Biochemistry : Identifying Amino Acids

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #21 : Identifying Amino Acids

Neurotransmitters such as monoamines are essential for proper neuronal functioning. Monoamines come from precursor aromatic amino acids. Knowing this information alone, which of the following amino acids could be precursors to monoamines?

I. Histidine

II. Tryptophan

III. Tyrosine

Possible Answers:

I, II, and III

I and II

II only

II and III

Correct answer:

I, II, and III

Explanation:

The question states that monoamines are synthesized from aromatic amino acids. Aromatic amino acids are defined as those that have one or more aromatic rings in their structure. There are three main aromatic amino acids: histidine, tryptophan and tyrosine. Tyrosine is the precursor for many of the catecholamines (such as norepinephrine, epinephrine and dopamine) whereas histidine is the precursor for histamine. Tryptophan is the precursor for tyrosine; therefore, a lack of tryptophan will result in decreased catecholamine synthesis.

Example Question #22 : Identifying Biochemical Molecules

After being placed in an acidic solution, the net charge on an amino acid is observed to be positive. Which of the following could be the identity of the amino acid?

I. Arginine

II. Lysine

III. Leucine

Possible Answers:

III only

II and III

II only

I and II

Correct answer:

I and II

Explanation:

The question states that the amino acid is placed in an acidic solution. This means that the amino groups and the carboxylic acid groups on the amino acids will be protonated (due to the excess hydrogen ions found in an acidic environment). When they become protonated, basic groups like amines become positively charged whereas acidic groups like carboxylic acids become neutral. Basic amino acids have extra basic groups (like amino groups) whereas acidic amino acids have extra acidic groups (like carboxylic acids). Of the listed amino acids, arginine and lysine are basic amino acids and will have a positive charge (due to the protonated basic groups). There are three basic amino acids (arginine, lysine and histidine) whereas there two acidic amino acids (aspartic acid and glutamic acid). Leucine is neither acidic nor basic.

Example Question #23 : Identifying Biochemical Molecules

Which of the following amino acids has a side chain that is capable of forming a hydrogen bond?

Possible Answers:

Isoleucine

Glutamate

Glycine

Valine

Leucine

Correct answer:

Glutamate

Explanation:

For this question, we need to identify which amino acid has a side group that is capable of forming hydrogen bonds. To do this, it is important to be familiar with all of the amino acids, and to be able to recognize their important characteristics.

From the answer choices, the amino acid glycine has a side chain that is simply just a hydrogen atom. Moreover, the amino acids leucine, isoleucine and valine have aliphatic side chains that are hydrophobic. All of these amino acids are incapable of forming hydrogen bonds with their side chain.

That brings us to the amino acid glutamate, which has a side chain with a carboxyl group. Due to this carboxyl group, glutamate's side chain is capable of forming hydrogen bonds.

Example Question #24 : Identifying Biochemical Molecules

Consider the following peptide.

At physiological pH (), what is the expected charge on the peptide?

Possible Answers:

Correct answer:

Explanation:

In this question, we're presented with the sequence of amino acids in a polypeptide at a given pH, and we're asked to determine what the charge of this peptide would be.

To begin, we can see that the peptide is given to us in terms of single letter amino acid abbreviations. Thus, these abbreviations need to be known in order to answer this question. The amino acid and its corresponding abbreviation for the ones shown in the question stem are as follows.

K - Lysine

T - Threonine

W - Tryptophan

D - Aspartate

G - Glycine

M - Methionine

R - Arginine

V - Valine

P - Proline

A - Alanine

Next, since we need to determine the overall charge of this peptide, we'll need to be familiar with which amino acids have a side group that is capable of carrying a charge.

The amino acids that are capable of having a negatively charged side chain are aspartate and glutamate.

The amino acids that are capable of having a positively charged side chain are lysine, arginine, and histidine.

