Oxygen Binding Curve for Myoglobin and Hemoglobin
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Hemoglobin and its role in the circulatory system. Created by Sal Khan.
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Hemoglobin and Myoglobin: structures, oxygen binding curves, T-state and R-state of Hemoglobin
The first part of the video is about the location, structures and the role of each of the oxygen binding proteins, hemoglobin and myoglobin.
In the second part of the video, the association-dissociation curves of hemoglobin oxygen and myoglobin are explained as well as the indication of the sigmoidal shape of the hemoglobin curve and the two conformations of hemoglobin
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Lecture - 12 Myoglobin and Hemoglobin
Lecture Series on BioChemistry I by Prof.S.Dasgupta, Dept of Chemistry, IIT Kharagpur. For more details on NPTEl visit
HEMOGLOBIN AND MYOGLOBIN BIOCHEMISTRY
Myoglobin and hemoglobin are oxygen-binding proteins. Hemoglobin is found in blood, and myoglobin is abundant in skeletal and cardiac muscle. Hemoglobin is an oxygen-transporter, and myoglobin is an oxygen-storer.
Myoglobin is a globular protein made up of a single polypeptide chain. Hemoglobin is also a globular protein, but it is a tetramer and is composed of 4 polypeptide chains. It is an α2β2-type tetramer, with two identical α chains and two identical β chains. Each of hemoglobin’s four subunits is very similar to the polypeptide chain making up myoglobin.
The myoglobin polypeptide chain consists of 8 α-helix sections, which are denoted A-H. Each polypeptide chain of the four hemoglobin subunits also consists of these 8 alpha-helix sections. Between these helices are connecting regions named after the helices they connect – e.g. AB region. Amino acids in each helix section are numbered – e.g. His F8.
Both myoglobin and hemoglobin have a prosthetic group. The prosthetic group found in both myoglobin and hemoglobin is the heme group, made up of a protoporphyrin ring and a central iron atom.
There is a heme group in each of hemoglobin’s subunits, as well as in myoglobin’s polypeptide chain, in the cleft between the E and F helices.
Iron can interact with 6 ligands, and four of these are provided by the nitrogen atoms of the pyrroles in the porphyrin ring. A fifth is provided by the imidazole side chain of His F8. When oxygen binds to the iron, that is a 6th ligand! Note that when oxygen is added on, it is tilted at 60° to the perpendicular.
A really cool conformational change happens when oxygen binds to the iron in the heme group. This cool phenomenon is of no consequence in myoglobin, but hemoglobin’s biological function depends on it. Before the binding of oxygen, steric constraints result in the ferrous iron lying 0.055 nm above the porphyrin plane. The binding of oxygen causes the iron to be drawn into the plane of the porphyrin ring, so that it is only 0.026 nm above it. The movement of the iron drags His F8 along with it and sets off a chain of conformational changes in hemoglobin that results in increased affinity of the heme groups of adjacent subunits for oxygen.
In hemoglobin, the four subunits – the two α subunits and the two β subunits – are arranged into two dimeric halves – one α1β1-subunit pair and one α2β2-subunit pair. Each of these dimeric halves moves as one rigid body. Subunits interact mostly with dissimilar chains – in other words, α subunits interact with β subunits, but not α subunits, and β subunits interact with α subunits, but not β subunits. There are two types of contacts between the two dimeric halves of hemoglobin – packing contacts and sliding contacts. Packing contacts do not shift during the conformational changes that occur after the binding of oxygen, while sliding contacts do.
When oxygen binds, the conformational change results in the dimeric halves rotating 15° relative to one another. Hemoglobin’s two conformations are called the T (for tense or taut) and R (for relaxed) forms. When hemoglobin is in the T form, oxygen is only accessible to the heme groups of the α-chains. Steric hinderance prevents it from binding to the chains. This steric hindrance is not present in the R conformational state. Hemoglobin resists oxygenation because its deoxygenated form, the T form, is stabilized by certain hydrogen bonds and interchain salt links. These interactions are broken in the oxygenated form, the R form, where hemoglobin is stabilized in a different conformation.
Meanwhile, myoglobin does not easily release oxygen. When myoglobin binds oxygen, it becomes oxymyoglobin. Oxymyoglobin releases oxygen during times of extreme oxygen deprivation, like when you’re exercising.
While Myoglobin’s O2-binding interaction displays classical Michaelis-Menten-type saturation behaviour, Hemoglobin’s interaction results in a sigmoid-shaped curve rather than a hyperbolic one. The sigmoid shape allows us to draw some conclusions. Binding of oxygen to one subunit of hemoglobin strongly enhances binding of oxygen to other subunits – a phenomenon called cooperativity.
Hemoglobin binds oxygen in the lungs, where the partial pressure of oxygen is around 100 torr. Here, 98% of hemoglobin has oxygen bound to it. In the capillaries of some tissues, the partial pressure of oxygen is 40 torr, and the hemoglobin releases oxygen. Here, 6% of hemoglobin has oxygen bound to it. The 92% difference is thanks to cooperativity. If hemoglobin’s curve was hyperbolic, then only 79% of hemoglobin would have oxygen bound in the lungs, and 28% of hemoglobin would have oxygen bound in the capillaries, for a difference of 51%. So the cooperativity means that hemoglobin is… 92/51% = 1.8 times more efficient at delivering oxygen!
