Question 1

The isoelectric point of alanine is pH = 6.15.It is mixed with proline (pHCOOH = 2.0;pHNH₂ = 10.6),and the mixture is placed in an electric field at pH 6.15.Which statement is true?
A)The two amino acids will be separated.
B)The two amino acids will not be separated.
C)Neither amino acid will move in the electric field.
D)Both amino acids will move from the origin and be separated.

Accepted Answer

The answer of The isoelectric point of alanine is pH...

Question 2

Which shows a proper peptide bond?
A) 
B) 
C) 
D)

Accepted Answer

Peptide bond is formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another amino acid. In the given options, the amino acid sequences are not visible, but the numbering system (11ec6d39) indicates that the same amino acid sequence was used in all options. Therefore, we should focus on the bond that connects the two amino acids. Peptide bonds have a characteristic double bond character between the carbon atom of the carboxyl group and the nitrogen atom of the amino group. This results in a planar and rigid structure for the peptide bond. Option B shows the correct bond between two amino acids with a characteristic flat and double bond appearance, indicating a peptide bond formation. Other options do not show this characteristic feature and hence are incorrect.

Question 3

What is the N-terminal for the pentapeptide Val-Ile-Glu-Arg-Tyr?
A)The NH₃+ group on the side chain of Arg.
B)Valine.
C)Tyrosine.
D)Tryptophan.

Accepted Answer

The N-terminal refers to the amino acid residue at the beginning of a peptide or protein chain. In this case, Valine is the first amino acid in the pentapeptide sequence and therefore the N-terminal. The NH₃+ group on the side chain of Arg is not part of the N-terminal, but rather a functional group on one of the amino acids within the sequence. Tyrosine and Tryptophan are not part of the sequence at all.

Question 4

In a biochemistry laboratory a student added ammonium sulfate to a tube containing a buffered protein solution.The student then centrifuged the solution.What was the student probably trying to do?
A)Change the pH of the buffer solution to solubilize all proteins.
B)Hydrolyze the proteins into their constituent amino acids to determine the percent composition.
C)Derivatize the N-terminal amino acid of all proteins.
D)Selectively precipitate and purify a certain protein.

Accepted Answer

The addition of ammonium sulfate to a buffered protein solution is a common method used in protein purification to selectively precipitate and purify a certain protein. Different proteins have different solubility levels at different concentrations of ammonium sulfate, allowing for the isolation and purification of a specific protein. Centrifugation is then used to separate the precipitated protein from the rest of the solution.

Question 5

What is the net charge on the dipeptide Arg-Pro at pH 9.0? The table below gives the pKa's of the ionizable groups on the free amino acids. 
A)-1
B)-0.5
C)0
D)+0.5
E)+1

Accepted Answer

The answer of What is the net charge on the...

Question 6

The peptide bond is which of the following?
A)An amide bond.
B)An ester bond.
C)An ether bond.
D)An amine bond.

Accepted Answer

The peptide bond is formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another amino acid through a condensation reaction, releasing a molecule of water. This results in the formation of an amide bond (-CO-NH-) between the two amino acids. Therefore, option A, an amide bond, is the correct choice.

Question 7

Ammonium sulfate is often used as a first step in protein purification because it
A)buffers solutions and protects the protein.
B)precipitates many proteins.
C)precipitates all proteins.
D)hydrolyzes proteins into their constituent amino acids.

Accepted Answer

The answer of Ammonium sulfate is often used as a...

Question 8

What is the net charge on the tripeptide Gly-Arg-Lys at pH 7? The table below gives the pKa's of the ionizable groups on the free amino acids. 
A)-1
B)0
C)+1
D)+2

Accepted Answer

The answer of What is the net charge on the...

Question 9

For the amino acid lysine,the Henderson-Hasselbalch equation can be applied to ________ ionization group(s).
A)one
B)two
C)three
D)four

Accepted Answer

Lysine has three ionizable groups: the alpha-carboxyl group, the alpha-amino group, and the side chain amino group. Each of these can be described by the Henderson-Hasselbalch equation at different pH values.

Question 10

The pKa of a certain weak acid is 4.0.Calculate the ratio of proton acceptor to proton donor at pH 7.0.
A)1000:1
B)20:1
C)3:1
D)1:1
E)The ratio cannot be calculated without knowing the structure of the weak acid.

