Skip to main content
Biology LibreTexts

Mb, Hb, Allostery, and Motors

  • Page ID
    1338
  • [ "Vet1", "article:topic" ]

    QUATERNARY STRUCTURES AND COMPLEX ENZYMES

     
    Advantages of complex (quaternary) protein structures
    • Stability: decreased surface-to-volume -> more hydrophobic interactions
    • New sites: e.g., immunoglobulin binding sites
    • Coupled reactions:
    trptophan synthetase: 
    Indoleglycerol-P --> G3P + indole 
    Indole + ser --> trp
    purine (A, G) synthesis: 10 reactions, 6
    enzymes, 1 complex (in purine depleted medium)
    • Cooperativity: e.g., allostery
     
    Example of Cooperativity: Mb/Hb (Myoglobin and Hemoglobin)
    • shows advantage of quaternary structure
    • shows examples of flexibility: low ΔG of shape change

    Myoglobin

    • MW ca 17,000 daltons
    • 75% -helix
    • Heme prosthetic group: protoporphyrin ring plus Fe2+
    • Heme binds O2 as porphyrin-Fe2+-O2, color change from brown --> red
    • Serves as an O2 buffer
    • Hyperbolic saturation curve shows that there is no coordinate activity
    3.pngMb curve.png

    Why is this an O2 buffer? High slope below the P50 means that considerable Mb is charged (or uncharged) for a small change in pO2 (as pO2 drops, MbO2 replenishes O2)

    Hemoglobin

    • Tetramer of myoglobin-like subunits, each with...
    • Heme prosthetic groups: protoporphyrin ring plus Fe2+
    • MW ca 4 x 17,000 daltons
    • 75% -helix
    • Complexed with O2, porphyrin-Fe2+-O2, brown --> red

    Hb.png

     
    • Better O2 buffer (at a higher [O2])
    • Sigmoid saturation curve shows that there is coordinate activity: “positive, homotropic, allosteric effector"
     
    hbmb.pngtorr.png

    Bohr effect: H+, CO2 promote dissociation of O2 from Hb-O2: "negative, heterotropic, allosteric effector." The Bohr effect in hemoglobin can also be depicted as an oxygen-binding curve. There is a proportional relationship between the affinity of pxygen and pH level. As the pH level decreases, the affinity of oxygen in hemoglobin also decreases. As hemoglobin approaches low pH, more oxygen is released.

    2,3-bisphosphoglycerate also promotes dissociation of O2. Purified hemoglobin binds much more tightly to the oxygen, making it less useful in oxygen transport. The difference in characteristics is due to the presence of 2,3-Bisphosphoglycerate(2,3-BPG) in human blood, which acts as an allosteric effector. An allosteric effector binds in one site and affects binding in another. 2,3-BPG binds to a pocket in the T-state (taut) of hemoglobin and is released as it forms the R-state (relaxed). The presence of 2,3-BPG means that more oxygen must be bound to the hemoglobin before the transition to the R-form is possible.

    bis.pngpH curve.png

    Lung conditions (Low H+, CO2) promotes O2 saturation; tissue conditions (high H+, CO2) promote O2 release; 2,4-BPG magnifies the allosteric effects. Allosteric effects match the saturation curve to the conditions in lung and tissue. 

    HbinRBCs.png

    Motor Proteins

    Types (substrate-motor)
    Linear:
    • M icrotubules (tubulin) – dynein (+ to -), kinesin (- to +, with exception—Science 1April 2011)
    • DNA – helicases
    • Microfilaments (actin) – myosin
    Rotary:
    • Bacterial flagella
    • F0F1 ATP synthases
    Motion depends on
    • Flexible 3o structure
    • Reversible binding
    • ATP hydrolysis affecting binding

    Microtubule--kinesin

    Microtubule: right handed hallow helix of tubulin α/β dimers

    beta.png

    kinesin: left-handed helix with two globular heads

    kinesin.png

    gfhdghfgh.png

    Each step depends on flexibility (“rotation”)
    Each step hydrolyzes one ATP (--> ADP + Pi)
    Each step involves an exchange reaction
     
    (There is another motor protein, dynein, which moves along microtubules.  
    Its 4-A crystal structure was recently reported (Science 331:1159, 3/4/11), 
    but its mechanism of action is still unknown.)

    Actin-Myosin

    Microfilaments: right-handed double helix of actin monomers

    micro.png

    myosin: left-handed coil- (alpha-helix) coil

    myosin.png

    bigpic.png

     
    Myosin is an ATPase. Would you expect the addition of actin to increase or decrease ATP hydrolysis activity?
    (Reaction rate: 0.05 s-1 --> 10 s-1)

    Actomyosin in Muscles

    acto.png

    Contraction: sliding in the A band from myosin-actin connections

    contraction.png

    Why rigor mortis? When there is a loss of ATP, the muscles cannot relax because it cannot be broken down into ADP

    Conclusion

    Flexibility in protein structures allows more complex functions
    • Reversible O2 and CO2 binding
    • Reversible protein-protein (kinesin-MT) binding
    Shows the importance of low ΔG in protein shape changes