Which one of the following most accurately describes the structure of a human haemoglobin molecule that can bind four oxygen molecules to form oxyhaemoglobin?
A. It has a primary, tertiary and quaternary structure only.
B. It has a primary, secondary and tertiary structure only.
C. It has a primary and secondary structure only.
D. It has a primary, secondary, tertiary and quaternary structure.
E. It has a primary, secondary and quaternary structure only.
The correct answer is D.
To understand this question, we have to know what the different structures of a protein are. So let’s review them one by one.
First, the primary structure : it is the sequence of amino-acids, linked to each other by peptidide bonds. It is a covalent chemical bond that is found between 2 successive amino-acids, more precisely, it is formed between the carboxyl and amino group of 2 distinct molecules. It has a defined polarity : the first amino-acid is, by convention, the one that has the free amino-function : it is the N-term of the protein. On the other end is the C-term, or free carboxyl group.
Secondary structure is the local folding of the protein chain. This phenomenon is a consequence of the different patterns of the hydrogen bonds. They result in the formation of different forms of proteins : helix, strand and sheets.
The tertiary structure is the protein’s 3D structure. For example, the tertiary structure of the hemoglobin is its globular structure. The integrity of the structure is ensured by different interactions :
covalent interactions in the form of disulfide bridges between 2 cysteines
electrostatic interactions in the form of ionic and hydrogen bonds
van der Waal interactions, which is a distant dependent interaction between 2 molecules.
Additionally, binding sites are little “pockets” that are found on the tertiary structure. Like the name suggests, its purpose is to bind another molecule or also known as ligand. It could be hormones, second messengers, enzyme substrates etc.
- Lastly, quaternary structure refers to the way each protein chain is organized ; in other words, it is the combination of 2 or more subunits. Hemoglobin is an example of that : it is made of 4 sub-units, α and β.
In this question, the hemoglobin structure can bind four oxygen molecules to form oxyhaemoglobin. We have to understand that as long as there is a quaternary structure, there is a tertiary, secondary and primary structure.
Knowing that, and the fact that we are talking about a globin structure (tertiairy structure) which can bind other molecules (which means it has binding sites = a tertiary structure), the only correct answer is D.