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!
MYOGLOBIN 3D MODELS: 3dprint.nih.gov/discover/3dpx...
3dprint.nih.gov/discover/3dpx...
Пікірлер: 192
Medical students really need these kind of short animated videos...as they hardly get time for long 1 hour lectures...my appreciation for your smart work..keep going like this
@nicks.3181
5 ай бұрын
Biotechnology Students approve that 🤝
Thank you for explaining the more complicated, in-depth concepts such as HOW the hemoglobin changes shape, instead of just saying that it does. SUPER helpful.
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Thank you for such a great, in-depth explanation for all us medical students!
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Just about 6 minutes of a very substantial discussion. The 2hr class lecture can never lol Amazing job!
Didn't know that I could learn so much and in an easy way in only 6 minutes. Thank you!
Absolutely brilliant video!...seriously can't thank you enough!!!!! It took you 4 minutes to explain what my lecturer has tried to do in 4 2 hour lectures!
Fenomenal video!!! You just made it perfectly clear for me, my teacher couldn’t do that in two hours while speaking about it
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Just stumbled on your videos as I'm going through an online, at your own pace, biochemistry class. This was very helpful, thank you!
@NeuralAcademy
5 жыл бұрын
I'm so glad to be of help ^_^ Thank you for your kind words! :-D
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@Mistfall254
4 жыл бұрын
IKR
@okoyes7151
4 жыл бұрын
What
@alixzandrazabal1671
3 жыл бұрын
YEP!
Really good job and deep explanation ! It's always difficult to find some explanations going so far. Most of the time it's too general: Thank you ;)
@NeuralAcademy
5 жыл бұрын
Thanks! I do my best to go as in depth as possible while keeping the videos to a reasonable length :-)
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@mollyhorse
3 жыл бұрын
Haha...that's what I was thinking,Alyssa!
@kingbobbie5196
3 жыл бұрын
I know right 😅😅
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Honestly. Reading from the book made me so confused!! And now everything is so clear! It’s way more easy to understand from this video.
Really good and very nicely explained.I'll recommend this video to my students. #Thanks a lot# from Bengaluru, India.
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SUCH A GOOOD VIDEO, THANK YOU!
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Great video as always!
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شكرا جزيلا لك يا دكتور
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Great videos! Will you please publish a series of videos on laboratory techniques in cell biology labs. Thanks
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I would appreciate it if you could talk about the fisher hypothesis on the respritory system
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Thank you, you just saved me from studying for hours on end. One question though, why do we need the non cooperative form at some point? Wouldn't it be much more efficient if it was always cooperative?
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Life saving video
Thanks I just remembered why I quit nursing