Quiz - Amino Acids, Peptides, Proteins | Amino Acids, Peptides and Proteins

1

Which of the following statements about amino acids is incorrect?

Amino acids are both basic and acidic

Amino acids have only two chemical functions: a carboxylic acid group and an amino group

α-amino acids are the most common amino acids in nature

α-carbon has generally a S configuration

Amino acids are carboxylic acids bearing an amino group but they can have additional chemical functions

2

What is the dominant form of an amino acid in a basic aqueous solution (pH > 13)?

NH3+-CHR-COOH

NH3+-CHR-COO-

NH2-CHR-COO-

NH2-CHR-COOH

Amine (pKa ~ 9-10) and carboxylic acid (pKa ~ 2-3) groups ⇒ amino acids are both basic and acidic. The structure of an amino acid depends on the pH:

3

What is the isoelectric pH of glycine (pKaCOOH = 2.3; pKaNH2 = 9.7)?

2.3

6.0

9.7

12.0

Isoelectric pH (or isoelectric point pI):

pI=pKa COOH + pKa NH22=2.3 + 9.72=6.0 

4

At pH > pI, the population of amino acids with neutral R groups in solution will have:

A net positive charge

A net negative charge

No charged groups

No net charge

5

Which of the following structures describes the overall 3D folding of a polypeptide?

Primary structure

Secondary structure

Tertiary structure

Quaternary structure

Plypeptides are organized in 4 different structures:

- primary structure ⇒ sequence of amino acids
- secondary structure (pleated sheet + α helix) ⇒ folding of the primary structure
- tertiary structure (pleated sheet + α helix) ⇒ further folding and coiling
- quaternary structure ⇒ aggregation of several polypeptides (overall 3D folding)

6

Which of the following statements about polypeptides is incorrect?

Amide bonds in polypeptides are called peptide bonds

Polypeptides are hydrolyzed in acidic condition to form amino acids

Polypeptides are characterized by their sequence of amino acid from the carboxy end (left) to the amino end (right)

Polypeptides synthesis is the coupling between an amino-protected amino acid and a carboxy-protected amino acid

Polypeptides are characterized by their sequence of amino acid from the amino end (left) to the carboxy end (right)