myoglobin & hemoglobin
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DESCRIPTIONMyoglobin & Hemoglobin. Structure, Function & malfunction of Biomolecules. Primary Structure. Sequence of amino acids in a protein connected via peptide linkage. Example –the enzyme lysozyme: 1 2 3 4 5 126 127 128 129 Lys-Val-Phe-Gly-Arg...Gly-Cys-Arg-Leu - PowerPoint PPT Presentation
Myoglobin & HemoglobinStructure, Function & malfunction of Biomolecules
Primary StructureSequence of amino acids in a protein connected via peptide linkage.
Example the enzyme lysozyme:
1 2 3 4 5 126 127 128 129 Lys-Val-Phe-Gly-Arg...Gly-Cys-Arg-Leu
Note: By convention, amino acid sequences are written starting with the amino terminus.
Secondary StructureRegular patterns of relatively small segments of a protein held together mainly by H-bonds
Tertiary StructureOverall 3-D shape of a protein. Two basic types are globular and fibrous.
Globular (Pepsin)Fibrous (Collagen)Examples:
Quaternary StructureOverall 3-D shape of a multi-subunit protein
Rabbit muscle glycogen phosphorylasehttp://bmbiris.bmb.uga.edu/wampler/tutorial/prot4.htmlExample:
Protein Function in CellEnzymes Catalyze biological reactionsStructural roleCell wallCell membraneCytoplasm
Protein: The Machinery of LifeNH2-Val-His-Leu-Thr-Pro-Glu-Glu-Lys-Ser-Ala-Val-Thr-Ala-Leu-Trp-Gly-Lys-Val-Asn-Val-Asp-Glu-Val-Gly-Gly-Glu-..
Oxygen Transport ProteinsMyoglobinExhibits Michaelis-Menten properties
HemoglobinExhibits allosteric properties
MyoglobinSingle polypeptide with 154(human) amino acids
C774H1224N210O222S5 17,183.8 daltons(human)
8 a helices (A-H) Located in skeletal & cardiac muscle
[high] in diving mammals like whale & seals
O2 Binding CurveMyoglobin has high affinity for O2.
P50 = 2.8 Torr
Allows myoglobin to act as O2 storage reserve.
Releases O2 when pO2 becomes low indicating O2 deprivation.
Heme (Fe2+) has affinity for O2.
Hematin (Fe3+) cannot bind O2.
Located in crevice where it is protected from oxidation.Heme Prosthetic Group
Oxygen Binding to MyoglobinO2 binds to only available coordination site on iron atom.
His 93 (proximal his) binds directly to iron.
His 64 (distal his) stabilizes the O2 binding site.
http://cwx.prenhall.com/horton/medialib/media_portfolio/text_images/FG04_44.JPGdistal histidineproximal histidine
CO binds tightly; linear. O2 binds less tightly, bent structure. Distal His forces bent binding of both, weakens CO binding. Myoblobin:CO complexes
Red Blood Cell (Erythrocyte)
Model Molecule: Hemoglobin
Hemoglobin Quaternary StructureTwo (141 AA/ )subunits and two (146 AA/ )subunitsa2b2
Hemoglobin StructureEach polypeptide chain resembles myoglobin tertiary structure but 1 sequence varies.
Invariant residues indicate importance of those residues in function.
Oxygen BindingHb exhibits + cooperativity.Eaton et al. Nature Struct. Biol. 1999, 6, 351
Water bound to heme IronO2 Binding to Hemoglobin
Oxygen bound to heme IronO2 Binding to Hemoglobin
Hb R-state - oxy
Hb Variantshttp://oregonstate.edu/instruction/bb450/stryer/ch10/Slide27.jpgHbA2a2d2Present in ~2% of adults
Embryonic Hba2e2Has affinity for O2
Fetal Hba2g2Has affinity for O2
Bohr Effect CO2 pHSome side groups remain protonated at lower pH.
Stabilizes T state and promotes unloading of O2 to active tissues.
Binding of CO2 also stabilizes T state.CO2 binds to a amino groups. http://cwx.prenhall.com/horton/medialib/media_portfolio/text_images/FG04_50.JPG
2, 3-BisphosphoglycerateStabilizes deoxyHb (T state)
Facilitates unloading of O2 in tissue.pO2 (partial pressure of O2) (Torr)20100saturation with O210050- BPG+ BPG
2,3-BPG Binding to Hbhttp://oregonstate.edu/instruction/bb450/stryer/ch10/Slide26.jpg
High Altitude and BPGAt higher altitudes, the [BPG] increases allowing Hb to unload O2 more easily.http://www.bio.davidson.edu/Courses/anphys/1999/Yusi/dpgoxyhbgraph.jpg
Stored Blood & BPG2,3-BPG becomes depleted in stored blood, so R state of Hb is stabilized.
If BPG depleted blood is used for a transfusion, the R state Hb doesnt release O2.
Add inosine to stored blood to maintain BPG levels.
CO PoisoiningCO is competitive inhibitor of O2.Affinity is 200X greater than that of O2.
CO also inhibits unloading O2 of in tissues.
Sickle Cell AnemiaNormal red blood cells are round like doughnuts, and they move through small blood tubes in the body to deliver oxygen.
Sickle red blood cells become hard, sticky and shaped like sickles. When these hard and pointed red cells go through the small blood tube, they clog the flow and break apart. This can anemia.
The origin of the disease is a small change in the protein hemoglobinThe change in cell structure arises from a change inthe structure of hemoglobin.
A single change in an amino acid causes hemoglobinto aggregate.
Scanning electron microscopic image of Red bllod cells
Differences in Red Blood Cells
Sickle Cell HemoglobinSignificant changein structure caused by the single mutation Hemoglobin S
L-Valine (Val/V)L-Glutamic acid (Glu/E)
Sickle Cell HemoglobinGUG CAC CUG ACU CCU GAG GAG AAGval his leu thr pro glu glu lys 1 2 3 4 5 6 7 8GUG CAC CUG ACU CCU GUG GAG AAGval his leu thr pro val glu lys 1 2 3 4 5 6 7 8Mutation (in DNA)Normal mRNANormal proteinMutant mRNAMutant proteinGlutamate (glu), a negatively charged amino acid, is replaced by valine (val), which has no charge.at 6