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Example Questions
Example Question #11 : Amino Acids
Polypeptides are molecules that contain multiple __________.
nucleic acids
lipids
monosaccharides
amino acids
phosphates
amino acids
Polypeptides are made from individual amino acids through formation of peptide bonds.
Monosaccharides are the fundamental units for carbohydrates, while fatty acids come together to form lipids. Nucleotides and phosphates are key components of the nucleic acids, RNA and DNA.
Example Question #13 : Proteins
What type of amino acid will have an isoelectric point above 7?
An amino acid with a basic side chain
An amino acid with a polar side chain
An amino acid that is positively charged at a pH of 5
An amino acid with no carboxylic acid
An amino acid with a basic side chain
The isoelectric point is the pH where the amino acid solution is electrically neutral. In acidic conditions, the carboxylic acid and amino terminus will both be protonated. This results in a positive charge (due to the amine being protonated). In basic conditions, both ends are deprotonated, resulting in a negative charge.
If an amino acid is basic, that means that the pH must be above 7 in order to deprotonate the amine in the side chain. Only then will the amino acid be electrically neutral. All basic amino acids (three of them) have an isoelectric point above a pH of 7. All other amino acids have an isoelectric point at a pH below 7.
Example Question #11 : Amino Acids
A polar amino acid in a highly basic solution is titrated with a strong acid. When will exactly half of the amino acid molecules be negatively charged?
At the first half equivalence point
At the isoelectric point
At the second half equivalence point
At the second equivalence point
At the first half equivalence point
The amino acid is polar, so we do not need to worry about charges in the side chain. Since the amino acid is starting in a highly basic solution, we know that the amino acid is deprotonated at both termini. That is, the amino terminus is neutral and the carboxyl terminus is negative. This results in a net charge of -1 (at the carboxyl end). The first half equivalence point upon titration will be seen when half of the amino acids are neutral and half of the amino acids are negatively charged. The full first equivalence point will show all molecules with a neutral charge, while the second full equivalence point will show all molecules with a positive charge due to protonation of the amine.
Example Question #1571 : Mcat Biological Sciences
A polar amino acid in a highly basic solution is titrated with a strong acid. Which pH is the best prediction for the isoelectric point of this amino acid?
8.2
7.0
1.0
5.4
5.4
Only basic amino acids have an isoelectric point above a pH of 7. All others will have an isoelectric point below a pH of 7.
Since the amino acid is polar, it will have an isoelectric point below a pH of 7. Since a pH of 1 is extremely acidic, we would expect both ends of the amino acid to be protonated at that pH. As a result, a pH of 5.4 is more appropriate when predicting the amino acid's isoelectric point.
Example Question #12 : Amino Acids
Which of the following amino acids contain(s) a hydrophilic functional group in its side chain?
I. Serine
II. Valine
III. Phenylalanine
IV. Tyrosine
V. Threonine
I, IV, and V
I and IV
II, III, and IV
I, II, III, IV, and V
I, IV, and V
Serine and threonine are classified as hydrophilic amino acids and contain hydroxyl (-OH) groups in their side chains. Tyrosine, although it is considered hydrophobic, does contain a hydrophilic hydroxyl group in its side chain. The answer is I, IV, and V, as all of these contain hydrophilic functional groups.
Example Question #12 : Amino Acids
All amino acids have at least two pKa values, one corresponding to the carboxylic acid, and one corresponding to the amine functionality. Some amino acids with polar side chains also have a pKa associated with the sidechain functionality.
Phenylalanine has pKa values of 2.58 (carboxylic acid) and 9.24 (NH2).
Arginine has pKa values of 2.01 (carboxylic acid), 9.04 (NH2), and 12.48 (side chain).
Valine has pKa values of 2.29 (carboxylic acid) and 9.72 (NH2).
For this problem, consider a molecule made of up of three amino acids, as described below.
HO-phenylalanine-arginine-valine-NH2
What would the overall charge of this molecule be at a pH of 7?
Phenylalanine:
Since the phenylalanine residue is at the C-terminus end of the molecule, only its carboxylic acid pKa is relevant, as its amine is involved in a peptide bond with arginine. At a pH of 7 (well above the carboxylic acid pKa of 2.58), the C-terminus carboxylic acid would be deprotonated and have a charge of .
Arginine:
For the middle amino acid, arginine, the only relevant pKa is that of its side chain since both its carboxylic acid and amino groups are involved in peptide bonds with neighboring amino acids. Since the side chain of arginine would be protonated at a pH of 7 (well below the sidechain pKa of 12.48), this amino acid would have a charge of .
Valine:
Finally, for valine, the relevant pKa to consider is the NH2 pKa of 9.72. At pH 7, this would also be protonated, resulting in a charge of .
The overall charge of the molecule at pH 7 would be .
Example Question #11 : Proteins
How many moles of base are required to fully deprotonate glutamic acid in its most acidic form (shown below)?
Four
Zero
Five
Three
Two
Three
One mole is base is required to deprotonate each carboxylic acid or amine, until the amino acid exists in its most deprotonated and basic form. First, two moles of base would be needed to deprotonate the two acid groups on glutamic acid (one depicted on each end). Next, a third mole would be needed to deprotonate the amine to its neutral state. Three total moles is the correct answer.
Example Question #14 : Macromolecules
Drain cleaners are a common household staple, used to open clogged drains in bathtubs and sinks. Human hair is a common culprit that clogs pipes, and hair is made predominately of protein. Drain cleaners are effective at breaking down proteins that have accumulated in plumbing. Drain cleaners can be either acidic or basic, and are also effective at breaking down fats that have accumulated with proteins.
A typical reaction—reaction 1—which would be expected for a drain cleaner on contact with human hair, would be as follows in an aqueous solution:
Another reaction that may occur, reaction 2, would take place as follows in an aqueous solution:
In reaction 1, an organic acid forms as a product of the reaction of the original protein and drain cleaner. What quality of the resulting anion contributes most to the acidity of the product?
Resonance stabilization
Electronegativity of the carboxy terminus
The lone pair of electrons on the original nitrogen
Basicity of the side chain
Polarity of the C-O bond
Resonance stabilization
The resonance of the two C–O bonds that result after deprotonation of an organic acid is the major contributor to anion stability.
Example Question #181 : Organic Chemistry, Biochemistry, And Metabolism
A depletion of amino acids in a cell would slow which immediate process?
Sugar metabolism
Fatty acid use
Protein and enzyme production
All of these
DNA replication and repair
Protein and enzyme production
A depletion of amino acids would immediately impact protein and enzyme production in the cell. All proteins, including enzymes, are made from chains of amino acids. Lack of nutrients, such as amino acids, would eventually impact other processes listed.
Example Question #22 : Amino Acids
Which of the following amino acid sequences would be found on the cytoplasm side of a transmembrane protein?
Valine–leucine–proline
Cysteine–leucine–proline
Threonine–valine–leucine
Proline–valine–asparagine
Glutamine–threonine–tyrosine
Glutamine–threonine–tyrosine
Transmembrane proteins are embedded within the phospholipid bilayer of the cell membrane; therefore, the protein is exposed to the nonpolar fatty acid tails, the polar phospholipid heads, and the polar environments of the cytoplasm and extracellular space.
The questions asks which amino acids would be found facing the cytoplasm. Because the cytoplasm is polar, the amino acids interacting with the cytoplasm must also be polar. Glutamine, threonine, and tyrosine are all polar amino acids, making this the best answer. The other answers contain nonpolar amino acids (proline, leucine, cysteine, valine) causing these answers to be incorrect.
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