Oxygen-Binding Proteins: Myoglobin, Hemoglobin, and Heme Mechanics

Name: Heme Group of Hemoglobin and Myoglobin
Uploaded: 2015-03-02T23:33:18+00:00
Duration: 13 min 4 s
Channel: Andrey K
Description: Summary and key takeaways on Heme Group of Hemoglobin and Myoglobin: Summary & Key Takeaways, covering Oxygen Transport Proteins Myoglobin consists of a single

Andrey K

Mar 02, 2015

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13 min video

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2 min read

YouTube video ID: L6vhDsi8O_g

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Myoglobin consists of a single polypeptide chain that stores one heme group and binds a single oxygen molecule. It releases oxygen when the cellular oxygen concentration drops, providing a local reserve for muscle cells.

Hemoglobin is composed of four polypeptide chains—two alpha and two beta—each attached to its own heme group. The quaternary structure enables cooperative binding: when one iron atom captures oxygen, the protein shifts conformation, increasing the affinity of the remaining sites. Hemoglobin carries oxygen from the lungs to tissues and returns carbon dioxide to the lungs for exhalation.

The Heme Group

The heme prosthetic group combines an organic protoporphyrin ring with a central inorganic iron atom. Protoporphyrin contains carbon, nitrogen, hydrogen, and oxygen atoms arranged in a planar macrocycle. The iron atom, in the ferrous (+2) oxidation state, sits at the center of this ring and serves as the direct binding site for oxygen.

Binding Mechanism

In the deoxy (unbound) state, the Fe²⁺ ion is coordinated to the four nitrogen atoms of protoporphyrin and to a nitrogen from a proximal histidine residue. Because the iron’s electron cloud is relatively large, the ion sits slightly below the plane of the porphyrin.

When diatomic oxygen approaches, its high electronegativity pulls electron density away from the iron. This reduction in electron density shrinks the iron radius, allowing the ion to move into the center of the porphyrin plane and form a bond with oxygen. The proximal histidine continues to anchor the iron, while the distal histidine on the opposite side of the ring forms a hydrogen bond with the bound oxygen, further stabilizing the complex.

Stabilization

The iron‑oxygen complex is resonance‑stabilized, existing as a hybrid of a neutral dioxygen‑ferrous iron state and a superoxide‑ferric iron state. The distal histidine’s hydrogen bond specifically stabilizes the negative charge on the superoxide component, preventing premature release of oxygen and ensuring efficient transport.

Takeaways

Myoglobin stores one oxygen molecule in muscle cells and releases it when oxygen levels fall.
Hemoglobin’s four-chain structure enables cooperative binding, allowing it to transport four oxygen molecules from lungs to tissues.
The heme group’s iron atom binds to four porphyrin nitrogens and a proximal histidine, positioning it below the porphyrin plane in the deoxy state.
Oxygen binding pulls electron density from iron, shrinking its radius so the iron moves into the porphyrin plane and forms a stable iron‑oxygen complex.
The distal histidine hydrogen‑bonds to bound oxygen, resonance‑stabilizing the superoxide‑ferric form and securing oxygen for transport.

Frequently Asked Questions

How does the distal histidine stabilize oxygen binding in hemoglobin?

The distal histidine forms a hydrogen bond with the bound oxygen molecule, stabilizing the negative charge of the superoxide‑ferric resonance form. This interaction helps keep oxygen securely attached while allowing release when needed.

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Biochemistry Molecular Model Kit Recommended

Provides a physical representation of protein structures and heme groups, helping to visualize the spatial relationships between iron, histidine, and protoporphyrin.

Amazon →

Lehninger Principles Of Biochemistry Textbook

A standard academic resource that provides detailed diagrams and explanations of hemoglobin and myoglobin structure, reinforcing the lecture content.

Amazon →

Periodic Table Of Elements Poster

Allows for quick reference of electronegativity and atomic properties, which are central to the lecture's explanation of oxygen binding.

