Transcript Document

Myoglobin & Hemoglobin
Structure, Function & malfunction
of Biomolecules
Primary Structure
Sequence of amino acids in a protein
connected via peptide linkage.
Example –the enzyme lysozyme:
1
2
3
4
5
126 127 128 129
Lys-Val-Phe-Gly-Arg...Gly-Cys-Arg-Leu
Note: By convention, amino acid sequences are
written starting with the amino terminus.
Secondary Structure
Regular patterns of relatively small segments of
a protein held together mainly by H-bonds
Examples:
α-helix
-structure
http://www.ultranet.com/~jkimball/BiologyPages/S/SecondaryStructure.html
Tertiary Structure
Overall 3-D shape of a protein. Two basic
types are globular and fibrous.
Examples:
Globular (Pepsin)
Fibrous
(Collagen)
Quaternary Structure
Overall 3-D shape of a multi-subunit protein
Example:
Rabbit muscle
glycogen
phosphorylase
http://bmbiris.bmb.uga.edu/wampler/tutorial/prot4.html
Protein Function in Cell
1.
Enzymes

2.
Catalyze biological reactions
Structural role



Cell wall
Cell membrane
Cytoplasm
Protein: The Machinery of
Life
NH2-Val-His-Leu-Thr-Pro-Glu-GluLys-Ser-Ala-Val-Thr-Ala-Leu-TrpGly-Lys-Val-Asn-Val-Asp-Glu-ValGly-Gly-Glu-…..
Oxygen Transport Proteins

Myoglobin


Exhibits Michaelis-Menten properties
Hemoglobin

Exhibits allosteric properties
Myoglobin

Single polypeptide with
154(human) amino acids

C774H1224N210O222S5
17,183.8 daltons(human)

8 a helices (A-H)

Located in skeletal &
cardiac muscle

[high] in diving mammals
like whale & seals
O2 Binding Curve
P50 = 2.8 Torr

Allows myoglobin to act
as O2 storage reserve.

Releases O2 when pO2
becomes low indicating
O2 deprivation.
50
2.8
20
arterial pressure

100
tissues
Myoglobin has high
affinity for O2.
saturation with O2

100
pO2 (partial pressure of O2)
(Torr)
Heme Prosthetic Group

Heme (Fe2+) has
affinity for O2.

Hematin (Fe3+)
cannot bind O2.

Located in crevice
where it is protected
from oxidation.
N
N
Fe
N
N
HO
O
Oxygen Binding to Myoglobin
distal histidine

O2 binds to only
available coordination
site on iron atom.

His 93 (proximal his)
binds directly to iron.

His 64 (distal his)
stabilizes the O2 binding
site.
proximal histidine
http://cwx.prenhall.com/horton/medialib/media_portfolio/text_images/FG04_44.JPG
Myoblobin:CO complexes
• CO binds tightly; linear.
• O2 binds less tightly, bent structure.
• Distal His forces bent binding of both,
weakens CO binding.
O
O
O
C
Fe
Fe
Red Blood Cell (Erythrocyte)
Model Molecule: Hemoglobin
Hemoglobin – Quaternary Structure
a22
Two α (141 AA/ α)subunits and two β (146 AA/ β)subunits
Heme
Hemoglobin Structure

Each polypeptide chain resembles
myoglobin tertiary structure but 1˚
sequence varies.

Invariant residues indicate importance
of those residues in function.
Oxygen Binding

Hb exhibits + cooperativity.
Eaton et al. Nature Struct. Biol. 1999, 6, 351
O2 Binding to Hemoglobin
Water bound to
heme Iron
O2 Binding to Hemoglobin
Oxygen bound
to heme Iron
Hb
T-state deoxy
Hb
R-state - oxy
Hb Variants

HbA2



Embryonic Hb



a2d2
Present in ~2% of adults
a2e2
Has  affinity for O2
Fetal Hb


a2g2
Has  affinity for O2
http://oregonstate.edu/instruction/bb450/stryer/ch10/Slide27.jpg
Bohr Effect

CO2 pH

Some side groups remain
protonated at lower pH.

Stabilizes T state and
promotes unloading of O2
to active tissues.

Binding of CO2 also
stabilizes T state.

http://cwx.prenhall.com/horton/medialib/media_portfolio/text_images/FG04_50.JPG
CO2 binds to a amino groups.
2, 3-Bisphosphoglycerate

Stabilizes deoxyHb (T
state)
Facilitates unloading of
O2 in tissue.
OO
OO
O
P
O
P
-O
O
O-
2, 3-bisphosphoglycerate
O-
100
saturation with O2

- BPG
50
+ BPG
20
100
pO2 (partial pressure of
O2) (Torr)
2,3-BPG Binding to Hb
http://oregonstate.edu/instruction/bb450/stryer/ch10/Slide26.jpg
High Altitude and BPG

At higher altitudes,
the [BPG] increases
allowing Hb to
unload O2 more
easily.
http://www.bio.davidson.edu/Courses/anphys/1999/Yusi/dpgoxyhbgraph.jpg
Stored Blood & BPG

2,3-BPG becomes depleted in stored blood,
so R state of Hb is stabilized.

If BPG depleted blood is used for a
transfusion, the R state Hb doesn’t release O2.

Add inosine to stored blood to maintain BPG
levels.
CO Poisoining

CO is “competitive inhibitor” of O2.


Affinity is 200X greater than that of O2.
CO also inhibits unloading O2 of in
tissues.
Sickle Cell Anemia
Normal red blood cells are round like doughnuts, and
they move through small blood tubes in the body to
deliver oxygen.
Sickle red blood cells become hard, sticky and shaped
like sickles. When these hard and pointed red cells go
through the small blood tube, they clog the flow and
break apart. This can anemia.
The origin of the disease is a small
change in the protein hemoglobin
The change in cell structure arises from a change in
the structure of hemoglobin.
A single change in an amino acid causes hemoglobin
to aggregate.
a

Scanning electron
microscopic image of
Red bllod cells
Differences in Red Blood Cells
Sickle Cell Hemoglobin
Hemoglobin S
Significant change
in structure caused
by the single
mutation
L-Glutamic acid
L-Valine
(Glu/ E)
(Val / V)
Sickle Cell Hemoglobin
at 6β
Normal mRNA
Normal protein
GUG CAC CUG ACU CCU GAG GAG AAG
val his leu thr pro glu glu lys
1
2
3
4
5
6
7
8
Mutation
(in DNA)
Mutant mRNA
Mutant protein
GUG CAC CUG ACU CCU GUG GAG AAG
val his leu thr pro val glu lys
1
2
3
4
5
6
7
8
Glutamate (glu), a negatively charged amino acid,
is replaced by valine (val), which has no charge.