Type I, 2A and 2X fibers (absent in 2B fibers). (B,C) Low-magnification (B) and high-magnification (C) transverse sections of adult mouse hindlimb skeletal muscle immunohistochemically stained for myoglobin expression. Note that myoglobin is expressed in a graded fashion in oxidative myofibers (Type I>2A>2X and absent in 2B fibers). 1, Type 1 fibers; 2, Type 2A fibers; 3, Type 2X fibers; 4, Type 2B fibers.

View larger version (12K): [in a new window]   Fig. 2. Myoglobin avidly binds oxygen. Myoglobin and hemoglobin function as oxygen transporters. Myoglobin displays a hyperbolic-shaped oxygen-binding curve whereas hemoglobin displays a sigmoidal-shaped oxygen-binding curve.

View larger version (34K): [in a new window]   Fig. 3. Myoglobin consists of a backbone and heme-binding domain. (A) Myoglobin was the first protein to be subjected to X-ray crystallography. The backbone of myoglobin consists of eight -helices (blue) that wrap around a central pocket containing a heme group (red), which is capable of binding various ligands including oxygen, carbon monoxide and nitric oxide. (B) The protoheme group is bracketed or stabilized by histidine residues above (His64) and below (His93).

View larger version (34K): [in a new window]   Fig. 4. Myoglobin is temporally expressed during muscle differentiation. (A) Immunohistochemical localization of myoglobin in differentiated C2C12 myotubes. Note that myoglobin (green) is uniformly expressed in the cytoplasm of differentiated myotubes and absent in the nuclear compartment (propidium iodide stains red and demarcates the nuclear compartment). (B) Western blot analysis of myoglobin expression following differentiation of C2C12 myotubes. Myoglobin (Mb) and myosin heavy chain (MHC) proteins increase with exposure of C2C12 myogenic cell line to differentiation media (DM).

View larger version (10K): [in a new window]   Fig. 5. Myoglobin gene disruption strategy. (A) The myoglobin (Mb) gene consists of three exons. To produce knockout mice that lack myoglobin, exon 2 was replaced with a neomycin cassette (neo), and homologous recombination technology was used as previously described (Garry et al., 1998). (B) Compared with the wild-type adult soleus muscle (above), which is an oxidative muscle group, the myoglobin-deficient soleus muscle (below) was depigmented.