Accepted Answer

The ratio of proton acceptor (A-) to proton donor (HA) in a solution of a weak acid can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). Given a pH of 7.0 and a pKa of 4.0, the equation becomes 7 = 4 + log([A-]/[HA]), which simplifies to log([A-]/[HA]) = 3. This means that [A-]/[HA] = 10^3 = 1000, so the ratio of proton acceptor to proton donor is 1000:1.

Question 11

The pKa's of the side chain group and the α-carboxyl group of glutamate are 4.1 and 2.1,respectively.Which statement accounts for this difference?
A)The side chain has more possible resonance structures.
B)The α-carboxyl group has less steric hindrance and is therefore ionized more easily.
C)The side chain is a different functional group than the α-carboxyl group.
D)The α-carboxyl group is closer to the α-amino group than the side chain is.

Accepted Answer

The α-carboxyl group is closer to the α-amino group, which can stabilize the carboxylate ion through intramolecular interactions, making it easier to ionize and thus lowering its pKa compared to the side chain carboxyl group.

Question 12

An enzyme works well at as a catalyst at a pH of 7.2.It is found that catalytic activity is significantly less at a pH of 8.4.Which could likely cause the decrease in activity?
A)The protein has been degraded into its amino acids at pH 8.4.
B)The protein has changed shape due to a change in charge.
C)The protonation state of amino acids involved in the catalytic mechanism has changed.
D)B and C above.
E)All of the above.

Accepted Answer

Option B is correct as the change in pH can affect the charge of the amino acid residues in the active site of the enzyme, which can change the shape of the protein and thus affect its catalytic activity. Option C is also correct since the catalytic activity of enzymes can be affected by the protonation state of amino acid residues in the active site. Option A is not correct because the degradation of the protein would result in a loss of protein structure and function, not just a change in catalytic activity. Option E is incorrect as it includes option A, which is not a likely cause of the decrease in activity. Therefore, the best answer is D.

Question 13

Dialysis during protein purification is a process to
A)concentrate proteins.
B)remove high molecular weight substances.
C)dissolve insoluble proteins.
D)remove low molecular weight solutes such as salts.

Accepted Answer

Dialysis is a process used in protein purification to remove low molecular weight solutes, such as salts, from a solution containing proteins. It involves the use of a semi-permeable membrane that allows small molecules to pass through while retaining larger molecules like proteins.

Question 14

The first treatment of a crude protein extract usually involves
A)electroplating of unwanted proteins.
B)acidification followed by neutralization.
C)fractionation with varying salt concentrations and centrifugation.
D)treatment with a great number of proteases.

Accepted Answer

Fractionation with varying salt concentrations and centrifugation is the most common method used to separate proteins based on their solubility and size. This process involves changing the salt concentration of the solution to cause different proteins to precipitate out of solution and be separated by centrifugation. It is a simple and effective way to remove unwanted proteins and purify the desired protein.

Question 15

Which amino acids are linked in phenylalanylglycine?
A)Phenyline,alanine and glycine.
B)Phenol,alanine and glycine.
C)Phenylalanine and glycine.
D)Phenol,adenine and glycine.

Accepted Answer

Phenylalanylglycine consists of the amino acids phenylalanine and glycine, connected by a peptide bond. The other options involve incorrect amino acids or chemical groups.

Question 16

The ionic charges associated with a protein molecule are ________.
A)mostly contributed by the side chains of constituent amino acids
B)determined by the contribution from the α-carboxyl group,α-amino group and side chain of every amino acid in the protein
C)contributed by only the N-terminal and C-terminal residues
D)independent of the amino acid composition and depend only on pH

Accepted Answer

The side chains of the constituent amino acids in a protein molecule can carry ionic charges depending on their pKa values and the pH of the environment. This contributes to the overall ionic charge of the protein molecule. The α-carboxyl group and α-amino group also carry charges but are neutralized in the formation of peptide bonds, and hence do not contribute significantly to the ionic charge of the protein molecule. The N-terminal and C-terminal residues may carry charges depending on their side chains, but these charges are not the sole contributors to the overall ionic charge of the protein molecule. Moreover, the ionic charges of the protein molecule are not independent of the amino acid composition and depend on the pH of the environment.

Question 17

How do the pKa values of an ionizable side chain compare when the amino acid is free versus when it is in a polypeptide chain?
A)The pKa of the side chain is independent of whether the amino acid is in a polypeptide chain or is free.
B)The pKa of the side chain is always lower for the free amino acid.
C)The pKa of the side chain may be lower or higher in a polypeptide chain due to weaker inductive effects and differences in their microenvironments.
D)The pKa of the side chain is usually higher in a polypeptide chain due to stabilization from nearby residues in the three-dimensional structure.

