Question about myoglobin

[unsung]

This is an EK Q fr the biology book:

Myoglobin is an 18 kilo Dalton peptide composed of 153 amino acid residues in a single chain. Which protein structure does a myoglobin molecule occuy?

A. primary
B. secondary
C. tertiary
D. quarternary

The answer is C. But how were we supposed to know that based on the given info ? The "single chain" info seems to suggest A.

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[RySerr21]

This is an EK Q fr the biology book:

Myoglobin is an 18 kilo Dalton peptide composed of 153 amino acid residues in a single chain. Which protein structure does a myoglobin molecule occuy?

A. primary
B. secondary
C. tertiary
D. quarternary

The answer is C. But how were we supposed to know that based on the given info ? The "single chain" info seems to suggest A.

its not A. that would imply you are looking at the amino acid sequence, which they don't describe. They are describing a completely functional protein. it just happens to be a single chain peptide. you know it has to be either tertiary or quarternary. as far as as pikcing between C and D most proteins are gonna be functional in their tertiary structure so it wouldnt be a bad idea to assume its tertiary. the only thing i can think of off the top of my head that requires a quarternary structure is the insulin receptor.

 

[rocuronium]

I think the idea here is that you can distinguish between hemoglobin (which has 4 subunits) and myoglobin which only has 1 "subunit".

The clue to this in the question is that there are "153 amino acid residues in a single chain." That gets rid of D as well. You know the protein is functional, so it has tertiary structure. Therefore the answer is C.

 

[unsung]

I think the idea here is that you can distinguish between hemoglobin (which has 4 subunits) and myoglobin which only has 1 "subunit".

The clue to this in the question is that there are "153 amino acid residues in a single chain." That gets rid of D as well. You know the protein is functional, so it has tertiary structure. Therefore the answer is C.

Okay, gotcha. I think I missed the point of the Q That makes sense. Thanks.

 

[Vihsadas]

Another way to think of this problem is to think about the differences between hemoglobin and myoglobin. Hemoglobin is composed of 4 heme binding subunits, while Myoglobin only consists of one heme binding subunit. The heme-binding subunit itself is the tertiary structure because it is composed of a folded melange of alpha-helices and beta-sheets.

Therefore, the entire hemoglobin molecule on the other hand would be an example of a quaternary structure because it is composed of 4 subunits (tertiary).

So, because myoglobin only has one subunit it is itself an example of tertiary structure.

 

[okcomputerha]

I had it between secondary and tertiary....I thought it may be secondary because it said single chain and I thought of the alpha helix..

Are alpha helix chains functional proteins or just subunits?

thanks

 

[Doctor D]

I agree with the above posters. Since we know myoglobin has 1 subunit it cannot be quarternary since by definition quaternary means more than 1 subunit. Also as stated before almost all functional proteins are either tertiary or quaternary. Primary structure is more descriptive. For example Tertiary structure depends on both secondary structure and primary structure. If you change an amino acid in the sequence the new amino acids R group may interact differently with the rest of the amino acids to produce a new shape for the tertiary structure. In essence tertiary encompasses both secondary and primary.

 

[Vihsadas]

Exactly. Let's make it more explicit:

Primary Structure: This is the actual sequence of amino acids. This is known as the 'spelling' of the sequence.

Secondary Structure: This is the initial folding of the chain of amino acids. Usually, the two structures that are secondary are 'alpha-helices' and 'beta-sheets'.

Tertiary Structure: This is the second level of folding of the chain of amino acids or multiple amino acid chains. The tertiary structure is a mix of alpha-helices and beta-sheets. This level is known as a 'subunit'.

Quaternary Structure: This is the interaction between multiple subunits. So a protein that has 4 subunits (Hemoglobin) is automatically quaternary.

Functional proteins are always composed of at least one subunit. Since myoglobin has one subunit it is 'tertiary'. Since hemoglobin has 4 subunits, it's 'quaternary'.