Thus, we need to go through the amino acids given to us in the question stem, and first spot which ones would have a negative charge. Then, we do the same thing for positive charge. Next, we consider the C-terminus and the N-terminus. Finally, we sum these all together to obtain our answer.

Looking through the residues in the peptide, only aspartate can carry a negative charge. Thus, there is one negatively charged residue.

Again, looking through the residues in the peptide, we can see that lysine and arginine are capable of carrying a positive charge. This means that we have two positively charged residues.

Next, let's consider the two ends of the peptide. At the C-terminal end is the alpha-carboxy group, which will carry a negative charge at physiological pH. Also, at the N-terminal end is the alpha-amino group, which will carry a positive charge at physiological pH.

Finally, we can sum everything together.

Thus, the net charge of our peptide is expected to be .

Example Question #21 : Identifying Amino Acids

Which of the following amino acids is negatively charged in a solution with pH 9.0?

Possible Answers:

Tyrosine

Glutamic acid

Arginine

Lysine

Methionine

Correct answer:

Glutamic acid

Explanation:

This problem tests knowledge of amino acid pKa values. Because the R-group of glutamic acid has a very low pKa of 4.25, which is less than the solution pH of 9.0, it will be negatively charged. Lysine and arginine are both positively charged at this pH, whereas tyrosine and methionine are both neutral.

Example Question #22 : Identifying Amino Acids

Which of the following amino acids is positively charged in a solution with pH 7.0?

Possible Answers:

More than one of these

None of these

Serine

Arginine

Tyrosine

Correct answer:

Arginine

Explanation:

The guanidino group of arginine is protonated in solutions with pH at or below the pKa of 12.5. It is a positively charged basic side chain that becomes neutral at a pH 12.5 or greater.

Serine and tyrosine both have uncharged polar side chains with one hydroxyl group, but only tyrosine has a pKa value for its side chain of 10.5. This indicates that tyrosine will lose its proton above pH 10.5 and is therefore neutral at pH 7.0.

Example Question #23 : Identifying Amino Acids

Which of the amino acids has a second asymmetric carbon?

Possible Answers:

Histidine

Isoleucine

More than one of these

Threonine

Leucine

Correct answer:

More than one of these

Explanation:

All the amino acids (except glycine) are chiral, but threonine and isoleucine are the only two with a second asymmetric carbon. As a result, they give two pairs of enantiomers that have different physical properties.

Example Question #21 : Identifying Biochemical Molecules

Which of the amino acids strongly absorb UV light?

Possible Answers:

G, A, M

None of these; amino acids do not absorb UV light.

S, T, M

F, Y, W

Y, H, I

Correct answer:

F, Y, W

Explanation:

Phenylalanine (F), tyrosine (Y), and tryptophan (W) all contain bulky aromatic side groups that cause proteins to absorb UV light at 280 nm.

Example Question #24 : Identifying Amino Acids

Which of the following describes amino acids with aromatic R-roups?

Possible Answers:

All other answers

Phenylalanine, tyrosine, and tryptophan

Can absorb ultraviolet light

Nonpolar

Hydrophobic

Correct answer:

All other answers

Explanation:

Phenylalanine, tyrosine, and tryptophan are amino acids with an aromatic (benzene) ring that are relatively nonpolar aand participate in hydrophobic interactions.

Example Question #25 : Identifying Amino Acids

Which of the following is a characteristic of selenocysteine?

Possible Answers:

All other answers

None of the other answers

A very rare amino acid residue introduced during protein synthesis

Contains selenium instead of sulfur normally found in cystiene

Derived from serine

Correct answer:

All other answers

Explanation:

Selenocysteine is a rare amino acid residue that is incorporated during protein synthesis (instead of during post-translational modifications as is the case with other rare residues). It is made from serine, but it looks like cysteine with a selenium atom in place of the sulfur atom. It is only found in a few known proteins including glutathione peroxidases. Selenocysteine has a lower pKa and lower reduction potential than cysteine.

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