Accepted Answer

The pKa of an ionizable side chain can be affected by the microenvironment in a polypeptide chain, such as weaker inductive effects and potential electrostatic interactions with nearby residues. Therefore, the pKa of the side chain may be either higher or lower in a polypeptide chain compared to when it is in a free amino acid form. However, it is not always lower for the free amino acid (choice B) or always higher in a polypeptide chain (choice D). Choice A is incorrect because the pKa of an ionizable side chain can be affected by its environment.

Question 18

Which substance is used to fractionate proteins based on differences in their solubility as a function of salt concentration?
A)Cellophane.
B)Sodium dodecyl sulfate.
C)Phenylisothiocyanate.
D)Ammonium sulfate.

Accepted Answer

Ammonium sulfate is a salt that can be used to fractionate proteins based on their solubility. As the concentration of ammonium sulfate is increased, the solubility of proteins decreases, allowing for separation based on differences in solubility. Cellophane and phenylisothiocyanate are not commonly used for protein fractionation, and sodium dodecyl sulfate is typically used for denaturing and separating proteins based on size.

Question 19

According to the Henderson-Hasselbalch equation,when the concentrations of proton acceptor and proton donor are the same,then
A)the carboxylic acid is totally neutralized.
B)only salt forms are present.
C)pH = pKa.
D)pKa = log[proton acceptor]/[proton donor].

Accepted Answer

The Henderson-Hasselbalch equation is pH = pKa + log([proton acceptor]/[proton donor]). When the concentrations of proton acceptor and proton donor are equal, then [proton acceptor]/[proton donor] = 1, and the log term becomes 0. Therefore, the equation simplifies to pH = pKa, which means that the pH of the solution is equal to the pKa of the acid. This is the point where the acid is half-neutralized and exists in its acidic and basic forms in equal proportions, and is called the acid's pKa. Therefore, choice C is the correct answer.

Question 20

The primary structure of a protein specifically describes the ________.
A)location of disulfide bonds
B)linear sequence of amino acids
C)overall three-dimensional shape
D)Φ and Ψ angles for each amino acid

Accepted Answer

The primary structure of a protein refers to the linear sequence of amino acids that make up the polypeptide chain. This sequence determines the ultimate three-dimensional shape and function of the protein. The location of disulfide bonds and Φ and Ψ angles are part of the secondary and tertiary structures, respectively.

Quiz 3: Amino Acids and the Primary Structures of Proteins

The isoelectric point of alanine is pH = 6.15.It is mixed with proline (pHCOOH = 2.0;pHNH₂ = 10.6) ,and the mixture is placed in an electric field at pH 6.15.Which statement is true?

Which shows a proper peptide bond?

What is the N-terminal for the pentapeptide Val-Ile-Glu-Arg-Tyr?

In a biochemistry laboratory a student added ammonium sulfate to a tube containing a buffered protein solution.The student then centrifuged the solution.What was the student probably trying to do?

What is the net charge on the dipeptide Arg-Pro at pH 9.0? The table below gives the pKa's of the ionizable groups on the free amino acids.

The peptide bond is which of the following?

Ammonium sulfate is often used as a first step in protein purification because it

What is the net charge on the tripeptide Gly-Arg-Lys at pH 7? The table below gives the pKa's of the ionizable groups on the free amino acids.

For the amino acid lysine,the Henderson-Hasselbalch equation can be applied to ________ ionization group(s) .

The pKa of a certain weak acid is 4.0.Calculate the ratio of proton acceptor to proton donor at pH 7.0.

The pKa's of the side chain group and the α-carboxyl group of glutamate are 4.1 and 2.1,respectively.Which statement accounts for this difference?

An enzyme works well at as a catalyst at a pH of 7.2.It is found that catalytic activity is significantly less at a pH of 8.4.Which could likely cause the decrease in activity?

Dialysis during protein purification is a process to

The first treatment of a crude protein extract usually involves

Which amino acids are linked in phenylalanylglycine?

The ionic charges associated with a protein molecule are ________.

How do the pKa values of an ionizable side chain compare when the amino acid is free versus when it is in a polypeptide chain?

Which substance is used to fractionate proteins based on differences in their solubility as a function of salt concentration?

According to the Henderson-Hasselbalch equation,when the concentrations of proton acceptor and proton donor are the same,then

The primary structure of a protein specifically describes the ________.