Hemoglobin Structure

1
Hemoglobin Structure

• Hemoglobin is a protein found in red blood cells
  and is the primary vehicle for transporting
  oxygen in the blood

(http//www.bloodless.it/hemoglobin.jpg)
2
Tetramer StructureThe Hemoglobin Molecule
iscomposed offour polypeptidesubunitsnon-coval
ently bonded to each other
Graphics on slides 2-6 are taken from
(www3.interscience.wiley.com8100/legacy/college/b
oyer/0471661791/structure/HbMb/hbmb.htm)
3
Class Question 1

• Are the strands named alpha (a) and beta (b)
  because of their secondary protein structures?

4
Answer

• NO! Actually all strands are alpha helices! The a
  a and b strands differ in number and sequence of
  amino acids.
• Each alpha and beta strand is composed of eight
  alpha helical substructures

5
Class Question 2

• If the strands are not covalently bonded, what
  types of bonds, or attractive forces are keeping
  them together?

6
Answer

• Hydrogen bonds
• Hydrophilic attractions
• Hydrophobic or lipophilic attractions

7
Heme

• At the center of each polypeptide unit is a heme
  group, which gives blood its red color, and
  contains one Fe2 ion

8
Class Question 3

• Why is a heme group a good oxygen binding group?
  Which is the Lewis acid and which is the Lewis
  base?

9
Answer

• The oxygen acts as a good electron donor (Lewis
  base) to the positively charged iron (II) ion,
  which is a Lewis acid.
• IN ADDITION
• Each hemoglobin molecule, with four heme groups,
  can bind four oxygen molecules
• Due to shape changes caused by binding, each
  additional oxygen adds on more easily

10
The heme is kept in place by a nitrogen atom of a
nearby histidine chain, called the proximal
histidine.
11
The side opposite to the proximal histidine is
where the oxygen binds
12
The distal histidine side chain blocks CO from
bonding to the Fe2, while allowing oxygen to
bind easily
13
Class Question 4

• Draw the Lewis structure of carbon monoxide
• How would the CO molecule bind to the Fe2 of the
  heme? Why might the heme have a greater affinity
  for the CO than for the O2?

14
Answer

• In the CO molecule

• The oxygen would act as a Lewis base
• The more electronegative oxygen has a greater
  electron density in a CO molecule than in an O2
  molecule, making it a better electron pair donor

www.uam.es
15
In people with the genetic disease Sickle Cell
anemia, thehemoglobin molecules clump together
16
Class Question 5

• Based on your knowledge of the primary,
  secondary, tertiary and quaternary structures of
  proteins, propose a hypothesis as to what could
  be different in sickle-cell hemoglobin that would
  cause it to clump together

17
Causes?

• In the activity, you will identify the genetic
  and chemical change in the protein structure that
  causes this disease.

medicine.osu.edu
www.pediatriceducation.org
18
Sources All accessed 7/17/07

• http//www.bloodless.it/hemoglobin.jpg
• http//college/boyer/0471661791/structure/HbMb/hbm
  b.htm
• www.uam.es
• www.medicine.osu.edu
• www.pediatriceducation.org