Amino Acids, Peptides, and Proteins

Which amino acid has three pKa values?

• leucine

• proline

• glycine

• threonine

• cysteine

Answer explanation

Cysteine has three pKa values:

  pK1 (—COOH): 1.96

  pK2 (—NH3+): 10.28

  pK3 (R group): 8.18  

Which statement is correct about peptides?

• Peptides have no α-carboxyl groups.

• Peptides have their amino acid sequences

       written from the N-terminus.

• Peptides do not have isoelectric points.

• Peptides are not biologically active.

• Peptide bonds are broken through

       condensation reactions.

Answer explanation

Peptides are named beginning with the amino-terminal (N-terminal) residue.

A new protein resembling myosin was reported. Unlike myosin, it binds calcium. Its isoelectric point and molecular weight are very similar to those of myosin. Which method would BEST separate the new protein from myosin if those two proteins were in the same buffer solution?

• ion-exchange chromatography

• size-exclusion chromatography

• affinity chromatography  

• dialysis

• fractionation

Answer explanation

Attaching calcium to the beads in the column would create an affinity matrix that could help purify the protein. Proteins that do not bind to calcium would flow more rapidly through the column than the new protein, which does bind calcium.

What measurement increases during purification of an enzyme?

• activity

• total protein

• specific activity

• fraction volume

Answer explanation

The specific activity is the number of enzyme units per milligram of total protein. The specific activity is a measure of enzyme purity: it increases during purification of an enzyme and becomes maximal and constant when the enzyme is pure.

The amino acid sequences of proteins:

• can be used to establish evolutionary relationships

  between organisms.

• are an example of tertiary structure.

• are similar within a given organism.

• are also referred to as “consensus sequences.”

Answer explanation

Protein sequences are a rich source of information about protein structure and function. Bioinformatics can analyze changes in the amino acid sequences of homologous proteins over time to trace the evolution of life on Earth and establish evolutionary relationships between organisms.