Quiz 7 Flashcards by Cynthia Olvera

myoglobin

with its single heme prosthetic group, exhibits a hyperbolic O2 binding curve

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hemoglobin can adopt

the deoxyribose (T) or oxy (R) conformation, which differ in O2 binding ability

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Hemoglobin (Hb) and myoglobin (Mb)

are oxygen-transport and oxygen-storage proteins

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Mb is

monomeric

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Hb is

tetrameric
- it is formed by a dimer of a B dimers

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Hb has

2 alpha chains of 141 residues, 2 Beta chains of 146 residues

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iron interacts with

six ligans in Hb and Mb

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four of these are the

N atoms of the heme porphyrin

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a fifth ligand

is donated by the imidazole side chain of amino acid residue His F8

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His F8

this residue is on the sixth or “F” helix, and it is the 8th residue in the helix

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When Mb or Hb bind oxygen,

the O2 molecule, itself, binds to the heme iron, as the sixth ligand.

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bound O2 molecule is

tilted relative to a perpendicular line to the heme plane

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Fe in Mb is

Fe2+, Fe(II)- ferrous iron- the form that binds oxygen

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mb with ferric iron Fe2+ is called

metmyoglobin and does not bind to oxygen

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when oxygen bind to Fe2+ in heme of mb,

the heme fe2+ is drawn toward the plane of the porphyrin ring

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for Mb, this small change has a little consequence

but a similar change in Hb initiates a series of conformational changes that are transmitted to adjacent subunits: very consequential

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Mb, an

oxygen-storage protein, has a greater affinity for oxygen at all oxygen pressures

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Hb becomes saturated with

O2 in the lungs, where the partial pressure of pO2 is high about 100 torr

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in capillaries pO2 is

low, about 40 torr, and oxygen is released from Hb

the binding of O2 to Hb

is cooperative, binding of oxygen to the first subunit is tough, but it makes binding to the other subunits more favorable: cooperatively

oxygen binding pulls the

Fe2+ into the heme plane

as fe2+ moves,

it drags His F8 and the F helix with it

this change is transmitted to the subunit interfaces, where the conformation changes leads to the

rupture of salt bridges between polypeptide subunits

changes occur at the

4th level, leading to conformational change

Hb must be able to

release oxygen in capillaries

bohr effect

explains the connection between CO2, H+ and conversion oxy-Hb(R) to deoxy-Hb(T)

thus as pH decreases

dissociation of O2 from hemoglobin is enhanced

binding of protons,

H+ diminishes oxygen binding on Hb

as the tissue-capillary interface, CO2 hydration

and glycolysis produces extra H+, promoting addition disassociation of O2 where it is needed most [tissues]

at the lung-artery interface, the reverse reaction,

bicarbonate dehydration [required for CO2 exhalation] consumes extra H+, promoting CO2 release and O2 binding

2,3 -BPG

an allosteric effector of hemoglobin

the sigmoid binding curve, which reflects high sensitivity for O2 binding and release,

is only observed in the presence of 2,3 BPG

since 2,3 BPG binds at a site distant from the F2+ where oxygen binds, it is called an

allosteric effector

in the absence of 2,3 BPG

oxygen binding to Hb would have a hyperbolic curve like Mb

where does 2,3-BPG bind?

- "inside" - one molecule binds, in the central cavity formed by the four subunits

what is special about 2,3 BPG?

negative charges interact with 8 positive charges in the cavity

fetal Hb(Hb F)

has lower affinity for 2,3-BPG and higher affinity for oxygen, so it can take oxygen from mothers Adult Hb (Hb A)

the ionic binding 2,3 BPG to the two

B subunits of Hb

gas exchange takes place across the

placenta

fetal Hb(Hb F) differs from adult Hb

- with y chains in place of B chains and this an a2y2 structure

Why does fetal Hb (Hb F) bind O2 more tightly than Adult Hb (Hb A)?

1. Fetal γ-chains have Ser instead of His at position 143 and thus lack two of the positive charges that stabilize BPG binding. 2. As a result, Hb F has a higher affinity for O2

sickle-cell anemia patients

have abnormally shaped red blood cells

the cause is a single amino acid substitution in the

B chains of hb Glucoses at position 6 of the B-chains is replaced by val

as a result, Hb S molecules

aggregate into long, chainlike polymeric structures that poke the erythrocyte membrane into a sickle shape

the polymerization of Hb S molecules arises because

val replaces glu on the surface of B chains

these can insert into

hydrophobic pockets in neighboring deoxy Hb S molecules