Chapter 4: Protein Function
Chapter 7, Voet, Voet & Pratt
* Link between protein structure and physiological function
* Focus on myoglobin and hemoglobin
* Both proteins reversibly bind to O2
* Extremely well-studied examples of structure/function relationships
* Behavior is a model for many other proteins, especially metabolic enzymes
Learning Objectives
* Compare myoglobin and hemoglobin
* Structures
* Properties
* O2 binding characteristics
* Understand structural basis for hemoglobin’s O2 binding characteristics
* Relate these characteristics to physiological role of hemoglobin
* Understand biochemical basis of ...view middle of the document...
8 torr
pO2 = 30 – 100 torr
O2 Binding Proteins: Hemoglobin (Hb)
* Is a tetrameric protein: 4 subunits
* Each has a similar 3° structure to Mb, though have different 1° structures
* 2 α chains + 2 β chains
* Form 2 αβ dimmers (protomers):
Myoglobin vs. Hemoglobin
Myoglobin Hemoglobin
Nomomeric Tetrameric (2α2β)
One heme Four hemes
Binds O2 in muscle Binds O2 in blood (RBCs)
Hyperbolic O2 binding curve Sigmoidal O2 binding curve
Non-cooperative O2 binding Cooperative O2 binding
Hb upon Binding O2
* Oxygenation causes the subunits to ‘slide’ past each other
* 1 protomer rotates by the other by ~15°
Deoxyhemoglobin (T form) Oxyhemoglobin (R form)
Hb Binding O2: A graphical Perspective
* O1 binding to Hb is described by a sigmoidal curve
* Such binding curves are indicative of a cooperative interaction between binding sites
* O2 bidning at 1 site increases the O2 affinity of another site
* Hb is an example of an allosteric protein
The Hill Equation and Hill Constant
Hb + nO2 Hb(O2)n
The Hill Equation
Yo2=pO2np50n+pO2n |
n = Hill constant
Is measured of the cooperativity between binding sites
* n = 1, ligand banding is non-cooperative
* n > 1, ligand binding is positively cooperative
* n < 1, ligand binding is negatively cooperative
What is the range of n?
Graphing the Cooperativity between O2 Binding Sites
A more convenient equation to plot:
logYO21-YO2=nlogpO2-nlogp50 |
For normal human Hb: n = 2.8 – 3.0
Communication Between O2 binding sites in Hb
Remember: T-state (deoxyHb) and R-state (oxyHb)
* Hemes in Hb are 25-37 Å apart
* So how is O2 binding at one heme communicated to all of the others?
1. Binding of O2causes the Fe(II) to move into the plane of the porphyrin.
2. Movement of the Fe(II) and flattening of the porphyrin forces Helix F to shift (~1Å)
3. The shift...
