Biochemistry Chapter 3

Which amino acid has three pKa values?

• leucine

• proline

• glycine

• threonine

• cysteine

Answer explanation

Cysteine is the only amino acid among the choices that has three pKa values due to its side chain containing a thiol group (-SH), which can ionize. The other amino acids listed do not have this characteristic.

Which amino acid would MOST likely be found in the interior of a globular protein?

• Alanine

• Glutamate

• Cysteine

• Lysine

Answer explanation

Alanine is a non-polar, hydrophobic amino acid, making it more likely to be found in the interior of globular proteins, away from the aqueous environment. In contrast, glutamate, lysine, and cysteine are polar or charged.

Given that all 20 common amino acids have at least two oppositely charged groups, which statement is NOT correct?

• Solutions of amino acids can act as buffers.

• All amino acids are zwitterions.

• All amino acids carry at least one charge across the full range of their titration curves.

• Amino acid isoelectric points should be near neutrality.

• It is not possible to fully deprotonate amino acids.

Answer explanation

The statement 'All amino acids carry at least one charge across the full range of their titration curves' is incorrect. Amino acids can be neutral at their isoelectric point, where they do not carry a net charge.

At what pH values will histidine have a net neutral charge?

• below 1.82

• between 1.82 and 6.0

• between 6.0 and 9.17

• above 9.17

• There are no pH values where histidine will have a net neutral charge.

Answer explanation

Histidine has a net neutral charge between its pKa values of 6.0 (imidazole group) and 9.17 (amino group). Below 6.0, it is positively charged, and above 9.17, it becomes negatively charged.

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

The correct statement is that peptides have their amino acid sequences written from the N-terminus. This is a standard convention in biochemistry, while the other statements are incorrect.

Which statement is correct about proteins?

• Proteins can consist of more than one polypeptide chain.

• Proteins consist solely of polymerized amino acids.

• Proteins all have similar amino acid compositions.

• Proteins can, by definition, consist of no more than 2,000 amino acid residues.

• Proteins do not have ionizable groups.

Answer explanation

The correct statement is that proteins can consist of more than one polypeptide chain, as many proteins are made up of multiple subunits. The other statements are inaccurate regarding protein structure and composition.

Which component is absolutely necessary for the purification of a protein?

• A. column chromatography

• B. the gene sequence of the protein

• C. a means of detecting the protein

• D. a centrifuge

Answer explanation

A means of detecting the protein is essential for purification, as it allows researchers to identify and quantify the protein of interest during the purification process. Without detection, purification cannot be effectively monitored.

Which protein would elute first from a gel filtration column?

• protein A, with Mr = 27,000

• protein B, with Mr = 58,400

• protein C, a homodimer with protomer Mr = 11,300

• protein D, with Mr = 15,600

Answer explanation

In gel filtration, larger proteins elute first. Protein B (Mr = 58,400) is the largest, followed by A (27,000), D (15,600), and C (22,600 as a homodimer). Thus, protein B elutes first.

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?

• A. ion-exchange chromatography

• B. size-exclusion chromatography

• C. affinity chromatography

• D. dialysis

• E. fractionation

Answer explanation

Affinity chromatography is the best method to separate the new protein from myosin because it can exploit the calcium-binding property of the new protein, allowing it to be selectively retained while myosin is washed away.

The specific activity of a protein in crude cellular extract is 15 units/mg. Following a purification process, the specific activity is 12,000 units/mg. What is the purification factor?

• 180,000

• 800

• 0.00125

• 15

• 12,000

Answer explanation

The purification factor is calculated by dividing the specific activity after purification (12,000 units/mg) by the specific activity before purification (15 units/mg). Thus, 12,000 / 15 = 800.