BIOCHEM Exam 2 Lecture 7

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BIOCHEM Exam 2 Lecture 7
2014-02-14 12:28:00

Exam 2 material
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  1. What percentage of protein does myoglobin account for in muscle?
  2. Myoglobin and hemoglobin are both what?
    Oxygen binding molecules
  3. How much of the erythrocytes (by weight) does Hemoglobin make up?
  4. Describe the structure of myoglobin
    myoglobin consists of a single polypeptide (globin) chain with 1 heme prosthetic group
  5. Describe the structure of hemoglobin
    Hemoglobin is a globular protein with quaternary structure

    4 globin chains (tetramer) - 2 alpha, 2 beta

    the 2 alpha monomers plus the 2 beta monomers make 2 alpha beta dimers resulting in a tetramer held together by hydrophobic interactions
  6. Hemoglobin's alpha and beta globin chains are each similar to...
    myoglobin! but only have 18% identical amino acids

    each globin chain binds 1 heme prosthetic group
  7. Distal His
    (in Hb)

    The distal His is at the top

    it maintains the iron II state when the O2 isn't bound

    Stabilizes oxygen's hydrogen bonding
  8. Proximal His
    (in Hb)

    The proximal His is at the bottom

    coordinately bound to iron
  9. What happens when Hb binds O2?
    The heme Fe moves into the plane of the porphyrin ring
  10. Describe the binding curves of Hb and Mb
  11. What does a sigmoidal curve mean?
    cooperative binding
  12. Differences in Mb and Hb saturation
    • Hb
    • -readily saturates in lung
    • -releases 66% of bound O2 in tissues

    • Mb
    • -remains near saturation (91%) in tissues
    • -not a transporter
  13. How does oxygen binding affect the Hb tertiary structure?
    • ligand binding affects iron position in the porphyrin ring
    • movemeny of iron affects proximal His (interacts with Fe)
    • affects helix position and tertiary structure
  14. Describe the 2 major conformations of Hb
    Deoxyhemoglobin --> T State

    • tense state
    • iron out of plane of porphyrin ring

    Oxyhemoglobin --> R State

    • relaxed state
    • iron in plane
  15. oxygen binding leads to changes in the Hb quaternary structure. How so?
    • Changes at the heme group cause changes in the tertiary structure
    • leads to changes in the quat structure
  16. What are the two models to explain the T to R transition? Explain them.
    • Concerted
    • -entire protein is either in the T or the R state
    • -ligand binding shifts the entire tetramer simultaneously from T to R

    • Sequential
    • -Binding of ligand to one subunit changes conformation of that subunit from T to R
    • -each monomer is in the T or R state

    **ligand binding in hemoglobin is most likely a combo of the two models
  17. Other models of cooperativity (name 4)
    • Hill
    • Adair
    • Symmetry Rule
    • Szabo-Karplus
  18. Describe Hill plots
    • Hill plots are used to describe cooperativity.
    • In a hill plot, nh is the hill coefficient, found by the slope of the line. 
    • if nh < 1 there is negative cooperativity (this would mean binding one prevents binding of next - not really observed)
    • if nh=1 there is no coop
    • if nh >1 positive coop (binding 1 facilitates binding of next)
  19. Why is cooperativity in Hb important?
    • it allows for the binding and release of oxygen at physiological conditions
    • (it is determined and affected by residues in the alpha and beta monomers)
  20. What regulates Hb oxygen binding and release?
    • 2,3-BPG
    • - stabilizes the T state (through H-bonds and electrostatic interactions)
    • - faciliatates release
    • - allosteric effector

    *note: 2,3 BPG keeps the t-state for a while and lowers oxygen affinity but eventually oxygen will bind again lower affinity doesn't mean never bind
  21. Allosteric effector
    small molecules that bind a protein and change conformation, altering protein function

    inhibits or activates binding of ligand
  22. What happens to the saturation curve with more or less BPG?
    • therefore less BPG = quicker saturation, larger keq
  23. Hemoglobin is constrained by?
    interactions between alpha and beta monomers
  24. What stabilizes the T state?
    Networks of ion pairs and H bonds. Breaking any of these interactions destabilizes the T state and increases ligand affinity.
  25. How does the affinity of Hb for oxygen change as pH decreases?

    • As pH decreases, protons are added to the key His residues that stabilize the T state. 
    • -stablizes interactions between monomers
    • -decreases oxygen affinity
  26. About 80-85% of COthat diffuses into the blood is converted to ...
    bicarbonate and H+ within RBCs
  27. What is the regulation of Hb ocygen binding by H+ ions and CO2 called?
    The Bohr effect
  28. The remaining ____ of COin blood reacts with N termini of alpha globin chains through a non - enzymatic reaction called ____.

    • carbamylation
    • -carbamylation further stabilizes the T state
    • -decreases Osaturation of Hb beyond that caused by a drop in blood pH
  29. What affects Hb/Obinding and release?
    • proton concentration
    • CO2 concentration
    • allosteric effectors
    • mutations -->hemoglobinopathies
  30. Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin
    • the beta adult subunit is replaced by a gamma subunit
    • -about 72% identity
    • -reduced affinity for 2,3 - BPG

    allows for Oto be transferred from mother to fetus
  31. Sickle Cell anemia
    • results from inheriting 2 alleles for a hemoglobin variant HbS
    • HbS molecules aggregate forming fibrils that cause RBS to form sickle shape
    • In HbS, Glue 6 on beta chains is replaced with a Val that can interact with hydrophobic areas on other beta subunits
    • the most severe sickle cell symptoms occur in low oxygen environments
    • the hydrophobic areas are only exposed in the T state - anything that stabilizes the T state will increase the amount of fibrillar HbS