›

Hemoglobin Cooperative Binding

Notes

Hemoglobin Cooperative Binding

Sections


Overview

Definitions

Cooperative binding

  • Describes unique interactions between heme groups in hemoglobin
  • Small movement of heme group propagates through hemoglobin's 3D structure

HEMOGLOBIN STRUCTURE

R-form (relaxed)

  • Alpha-alpha interactions: weak ionic and H-bonds form salt bridges
  • Beta-beta interactions: no interactions; move apart upon oxygenation
  • Alpha-beta dimers: strong hydrophobic interactions within each dimer

T-form (tense)

  • 3D structure changes between oxygenated and deoxygenated states

HEME SITE

T-form (tense)

  • Heme site: when O2 leaves, iron center moves out of porphyrin plane & proximal histidine moves away from iron center
  • Small movement in heme group makes O2 binding unfavorable

SALT BRIDGES

  • Salt bridges break and reform upon oxygen binding --> peptide wiggle-room
  • Alpha chain salt bridges:
    – Alpha1: arginine carboxy terminus (-) and arginine side group (+)
    – Alpha2: lysine (+) and aspartate (-)
  • Salt bridges regulate cooperativity: iron centers move into porphyrin planes

Dissociation curve

  • % oxygen saturation vs. oxygen partial pressure (torr)
  • Cooperative binding produces sigmoidal binding curve
  • After hemoglobin reaches 50% saturation: saturation increases rapidly (steepest point of curve)
  • Hemoglobin O2 affinity rapidly increases at half saturation

Full-Length Text

  • Here we'll learn about cooperative binding, a key allosteric effect that facilitates hemoglobin function in the body.
  • To begin, start a table to define cooperative binding.
  • Denote that it describes the unique interactions between the heme groups in hemoglobin.

First, let's draw the three dimensional structure of hemoglobin in its oxygenated form (R-Form or "relaxed form").

  • To begin, draw two polypeptides weakly bound together.
  • Shade them the same color and label them alpha subunits.
  • Write that weak ionic and hydrogen bonds form salt bridges between these polypeptides.
  • Now, on top of these subunits, draw two beta-subunits that do not interact with each other.
  • Indicate that they form dimers with the alpha-subunits below them.
    • Strong hydrophobic interactions hold these dimers together.
  • Then, indicate that each subunit in hemoglobin has a heme active site.
  • Now, indicate that the three-dimensional structure of hemoglobin changes as it moves between oxygenated and deoxygenated states.
  • Write that the deoxygenated state is the T-form for "tense."

Let's learn how.

  • Draw a representative heme active site as a straight line with an iron center in the middle.
  • Indicate that the line specifically represents the "porphyrin ring" and show that the iron center binds oxygen.
  • Draw a histidine group above the iron center and label it proximal histidine, it is an amino acid in the protein portion of each hemoglobin subunit.

Now, let's illustrate what happens when we remove oxygen.

  • Redraw the porphyrin ring.
  • Show that as oxygen leaves, the iron center moves out of the plane of the porphyrin ring and the proximal histidine moves away from the iron center; the porphyrin ring, itself, also moves.
    • Write that this small movement in the heme group makes oxygen binding unfavorable.
    • This brings us to cooperative binding, in which the tiny movement of a single iron center propagates throughout hemoglobin's three-dimensional structure!
  • Redraw our 3D hemoglobin in its oxygenated state.
  • Draw an alpha helix in each alpha-subunit and show that they form two salt bridges.

Let's take a closer look at one of them.

  • Draw a portion of a helix and label the carboxy-terminus.
  • Indicate that it is negatively charged.
  • Show that an arginine side group emerges from the helix and that it is positively charged.
  • Next, draw a portion of the helix from the second alpha-subunit.
  • Show that aspartate (negative) and lysine (positive) side groups protrude from this helix.
    • Show that the oppositely charged groups form weak ionic bonds:
    • The carboxy-terminus and lysine.
    • The arginine side group and aspartate.
    • These weak ionic bonds are the only salt bridges that form between alpha-subunits in the R-form.
  • Why?
    • Write that oxygen binding disrupts the other salt bridges in hemoglobin, which allows for more polypeptide wiggle-room.
    • Thus, the "R-form" is "relaxed."
    • Oxygen binding, in turn allows the remaining iron centers to move into the planes of their porphyrin rings, which is more favorable for oxygen binding!

Now let's draw hemoglobin's dissociation curve to visualize this.

  • Draw a graph and label the x-axis oxygen partial pressure (torr).
  • Number it 0 to 100.
  • Next, label the y-axis % oxygen saturation and number it 0 to 100
  • Draw a sigmoidal curve and label it cooperative binding.
  • Mark the portion of the curve at which hemoglobin is half-saturated (50%).
    • Note that at this point, hemoglobin saturation increases rapidly (the steepest point in the curve).
  • Thus, write that hemoglobin's affinity for oxygen rapidly increases when it is half saturated; when half of its heme groups are already bound to oxygen.