Question

Which of the following statements are correct regarding the structures of myoglobin and haemoglobin and their abilities to bind oxygen?

Select one or more:

a. When haemoglobin has two oxygen molecules bound, it effectively has a lower affinity for oxygen than when it has no oxygen molecules bound.

b. Myoglobin and haemoglobin have very similar tertiary structures.

c. Myoglobin and hemoglobin have very similar quaternary structures.

d. Haemoglobin is a tighter oxygen binder than myoglobin because it has a different prosthetic group.

e. Haemoglobin can alter its oxygen affinity due to its quaternary structure and its ability to change between two different conformational states.

Answer 1

Answer: option (e) is the correct answer. Haemoglobin can alter its oxygen affinity due to its quaternary structure and its ability to change between two different conformational states.

Myoglobin and hemoglobin are oxygen-binding proteins. Hemoglobin is found in blood, and myoglobin is abundant in skeletal and cardiac muscle. Myoglobin has higher affinity for oxygen than Hemoglobin and its primary function is to store oxygen whereas Hemoglobin transports oxygen. Myoglobin binds one oxygen molecule whereas Hemoglobin binds 4. Myoglobin binds oxygen more tightly than does hemoglobin.

Myoglobin is a globular protein made up of a single polypeptide chain (monomer). 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. 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. A conformational change happens when oxygen binds to the iron in the heme group. This phenomenon is of no consequence in myoglobin, but hemoglobin’s biological function depends on it. Binding of oxygen to one subunit of hemoglobin strongly enhances binding of oxygen to other subunits – a phenomenon called cooperativity.

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 behavior, Hemoglobin’s interaction results in a sigmoid-shaped curve rather than a hyperbolic one.

BASIS FOR COMPARISON         

	HEMOGLOBIN	MYOGLOBIN
Number of chains	Hemoglobin has 4 chains of two different types- alpha and beta, delta, gamma, or epsilon (depending on the type of hemoglobin).	It contains single polypeptide chains.
Type of structure	A tetramer composed of 2 α and 2 β subunits.	A monomer and lack quaternary structure.
Binding	Bind with 4 oxygen molecules and exhibits cooperative binding. It can binds CO2, CO, NO, and H+ also.	Bind with single oxygen molecules, tightly and firmly. Does not exhibits cooperative binding.
Binding affinity	Has a low affinity to bind with oxygen.	Has a high affinity to bind with oxygen. Which does not depends on the oxygen concentration.
Their presence	Systemically all over the body.	In muscles cells.
Types of curve	Sigmoid binding curve.	Hyperbolic curve.
Also known as	Hb.	Mb.
Role	Hemoglobin is transported along with blood to whole body and carries oxygen.	Myoglobin supplies oxygen to muscles only, which is helpful at the starving time of oxygen.
Concentration in blood	High in RBC.	Low.