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NOTE: Correction: The histidine that anchors the heme group is the PROXIMAL* histidine, while the histidine that helps O2 bind is the DISTAL* histidine. In the video, I mistakenly switched that. My apologies.
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In this video, I discuss the structures, functions, and key characteristics of myoglobin and hemoglobin, as well as compare and contrast the two.
Myoglobin (Mb) is a globular protein in muscle cells that functions in binding oxygen arriving to the muscles from the blood so as to deliver said oxygen to the muscle cells. Myoglobin has a single polypeptide chain with 8 alpha helices and no beta sheets. Since it has only one polypeptide chain, its highest level of protein structure is tertiary. It has one heme group (a prosthetic group) with an iron ion. It has two histidine residues in its interior, one of which is the proximal histidine, which anchors the heme group, while the other histidine residue, the distal histidine, helps myoglobin bind oxygen by reducing the binding affinity of carbon monoxide. Myoglobin’s oxygen binding curve is hyperbolic, characteristic of non-allosteric proteins.
Hemoglobin (Hb) is a globular protein in red blood cells that functions in taking oxygen from the lungs to the tissues (including muscle tissue) via the bloodstream. Hemoglobin has quaternary structure, as it is made up of 4 polypeptide subunits, two alpha subunits, two beta subunits, each of which can bind one oxygen molecule (each subunit has a heme group). Hemoglobin displays cooperativity, a form of allosteric regulation; essentially, once one oxygen molecule is bound to one subunit of hemoglobin, the affinity for oxygen at the other subunits increases. It becomes easier for each successive oxygen molecule to bind hemoglobin. Hemoglobin’s oxygen binding curve is sigmoidal, characteristic of allosteric proteins.
Between the two, Myoglobin has a higher affinity for oxygen than Hemoglobin. This can be see in their oxygen binding curves (shown in the video). This makes intuitive sense, though. If myoglobin functions by taking oxygen from hemoglobin in the blood to give to the muscle cells, then it MUST have a higher affinity for oxygen than hemoglobin. Otherwise, myoglobin wouldn’t be able to take the oxygen away from hemoglobin, and the muscle cells wouldn’t be able to be supplied with oxygen.
NOTE: Correction: The histidine that anchors the heme group is the PROXIMAL* histidine, while the histidine that helps O2 bind is the DISTAL* histidine. In the video, I mistakenly switched that. My apologies.
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Hemoglobin and myoglobin
This lecture explains about Hemoglobin and myoglobin structural and functional similarities and differences.
Myoglobin and hemoglobin are hemeproteins whose physiological importance is principally regarding their ability to bind molecular oxygen. Myoglobin is a monomeric heme protein found usually in muscle tissues the place it serves as an intracellular storage website for oxygen. Throughout periods of oxygen deprivation oxymyoglobin releases its certain oxygen which is then used for metabolic purposes.
The tertiary constitution of myoglobin is that of a typical water soluble globular protein. Its secondary constitution is amazing in that it contains an extraordinarily high proportion (75%) of α-helical secondary structure. A myoglobin polypeptide is comprised of eight separate correct passed α-helices, targeted A via H, which are connected by quick non helical regions. Amino acid R-agencies packed into the internal of the molecule are predominantly hydrophobic in character whilst those exposed on the surface of the molecule are ordinarily hydrophilic, accordingly making the molecule moderately water soluble.
Every myoglobin molecule comprises one heme prosthetic staff inserted right into a hydrophobic cleft in the protein. Every heme residue comprises one relevant coordinately certain iron atom that is in most cases within the Fe2+, or ferrous, oxidation state. The oxygen carried with the aid of hemeproteins is sure instantly to the ferrous iron atom of the heme prosthetic group. Oxidation of the iron to the Fe3+, ferric, oxidation state renders the molecule incapable of traditional oxygen binding. Hydrophobic interactions between the tetrapyrrole ring and hydrophobic amino acid R groups on the inner of the cleft within the protein strongly stabilize the heme protein conjugate. Furthermore a nitrogen atom from a histidine R crew placed above the airplane of the heme ring is coordinated with the iron atom further stabilizing the interplay between the heme and the protein. In oxymyoglobin the remaining bonding web site on the iron atom (the 6th coordinate role) is occupied with the aid of the oxygen, whose binding is stabilized by using a 2d histidine residue.
Carbon monoxide also binds coordinately to heme iron atoms in a fashion just like that of oxygen, but the binding of carbon monoxide to heme is far improved than that of oxygen. The preferential binding of carbon monoxide to heme iron is largely liable for the asphyxiation that outcome from carbon monoxide poisoning.
Grownup hemoglobin is a [α(2):β(2)] tetrameric hemeprotein discovered in erythrocytes where it's liable for binding oxygen within the lung and transporting the bound oxygen throughout the physique where it is used in cardio metabolic pathways.
