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Protein Function (5.1 Reversible Binding of Protein to Ligand: O2-Binding…

- - - - two alpha chains
      - two beta chains
      - same naming convention as myoglobin
        
        E and F helices make up heme-binding pocket
    - - interactions of unlike subunits
        
        alphabeta interfaces
        
        stabilized by hydrophobic effect
    - - T state
        
        more stable
      - R state
        
        high o2 affinity
      - cooperation
  - - - acts as gate to control access to heme
  - - - P + L ⇌ PL
      - Ka = [PL]/[P][L]
        
        association constant, NOT acid dissociation constant
        
        measures affinity of L for P.
        
        units = 1/M
        
        higher Ka, higher affinity
        
        =ka/kd
        
        ka = association constant
        
        kd= dissociation constant
        
        order
        
        1 molecule = first
        
        2 molecule = second
        
        The [L] @ which 1/2 of ligand-binding sites are occupied = 1/Ka
        
        1/Ka = Kd - dissociation constant
        
        lower Kd, higher affinity
        
        Kd is equivalent to the molar concentration of ligand at which half of the available ligand-binding sites are occupied
        
        Y represents fraction of binding sites occupied by ligand
  - - - 153 AA residues
      - One heme
      - 8 alpha helical segments connected by bends
        
        contains 78% of AA
        
        designated by location
        
        ex: his F8 - 8th residue in alpha helix F
        
        named A-H
        
        bends named ex: AB, CD, EF, FG
  - - - o2 storage
      - o2 transport
    - - myoglobin - found in muscles
      - neuroglobin- found in neurons
      - cytoglobin- found in walls of blood vessles, regulates levels of nitric oxide.
  - - - Transition metals like iron can
        
        Heme - iron incorporated in protein-bound prosthetic group
        
        Found in many o2 transporting proteins
        
        porphyrin ring - contains four N's that bind to Fe.
        
        Fe2+ ferrous binds to o2
        
        Fe3+ Ferric does not bind to o2
        
        proximal HIS occupies one coordination bond
        
        Fe kept at center to prevent o2 rxn converting ferrous to ferric.