Lect. 8-1

1
Globular Proteins

• Some design principles
• Globular proteins fold so as to "bury" the
  hydrophobic side chains, minimizing their contact
  with water
• Most polar residues face the outside of the
  protein and interact with solvent
• Most hydrophobic residues face the interior of
  the protein and interact with each other
• Packing of residues is close, but protein
  interiors contain some empty space
• The empty space is in the form of small cavities

2
Globular Proteins

• More design principles
• "Random coil" is not random
• Structures of globular proteins are not static
• Various elements and domains of protein move to
  different degrees
• Some segments of proteins are very flexible and
  disordered.
• Myoglobin and hemoglobin are typical examples of
  globular proteins.
• Both are heme-containing proteins and each is
  involved in oxygen metabolism.

3
Myoglobin 2o and 3o aspects

• Myoglobin is a single peptide chain of 153
  residues arranged in eight a-helical regions
  labeled A-H.
• The heme cofactor is the oxygen binding site so
  it is necessary for myoglobins function, oxygen
  storage in mammalian muscle tissue.
• His E7 and F8 are important for binding the heme
  group within the protein and for stabilizing
  bound oxygen.

4
Myoglobin 2o and 3o structure
5
The Heme Group
-
-
C
H
C
H
C
O
O
C
H
C
H
O
O
C
2
2
2
2
C
H
C
H
3
3
N
N
Fe(II)
C
H
N
N
2
C
H
C
H
3
C
H
C
H
C
H
3
2
6
Binding Site for Heme

• Oxygen binds to 6th coordination site on heme iron

7
Oxygen binding to heme
His E7 acts as a gate to favor oxygen binding
over carbon monoxide.
8
Hemoglobin

• A tetrameric protein
• two a-chains (141 AA)
• two b-chains (146 AA)
• four heme cofactors, one in each chain
• The a and b chains are homologous to myoglobin.
• Oxygen binds to heme in hemoglobin with same
  structure as in Mb but cooperatively as one O2
  is bound, it becomes easier for the next to bind.

9
Hemoglobin ribbons hemes

• Each chain is in ribbon form.
• The heme groups are in space filling form

10
Oxygen Binding Curves

• Hemoglobin and myoglobin respond differently to
  increase in O2 concentration.
• Myoglobin shows normal saturation behavior while
  hemoglobin shows cooperative behavior. Each
  oxygen added to a heme of Hb makes addition of
  the next one easier.
• The myoglobin curve is hyperbolic.
• The hemoglobin curve is sigmoidal.

11
Oxygen Binding Curves-2
12
Oxygen binding by hemoglobin
A Quaternary Structure Change One alpha-beta
pair moves relative to the other by 15 degrees
upon oxygen binding This large change is caused
by movement of Fe by only 0.039 nm when oxygen
binds
13
Oxygen binding by hemoglobin
14
The Bohr Effect

• Competition between oxygen and H
• Discovered by Christian Bohr
• Binding of protons diminishes oxygen binding
• Binding of oxygen diminishes proton binding
• Important physiological significance-O2
  saturation of Hb responds to pH

15
The Bohr Effect
16
Bohr Effect II

• Carbon dioxide diminishes oxygen binding
• CO2 produced in metabolically active tissue
  (requires oxygen)
• Hydration of CO2 in tissues and extremities leads
  to proton production
• CO2 H2O ? HCO3- H
• These protons are taken up by Hb forcing more
  oxygen to dissociate
• The reverse occurs in the lungs

17
2,3-Bisphosphoglycerate
An Allosteric Effector of Hemoglobin The sigmoid
binding curve is only observed in the presence of
2,3-BPG Since 2,3-BPG binds at a site distant
from the Fe where oxygen binds, it is called an
allosteric effector
18
2,3-bisphosphoglycerate (2,3-BPG) is a negative
allosteric effector of O2 binding to Hb - binds
tighter to deoxyHb
2,3-BPG