Amazon →

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Full Transcript YouTube

inside the cells of our body a process
we call aerobic cellular respiration
uses oxygen to produce ATP molecules and
these ATP molecules are used by
ourselves as an energy
source now what exactly delivers the
oxygen to the cells of our body well we
have two proteins inside our body that
play this role they deliver oxygen to
the cells of our body and these two
proteins are myoglobin and hemoglobin
globin is a protein that consists of a
single polypeptide chain and it is found
in the muscle cells of our body it is
used by our muscle cells to store oxygen
and give the muscle cells the oxygen
when the concentration of oxygen becomes
very very low on the other hand
hemoglobin is a protein that consists of
four individual polypeptide chains we
have alpha 1 and Alpha 2 which are two
identical Alpha chains and we have beta
1 and beta 2 subunits which are also
identical subunits and so these four
polypeptide chains give the hemoglobin
molecule quinary structure and that's
exactly what gives that hemoglobin the
ability to bind oxygen cooperatively and
we'll see what that means in the next
lecture so what hemoglobin does is it
essentially continually delivers the
oxygen from the lungs and the tissues of
cells of our body and it also binds CO2
and brings the carbon dioxide back to
the lungs so that the carbon dioxide can
be expelled by our body now in this
lecture what I'd like to focus on is how
these two proteins actually are capable
of binding to oxygen in the first place
so these two proteins contain a special
prosthetic group known as the heem group
that assists the protein in actually
binding the oxygen and the heem group
has the following structure so the heem
group consists of two components it has
the organic component known as
protor that contains the carbon atoms
the nitrogen atoms the hydrogen atoms
and the oxygen atoms and this entire
region shown in black is the protop
porer it's the organic component of that
heem group now at the center of that
protopine is an inorganic atom a metal
atom the iron atom and this is what
makes up the inorganic component of that
heem group and it's this Fe atom that is
actually responsible to not only binding
to the protein but also to binding to
that oxygen as we'll see in just a
moment now notice as shown in this
diagram this Fe atom is bound to four
nitrogen atoms we have 1 2 3 4 now Fe
can have an oxidation state of postive 6
and in this particular case because we
have four bonds what that means is this
Fe is in its fairest State and what that
means is it has a state an oxidation
number of positive2 and so our Fe atom
at the center of the heem group can form
two other bonds now one of the bonds
is formed between one of the amino acids
of that polypeptide chain and this is
shown in the following diagram so if we
take the heem group and we flip it this
way so if the heem group lies on the
plane of the board and we take it and we
flip it this way then at the bottom
portion of that heem group we have an
amino acid more specifically we have a
histadine amino acid that is part of the
protein either myoglobin or that
hemoglobin that is bound onto that Fe
atom so this purple circle is the Fe
atom the green circle are the nitrogen
atoms the blue circles are the oxygen
atoms and the purple circle is that
metal atom that Fe atom so on one side
of the protor and plane the iron atom is
boun to the histadine residue of that
polypeptide chain that polypeptide chain
can can be part of the myoglobin or it
can be part of the hemoglobin molecule
so because each polypeptide chain
contains a single heem group myoglobin
contains a single heem group but
hemoglobin contains four different heem
groups and that means myoglobin can only
bind onto one oxygen while hemoglobin
can bind four different oxygen atoms as
we'll see in just a moment so on the
bottom of that Fe we have the bond form
between the nitrogen of this histadine
residue and this metal atom found in
that heem group now in this particular
State the electron density around that
Fe is simply too large for that Fe to
actually fit inside the center of that
plane and that's exactly why this Fe
atom will be found slightly below the
protop porer plane as shown in the
following diagram so in
deoxyhemoglobin or deoxy myoglobin when
the protein is not Bal to the oxygen the
iron adal remains unbal to oxygen and in
this case the F metal atom is simply too
large it has too large of an electron
density around that proton nucleus for
that entire metal atom to actually fit
Snuggly in the center of that protop
porphin and so what that means is this
Fe atom will be found slightly
below now in this particular case this
Fe atom has 1 2 3 four five bonds and
what that means is it can form one more
bond with some other atom and so on the
top portion of that Fe that is exactly
