Hemoglobin

Hemoglobin

1. Two stable states

   • T-deoxy
     • subunits are very tightly coupled
       • Issue:
         • change in one subunit cannot occur without change to the entire quaternary structure
     • N term amino group & C term His of the B
       • Participate in ion pairs
         • Increases pK values
           • more basic environment
             • groups will not act like protons, where they donate a proton
     • Low affinity
       • oxygen does not bind well, and instead gets RELEASED
       • p50 is high
     • decreases Hb's affinity to O b/c ____ binds & helps stabilize the T state
       • high BPG
         • byproduct of body's catabolism
           • higher the concentration the more glycolysis is running
           • to help unload BPG, it binds to deoxyHb in B1 & B2 subunits
             • within the subunit 2 His residue & 1 Lys residue interacts
         • BPG does not READILY bind to fetal residue b/c His143 is replaced by Ser
           • So instead of O being released it is retained
       • CO2
         • binds N-term of Hb
           • forms carbamates & stabilizes T state
     • 1 peak: 560 nm
   • R-oxy
     • no ion pairs
       • decrease pK
         • more acidic environment
           • groups will likely donate protons
             • for each O2 Hb binds, it will release protons
               • Stimulates Hb to bind more O2
               • protons recombine with CO2 to form bicarbonate
                 • Hb takes this to lungs via bloodstream for exhalation
                 • Other CO2 bind to N-term of amino groups of Hb to form carbamates
     • high affinity
       • High affinity in lungs
         • pO2 is high
           • O2 binding causes T-state ion pairs to form R-state
       • environment is favorable for O binding
         • Low p50 value
     • 2 peaks: 540 & 580

2. Binds to Oxygen in a cooperative fashion

   • Oxygen affinity increases as more O2 binds
     • Hill's equation
       • • n
           • >1
             • Positive cooperativity
           • =0
             • non-cooperative
               • single binding site ex. Mb
               • hyperbolic
           • <1
             • Negative cooperativity
     • Allows Hb to function as O2 transport
       • from lungs to respiring tissues
       • Bohr Effect
         • transport CO2 produced by respiring tissues back to the lungs to be exhaled
           • Tissues--> Capillaries
             • pO2 is low
           • dissolved CO2 produces bicarbonate
     • shift of curve to left
       • low p50 value
   • p50 value for O binding is 30 torr
     • the 2nd and 3rd bind at p50 = 12 torr
   • p50 value decreases as O binds better
   • binding of O causes Fe II to move into the plane of the ring
     • His F8 gets pulled towards heme
       • causes shift to F helix
         • change in 3 structure communicates change to other subunits via changes of electrostatic and hydrostatic binding
           • results in a change to quaternary structure
             • goes from T->R state
   • Sigmoidal shape

3. Tetrameric protein

4. Allosteric enzyme

   • O2 binding to Hb involves a T->R reaction

5. Mutations

   • Hemolytic anemia
   • Cyanosis
   • Polycethemia
     • high affinity to O
       • Issue: Hb won't be able to release O, leading to reddish complexion
   • Sickle cell anemia

6. along with Mb contain mainly A helices which are 2dary structures

7. Cofactor

   • heme
     • protoporphyrin ring
     • Fe2+
       • 6 ligands control Fe2+ in oxy-Hb & oxy-Mb
         • 4 N's from the protoporphyrin ring
         • proximal His residue His F8
         • O2