Download Lecture 14

Protein Function
Globins and Antibodies
3/12/2003
Quaternary structure of deoxy- and oxyhemoglobin
T-state
R-state
D-2,3-bisphosphoglycerate (BPG)
O
O
C
H
C
OPO3--
H
C
OPO3--
BPD binds to hemoglobin and decreases
the oxygen affinity and keeps it in the
deoxy form.
H
BPG binds 1:1 with a
K=1x10-5 M to the
deoxy form but weakly
to the oxy form
BPG binds specifically to
the deoxy state and
stabilizes it in the T state.
In the R state the central
cavity is too narrow for
BPG to fit or bind.
The P50 value of stripped hemoglobin increases
from 12 to 22 torr by 4.7 mM BPG
At 100 torr or arterial blood, hemoglobin is 95%
saturated
At 30 torr or venous blood, hemoglobin is 55%
saturated
Hemoglobin releases 40% of its oxygen. In the
absence of BPG, little oxygen is released. Between
BPG, CO2, H+, and Cl- all O2 binding is accounted
for.
BPG levels are partially responsible
for High-Altitude adaptation
BPG restores the
37% release of O2
at higher
elevations
between arterial
and venous blood
Hemoglobin mutants
There are about 500 variants of hemoglobin 95% are single
amino acid substitution.
5 % of the worlds population carries a different sequence
form the normal.
•Changes in surface charge
•Changes in internally located residues
•Changes stabilizing Methemoglobin
•Changes in the a1-b2 contact
Changes in surface rarely change the function of hemoglobin
with the exception of the sickle cell mutation.
Internal residues cause the hemoglobin to contort to different
shapes and alter its binding properties. Heinz bodies are
precipitated aggregates of hemoglobin. Usually cause
hemolytic anemia characteristic by cell lysis.
Hb Hammersmith Phe CD1(42)b  Ser. The Phe wedges the
heme in place, without it the heme falls out of the protein.
Hb Bristol Val E11(67)b  Asp occludes O2 from the pocket.
Hb Bibba substitutes a Pro in the middle of H helix kinks the
chain.
Hb Savannah replaces Gly B6(24)b  Val where the B helix
crosses the E helix.
Stabilizing Methemoglobin. Or the Fe(III) state eliminates
the oxygen binding to the heme. Diagnostic of a blue color
or cyanosis.
Hb Boston His E7(58)a  Tyr (the distal His)
Hb Milwaukee Val E11(67)b  Glu
Hb Iwate His F8(87)a  Tyr “black mouth disease Japan
They have chocolate brown blood.
Changes at the a1-b2 interface usually have lower Hill
coefficients. Stabilize either the T or R state
Polycythemia, a ruddy complexion
Hb Yakima Asp G1(99)b  His eliminates H-bonding
that stabilizes the T form at the a1-b2 interface and
stabilizes the R state P50 =12 torr
Hb Kansas Asn G4(102)b  Thr eliminates H-bonding
that stabilizes the T form at the a1-b2 interface and so
when O2 binds it remains in the T state P50 =70 torr
Fetal Hemoglobin
•Fetal hemoglobin has a different b subunit called a g
subunit or a2g2.
•In Fetal hemoglobin, BPG does not affect this variant
and the baby’s blood will get its oxygen from the
mothers hemoglobin.
•The transfer of oxygen is from the mother (less tightly
bond) to the baby (more tightly bond).
Antibodies and the immune response