where that Bond will be formed between
the datomic oxygen and that metal atom
remember it's this metal iron atom of
the heem group that binds directly and
holds onto that datomic oxygen so if
this is the Unbound state of our protor
then when the oxygen actually binds what
happens is this datomic oxygen moves
from the top position of that Fe and it
begins to pull away some of that
electron density from the that metal
atom remember oxygen is the second most
electronegative atom on the periodic
table and it is much more electr
negative than this metal iron atom and
so what that means is the electron
density will be pulled away from that
metal atom decreasing the radius and the
size of that metal atom and so what
happens is because the size of this iron
atom decreases it now is able to fit at
the center of that protor Forin group
and so this is what it will look like
when it will be bound to that datomic
oxygen so this is the datomic oxygen
that is bound to this metal atom and it
pulls away the electron density
decreasing the size of the metal atom
and now the metal atom is able to fit
into the center of that protop porer
plane now what exactly is the structure
that describes this complex here well
this complex between the metal atom and
the datomic oxygen can be described by a
resonance stabilized structure as shown
on the board so these are the two
electron structures that describe the
resonance stabilized complex between our
Fe atom and the datomic oxygen so on
this side on this electron configuration
we have a DI oxygen that contains a
neutral charge and we have the iron that
contains a positive two a positive two
charge so it is in its fairest state but
what happens is because the dioxygen
because the datomic oxygen consists of
these two electronegative atoms they can
pull away an electron readily from that
feris atom and create a feric ion that
contains a positive three charge and so
one of the electrons will be pulled away
from the metal atom and onto the oxygen
giving this datomic oxygen a negative
charge and this is called a super oxide
ion in fact this super oxide feric ion
complex is the resonance structure that
more closely describes what the
structure is between these uh two atoms
the two oxygen atoms and the single
metal atom so we see that when the
datomic oxygen binds onto that Fe from
the top that datomic oxygen develops a
negative charge because it is more
electr negative and it pulls away those
electrons that electron from the metal
atom and that's precisely what moves
this entire residue up and what allows
this metal atom to fit into the center
portion of that protor group
now because the datomic oxygen gains a
negative charge it becomes slightly less
stable and so we have to be able to
somehow stabilize this datomic oxygen
inside that heem group and in fact what
happens is we have another histadine
residue that is part of the polypeptide
chain inside that protein either
myoglobin or hemoglobin that binds
creates a hydrogen bond with that
negative charge and this is shown on the
follow in the following diagram so the
actual structure of the iron oxygen
complex is resonance stabilized as shown
in this diagram one of it consists of
this structure and the other one
consists of this structure here and
notice that the super oxide oxygen form
has a negative charge on that oxygen and
that destabilizes it and to stabilize
this structure we see that a region of
the protein another histadine amino acid
forms a hydrogen bond with this oxygen
as shown in the following diagram and
this residue is known as the disal
histadine so this here is known as the
proximal histadine and the proximal
histadine is the amino acid of that
polypeptide chain that forms a bond with
that Fe atom that holds the he group to
that protein and it's the disal
histadine that is found on the opposite
side on the opposite plane of the
protopine that is responsible in forming
a hydrogen bond between this oxygen here
of this datomic oxygen and this nitrogen
of that histadine and this stabilizes
that diatomic oxygen so we see that if
we're talking about myoglobin or or
hemoglobin both of these proteins
contain a heem group that are that is
responsible for binding that datomic
oxygen and it's the iron atom the metal
atom at the center of that heem group
that is actually responsible for
directly binding onto that datomic
oxygen now the other side of that iron
atom is actually bound onto the amino
acid found in that protein and it's that
bond that hold the he gr inside that
protein in the first place we see that
we have the distal histadine which is
another Amino Acid found inside that
protein that is able to actually
interact and stabilize that negative
charge on the super oxide ion and as
we'll discuss in a future lecture it's
this stabilization that also allows the
unloading the release of this datomic
oxygen in its dioxygen form to the cells
and the tissues of our body