Myoglobin and Hemoglobin 2

what are hemoglobin and myoglobin?

they are NOT enzymes; they are proteins. Myoglobin is for oxygen storage (muscle). Hemoglobin is for oxygen transport (blood).

myoglobin is

monomeric; 153 AAs and 17,200 MW

hemoglobin is

tetrameric; two α chains (141 residues) and two β chains (146 residues); shows cooperativity and allostery (KNF + MWC features)

how many ligands does iron interact with?

5 or 6; four are nitrogen atoms of the porphyrin ring

what does myoglobin contain in order to transport oxygen?

heme

is heme a cofactor, coenzyme, or prosthetic group?

all of the above (it fits all definitions)

what is the fifth ligand that iron interacts with?

the imidazole side chain of His F8

what is F8?

the 8th residue on the F (sixth) helix

what happens when myoglobin or hemoglobin bind oxygen?

O₂ becomes the sixth ligand; O₂ is tilted relative to a perpendicular to the heme plane

in deoxymyoglobin, the ferrous ion lies

0.055 nm above the plane of the porphyrin ring

when oxygen binds to iron in the heme of myoglobin

the iron atom moves toward the porphyrin ring plane

with oxygen bound, the iron atom in myoglobin is only

0.026 nm above the plane; little effect in Mb, but major cooperativity effects in Hb

what is ferrous iron?

Fe²⁺ (the oxygen-binding form of iron in myoglobin)

what does oxidation of iron do?

converts Fe²⁺ to Fe³⁺; forms metmyoglobin, which cannot bind oxygen

cooperativity in oxygen binding is not due to

direct heme-heme interactions

what do changes in the position of the heme iron atom upon oxygenation lead to?

conformational changes in hemoglobin

upon binding, the iron atom moves how much closer to the plane of the heme?

0.039 nm closer

as iron moves toward the plane

it drags His F8 and the F helix; transmits conformational change to subunit interfaces; breaks salt bridges

the deoxy (T state) is stabilized by how many salt bridges?

8

when are salt bridges broken?

in the transition from T (deoxy) to R (oxy)

where does oxygen bind first in hemoglobin?

the two α subunits

when does the large conformational change occur in hemoglobin?

when two oxygens are bound, increasing β-subunit affinity

does deoxyhemoglobin have high or low affinity for oxygen when only one oxygen is bound?

low; β-subunit hemes are inaccessible

what do small changes in tertiary structure of the other α subunit cause?

increases its oxygen affinity 3-fold (KNF model)

hemoglobin with two oxygens bound is in equilibrium with what?

the form where all four subunits are in the R state (MWC all-or-none model)

does myoglobin have a greater or lesser affinity for oxygen?

greater; it stores oxygen

how is hemoglobin different from myoglobin?

hemoglobin binds O₂ in lungs and releases it in capillaries

where does hemoglobin become saturated?

in the lungs at ~100 torr O₂

the binding of O₂ to hemoglobin is

cooperative (ideal for physiological function)

what is the Bohr effect?

protons antagonize O₂ binding; decreased pH lowers affinity

what does the protonation of histidine 146 do?

induces the R→T transition; decreases O₂ affinity

in deoxyhemoglobin, how many amino acid residues are used to form how many salt bridges?

3 residues form 2 salt bridges; stabilize the T (quaternary) structure

what does carbon dioxide do in hemoglobin?

promotes O₂ release; CO₂ hydration makes protons; diminishes O₂ binding

as oxygen dissociates

protons are taken up by hemoglobin (reverse occurs in lungs)

what do CO₂ hydration and glycolysis do?

produce extra protons, promoting O₂ dissociation in tissues

what does bicarbonate dehydration do?

consumes protons, allows CO₂ exhalation, promotes O₂ binding

what does CO₂ favor?

oxygen release; stabilizes deoxyhemoglobin; forms carbamate groups

what is the carbamate group?

negatively charged group formed on terminal amino groups; participates in salt bridges; stabilizes T state

What is 2,3-bisphosphoglycerate (2,3-BPG)?

a negative allosteric effector of hemoglobin

when 2,3-BPG is absent

hemoglobin binds oxygen like myoglobin (hyperbolic curve)

when is sigmoid binding curve observed?

when 2,3-BPG is present

where does 2,3-BPG bind?

a site far from the heme iron

where does BPG lie?

in the central cavity between the two β-subunits

what does BPG binding do?

stabilizes the T state and lowers oxygen affinity

what do negative charges interact with in BPG?

8 positive charges in the central cavity

is fetal hemoglobin's affinity higher or lower for O₂?

higher

how does fetal hemoglobin differ from adult hemoglobin?

γ-chains replace β-chains → α₂γ₂

is fetal hemoglobin's affinity higher or lower for 2,3-BPG?

lower

what amino acid do fetal γ-chains have at position 143?

serine (lacks 2 positive charges → reduced BPG binding)

because BPG binds less tightly

fetal Hb behaves more like myoglobin (higher affinity)

how can enzyme activity be regulated?

many mechanisms

allosteric effectors can regulate activity

positively or negatively

allosterically regulated enzymes are typically

oligomers that display cooperativity

cooperativity is a special case of

allostery

what can be used to explain allostery and cooperativity?

MWC and KNF models; enzymes may show features of one or both