For an outline of the special forms of hemoglobin tetramers see the part below on Hemoglobin Genes. Each subunit of a hemoglobin tetramer has a heme prosthetic workforce same to that described for myoglobin. The usual peptide subunits are specific α, β, γ and δ which might be arranged into probably the most most of the time happening functional hemoglobins.
Even though the secondary and tertiary constitution of various hemoglobin subunits are an identical, reflecting huge homology in amino acid composition, the versions in amino acid composition that do exist impart marked differences in hemoglobin's oxygen carrying residences. Furthermore, the quaternary constitution of hemoglobin leads to physiologically most important allosteric interactions between the subunits, a property missing in monomeric myoglobin which is in any other case very similar to the α-subunit of hemoglobin.
Evaluation of the oxygen binding residences of myoglobin and hemoglobin illustrate the allosteric houses of hemoglobin that outcome from its quaternary constitution and differentiate hemoglobin's oxygen binding properties from that of myoglobin. The curve of oxygen binding to hemoglobin is sigmoidal typical of allosteric proteins wherein the substrate, in this case oxygen, is a positive homotropic effector. When oxygen binds to the primary subunit of deoxyhemoglobin it increases the affinity of the remaining subunits for oxygen. As further oxygen is sure to the 2d and 0.33 subunits oxygen binding is further, incrementally, bolstered, in order that on the oxygen anxiety in lung alveoli, hemoglobin is completely saturated with oxygen.
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Biochemistry and Cell Biology: Haemoglobin and Myoglobin
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Hemoglobin vs Myoglobin: Oxygen Dissociation Curves
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1. Myoglobin is a protein that acts like a 'battery', storing oxygen in muscles until it is needed. It is related to the oxygen transport protein called hemoglobin, but does not have a signicant function in transporting oxygen - only storing it.
2. Myoglobin contains a functional group called 'heme'. Heme is composed of a ring called Protoporphyrin IX plus an iron atom. Heme is closely related to chlorophyll.
3. The iron group in heme is held in place by 4 nitrogens of the heme and one N of a histidine above the ring. In addition to molecular oxygen, carbon monoxide can bind to the iron of the heme group.
4. Hemoglobin differs from myoglobin in containing four polypeptide subunits instead of one. Interactions between multiple polypeptide subunits of a protein are called quaternary structure.
5. Hemoglobin contains two alpha and two beta subunits, each carrying one heme molecule. Binding of an oxygen molecule by one subunit causes a slight conformational change in the subunit that causes a slight quaternary change that causes an adjacent subunit to bind oxygen with greater affinity. This is referred to as cooperativity. When hemoglobin is in the state of high affinity for oxygen (wants to bind oyxgen), we say it is in the R state. When it is in the low affinity state for oxygen (wants to release oxygen), we say it is in the T state.
6. Cooperativity requires multiple subunits. Myoglobin, which has only one subunit, does not exhibit cooperativity. Thus, myoglobin's affinity for oxygen does not change as the oxygen concentration changes. This is not a good property for carrying oxygen, but is great for storing oxygen. It is for these reasons that myoglobin is used to store oxygen and hemoglobin is used to carry oxygen.
7. In the lungs, the oxygen concentration is high, so hemoglobin easily gets loaded up with oxygen. In tissues, where oxygen concentration is low, one of the oxygens comes off of hemoglobin and the reversal of what happened above occurs. Loss of one oxygen by hemoglobin favors loss of the others and oxygen is dumped where it is needed.
8. The Bohr effect relates to the fact that hemoglobin loses affinity for (lets go of) oxygen the more acidic the environment in which the hemoglobin is found. Any tissue, such as muscles, when actively using energy, produces acid. Active tissues require more oxygen than non-active tissues, as we will see later in the course. Thus active tissues get more oxygen dumped on them by hemoglobin, due to the Bohr effect.
9. Adult hemoglobin contains a tiny pocket in the middle of it that can bind a molecule called BPG (also called 2,3BPG). BPG is produced by actively respiring tissues. When it is bound, hemoglobin loses some affinity for its oxygen and lets it go. Hemoglobin drops BPG before it gets to the lungs and it is broken down readily, if you're not a smoker. I
10. In smokers, BPG is in greater abundance in the blood, so hemoglobin has reduced oxygen carrying capacity.
11. Hemoglobin can also carry carbon dioxide. CO2 is a byproduct of active tissues. When hemoglobin picks up protons near active tissues, it dumps the oxygen and picks up CO2, which it transports it back to the lungs. In the lungs, the pH is low, the protons and CO2 come off. They are actually forced off due to the high oxygen concentration. When CO2 comes off, it is exhaled.
12. Fetal hemoglobin differs from adult hemoglobin in that the two beta subunits are replaced by two gamma subunits. This changes hemoglobin's structure very slightly so that 2,3BPG can't bind. Consequently, fetal hemoglobin spends more time in the R state and can take oxygen away from adult hemoglobin.
13. Sickle cell anemia arises from a mutation in one of the hemoglobin subunits. This mutation causes hemoglobin to polymerize under low oxygen conditions and converts the blood cells into a sickle shape. They get stuck in capillaries when this happens. The mutation appears to convey a bit of protection against malaria when present heterozygously
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