Biochemistry : Quaternary Structure

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Quaternary Structure

Which of these macromolecules has quarternary structure?

Possible Answers:

Chymotrypsin

Hemoglobin

Lactose

Sucrose

Myoglobin

Correct answer:

Hemoglobin

Explanation:

Hemoglobin is the only available example of a macromolecule composed of multiple subunits. Hemoglobin has frou subunits, each capable of binding and transporting one molecule of oxygen in the blood.

Chymotrypsin and myogblobin are both simple proteins, each consisting of a single polypeptide. These proteins do not have multiple subunits; thus their highest level of structure is tertiary (three-dimensional). Lactose and sucrose are disaccharides, each composed of two carbohydrate monomers (monosaccharides).

Example Question #2 : Quaternary Structure

Hemoglobin is a protein that possesses more than one polypeptide subunit, therefore it has a __________ structure.

Possible Answers:

quaternary

complex

primary

tertiary

secondary

Correct answer:

quaternary

Explanation:

Hemoglobin is a tetramer that possesses a quaternary structure containing multiple folded polypeptide structures (tertiary structures). A tertiary protein will commonly contain a single polypeptide chain with one or more secondary structures.

Example Question #3 : Quaternary Structure

Which of the following is true about quaternary structure?

Possible Answers:

A protein with multiple identical subunits does not have a quaternary structure.

None of these

When ligands bind to proteins, their 3D-structures sometimes change

The main forces holding together oligomeric subunits are disulfide bonds

Correct answer:

When ligands bind to proteins, their 3D-structures sometimes change

Explanation:

A protein with multiple identical subunits does indeed have a quaternary structure; in these cases, dimers and tetramers are common. The main forces holding together oligomeric subunits are weak, non-covalent interactions, specifically, hydrophobic ones, as well as electrostatic forces. Subunits do not necessarily form separate domains within a protein; in a potassium channel protein, for example, there are identical subunits which come together to form the single channel. Proteins’ 3D-structures do indeed sometimes change when ligands bind; this change help regulate the proteins’ biological activity.

Example Question #4 : Quaternary Structure

Which of the following proteins do not have quaternary structure?

Possible Answers:

Myoglobin

p53

Hemoglobin

DNA polymerase

Correct answer:

Myoglobin

Explanation:

Quaternary structure of a protein involves the assembly of subunits. Hemoglobin, p53 and DNA polymerase are all composed of subunits, while myoglobin is a functional single sequence. Since myoglobin does not have multiple subunits, it does not have quaternary structure.

Example Question #43 : Protein Folding

Which of the following best describes the quaternary structure of a protein?

Possible Answers:

The sequence of nucleic acids

The polypeptide chain's complete 3-D structure

The four parts of a protein's amino acid sequence

The layout of alpha-helices and beta-sheets

How the polypeptide chains fit together

Correct answer:

How the polypeptide chains fit together

Explanation:

Quaternary structure describes how polypeptide chains fit together to form a complete protein. Quaternary protein structure is held together by hydrophobic interactions, and disulfide bridges. The sequence of amino acids is known as primary structure; helices, sheets, and similar features are part of the secondary structure; and the 3-D organization is tertiary structure. "The four parts of a protein's amino acid sequence" does not refer to anything in particular.

Example Question #1 : Quaternary Structure

Which of the following best explains a quaternary structure of a protein?

Possible Answers:

Hydrogen bonds, alpha-helices and beta-pleated sheets

Hydrogen and disulfide bonds, two or more polypeptide chains

Collagen and myoglobin 

Disulfide bonds, single polypeptide chain

Linear sequence of amino acids

Correct answer:

Hydrogen and disulfide bonds, two or more polypeptide chains

Explanation:

Primary structure: linear sequence of amino acids

Secondary structure: hydrogen bonds, alpha-helices and beta-pleated sheets

Tertiary structure: disulfide bonds, single polypeptide chain

Myoglobin is a monomer, and is made of a single polypeptide chain. Thus, its highest level of protein structure is tertiary. While collagen does contain different polypeptide chains, it is an example of a protein with quaternary structure, not an explanation of what this means.

Example Question #2 : Quaternary Structure

What form of protein structure includes disulfide bonds?

Possible Answers:

Only quaternary structure

Only secondary structure

Only tertiary structure

Tertiary and quaternary structures

Only primary structure

Correct answer:

Tertiary and quaternary structures

Explanation:

In this question, we're asked about how disulfide bonds relate to protein folding. Let's go through each form of structure.

Primary structure refers to the sequence of amino acids in the polypeptide, from the N-terminal end to the C-terminal end.

Secondary structure refers to local conformations of protein folding. There are a number of commonly found motifs that have been recognized, such as alpha-helices and beta-pleated sheets. These motifs are stabilized by intermolecular interactions between amino acid side-chains and also between alpha-carboxy and alpha-amino groups of the peptide backbone. Some of these intermolecular interactions include hydrogen bonding, van der Waals interactions, dipole interactions, and ionic bonding.

Tertiary structure refers to the overall three-dimensional structure of the folded polypeptide. This form of structure relies on the same intermolecular interactions found in secondary structure. In addition, tertiary structure also includes disulfide bonds that are found between cysteine residues.

Quaternary structure refers only to proteins that are composed of multiple polypeptides. These separate polypeptides are held together by the same intermolecular forces found in secondary and tertiary structures. In addition, disulfide bonds are also found in quaternary structure, just like in tertiary structure.

Thus, tertiary and quaternary structure both include disulfide bonds.

Example Question #6 : Quaternary Structure

What is the primary driver of protein folding on a macro level?

Possible Answers:

Ionic bonding

Entropic force

Hydrogen bonding

Van der Waals forces

Covalent bonding

Correct answer:

Entropic force

Explanation:

While covalent bonds create the primary structure of a protein, and hydrogen bonding and Van der Waals forces have a large impact on the secondary structure of a protein, they are not the main contributors to overall folding of a protein. This has more to do with solvation costs, hydrophobicity, and entropy. The hydrophobicity and hydrophobic portions of the protein must fold to minimize entropic costs.

Example Question #7 : Quaternary Structure

Which of the following statements is true about proteins with quaternary structures?

I. Proteins are composed of multiple polypeptide chains.

II. Proteins are composed of subunits that interact through weak forces (noncovalent) only.

III. Sub-units may work cooperatively,one sub-unit binding to a molecule increases the affinity of the other sub-units for the same ligand.

IV. Hemoglobin is a protein displaying a quaternary structure composed of 4 sub-units.

Possible Answers:

II and III

I, III, and IV

III and IV

I, II, III, and IV

I and II

Correct answer:

I, III, and IV

Explanation:

Hemoglobin is a classic example of protein with a quaternary structure. The binding of oxygen to one sub unit increases the affinity of the other sub units for oxygen (cooperativity). Adult hemoglobin is made of two alpha globin and two beta globin polypeptides. Protein quaternary structure may involve both noncovalent and covalent forces. 

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