STRUCTURE & FUNCTION OF MYOGLOBIN
DR AMINA TARIQBIOCHEMISTRY
Hemeproteins
These are specialized proteins that containhemeas a tightly bound prosthetic group.Role ofhemeis different in different tissues,e.g.hemegroup incytochromes- electron transport carrier.catalasepart of the active site.hemoglobin andmyoglobin- bind oxygen.
Myoglobinhas been investigated intensely and is the first protein molecule to have been completely described in terms of its three-dimensional geometry.This achievement won the British scientist John Kendrew a share in the 1962 Nobel Prize for Chemistry.
MYOGLOBIN
It is present in heart and skeletal muscle.Function as a reservoir of oxygen and as an oxygen carrier.It increases the rate of transport of oxygen within the muscle cell.
Is both a structural and functional relative of hemoglobin.The oxygen-transport protein of the blood of higher animals.Has the ability to store oxygen by binding it to an iron atom.Myoglobinis composed of a single polypeptide chain of153 amino acidresidues.
Non polar amino acids are present in the interior of molecule packed by hydrophobic interactions.Polar amino acids are present at the surface of molecule and held together by hydrogen bonds, which they form with each other and with water.
Structure and bonding
AHemegroup is a flat ring molecule containing carbon, nitrogen and hydrogen atoms, with a single Fe2+ion at the center.Without the iron, the ring is called aPorphyrin.In ahememolecule, the iron is held within the flat plane by four nitrogenligandsfrom theporphyrinring.
Theglobinportion provides an environment for thehemethat can bind only one oxygen molecule.It has eight alpha helices and a hydrophobic core.It is the primary oxygen-carrying pigment of muscle tissues.
Myoglobincontains aporphyrinring with an iron center.There is aproximalhistidinegroup attached directly to the iron center.And adistalhistidinegroup on the opposite face, not bonded to the iron, but it stabilizes the binding of oxygen to the iron.
This protein does not exhibit cooperative binding of oxygen, since positivecooperativityis a property ofmultimeric/oligomericproteins only.
Myoglobincan bind one molecule of oxygen, because it contains onehemegroup.The oxygen disassociation curve is Hyperbolic for Mb.It means that Mb has high affinity for oxygen at all pO2.
pO2needed to achieve half saturation of the binding site is approx: 1 mm of Hg.( 26 mm of Hg -Hb).Advantage: Mb can bind O2released by theHbin the tissues at low pO2, and then release it within the muscle cell in response to O2demand.
High concentrations ofmyoglobinin muscle cells allow organisms to hold their breaths longer.Oxygen binds tomyoglobinand is released only when the hemoglobin can no longer supply adequate oxygen to muscle cells.
The distribution ofmyoglobinamong the higher animals is a reflection of its physiological function. It is found abundantly in the tissues of diving mammals, e.g., the whale, the seal, and the dolphin. High concentrations ofmyoglobinin these animals presumably allows them to store sufficient oxygen to remain underwater for long periods.
Myoglobinis found abundantly in man in cardiac muscle, which, by virtue of its essential function, must possess the capacity for continued activity when environmental oxygen concentrations are low.
Role in disease
Myoglobinis released from damaged muscle tissue (rhabdomyolysis), which has very high concentrations ofmyoglobin.The releasedmyoglobinis filtered by the kidneys but is toxic to the renal tubular epithelium and so may cause acute renal failure.
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