Question 1

Basic amino acids are positive, negative at pH 7 and acidic R group amino acids are positive, negative at pH 7.

Accepted Answer

Basic amino acids have a positive charge at pH 7 because they are protonated. Acidic R group amino acids have a negative charge at pH 7 because they have donated a proton (H+) to the solution.

Question 2

Disulfide bridges can form in proteins .

Accepted Answer

Disulfide bridges are formed between two cysteine residues that are close in three-dimensional space, but not necessarily close in the primary structure. In other words, the cysteine residues may be far apart in the linear sequence of amino acids but come closer together as the protein folds into its 3D shape. Thus, disulfide bridges are an important stabilizing factor in the 3D structure of proteins.

Question 3

An amino acid named for a plant from which it was first isolated is

Accepted Answer

Asparagine was the first amino acid to be isolated from asparagus in 1806, hence its name is derived from that plant.

Question 4

Ultraviolet UV) light can be used to estimate protein solution concentrations because

Accepted Answer

Tryptophan and tyrosine absorb at 280 nm and are commonly found in proteins, making this wavelength useful for estimating protein concentrations using UV light. Phenylalanine also absorbs at 260 nm, but this is less commonly used for protein quantification. Aromatic amino acids such as tyrosine, tryptophan, and phenylalanine absorb UV light, but not all amino acids do. Therefore, choice B is incorrect. Choice C is partially correct but does not include tryptophan, which is also an important amino acid for protein quantification. Therefore, the best choice is D.

Question 5

The overall shape of a protein is greatly influenced by

Accepted Answer

The overall shape of a protein is determined by the sequence of amino acids, and each amino acid has a unique side chain (R group) that imparts certain properties, such as hydrophobicity or hydrophilicity. These properties influence the way the protein folds into its three-dimensional shape.

Question 6

An amino acid with two chiral carbon atoms

Accepted Answer

A chiral carbon atom generates two stereoisomers, either R or S configuration. Amino acid with two chiral carbon atoms can exist in four possible stereoisomers, RR, SS, RS, and SR.

Question 7

Amino acids with non-ionizable side chains are zwitterions when they are .

Accepted Answer

Amino acids with non-ionizable side chains are zwitterions when they are at physiological pH, pH = 7.4. This is because at this pH, the amino group (which has a pKa of around 9-10) is mostly protonated and the carboxyl group (which has a pKa of around 2) is mostly deprotonated, resulting in an overall neutral zwitterionic form. In acidic solutions, the amino group will be mostly protonated and the carboxyl group will also be mostly protonated, resulting in a positively charged form. In alkaline solutions, both the amino and carboxyl groups will be mostly deprotonated, resulting in a negatively charged form.

Question 8

The RS system of nomenclature describes

Accepted Answer

The RS system of nomenclature is used to describe the absolute configuration of chiral centers, including those in amino acids, by assigning priorities to substituents around the chiral carbon.

Question 9

Arginine is the most basic of the 20 amino acids because its side chain is under most cell conditions.

Accepted Answer

The side chain of arginine has a terminal guanidinium group which contains a positively charged nitrogen atom. This nitrogen atom is able to accept a proton, making the side chain protonated under most cell conditions. This gives arginine a net positive charge at physiological pH, making it basic.

Question 10

If the R group of an amino acid is -CH₃, then the name of this compound is

Accepted Answer

The answer of If the R group of an amino...

Question 11

The amino acids in polypeptide chains which contain sulfur S) are

Accepted Answer

The amino acids that contain sulfur are cysteine and methionine. Methionine contains a sulfur atom in the side chain whereas cysteine contains a sulfur atom in a functional group called a thiol (-SH). Therefore, the correct answer is E, which includes both cysteine and methionine.

Question 12

Concentrations of some proteins cannot be estimated by UV spectrophotometry because they are

Accepted Answer

Proteins low in tryptophan and tyrosine cannot be estimated by UV spectrophotometry because these amino acids absorb UV light, and their absence results in low absorbance.

Question 13

The R group of an amino acid determines if it is

Accepted Answer

The R group, or side chain, of an amino acid determines its properties, including whether it is hydrophilic or hydrophobic, polar or nonpolar, charged or uncharged, and whether it acts as an acid or a base.

Question 14

Amino acids are named that because each one

Accepted Answer

Amino acids are named that because of their structure, which includes an amino group (-NH2) and a carboxyl group (-COOH) on the same carbon atom. This structure makes them amino derivatives of carboxylic acids. Therefore, choice D is the correct answer.

Question 15

The last common ancestor of modern organisms must have used

Accepted Answer

The last common ancestor of modern organisms likely used L amino acids, as this is the form predominantly found in proteins of all known life forms today.

Question 16

Although the hydroxyl groups in serine and threonine are uncharged, they can react within active sites of some enzymes .

Accepted Answer

The hydroxyl group in serine and threonine is polar, meaning it has a partial positive charge on the hydrogen and a partial negative charge on the oxygen. This polarity makes it more likely to interact with other atoms in the enzyme's active site, allowing for catalysis to occur. However, the fact that the hydroxyl groups are uncharged is also important, as it allows them to participate in bonding interactions without disrupting the overall charge balance of the enzyme. Size may also play a role in some cases, but it is not the primary reason why hydroxyl groups in serine and threonine are reactive in enzyme active sites.

Question 17

Proline is distinct among the 20 commonly found amino acids because

Accepted Answer

Proline is distinct because the nitrogen of its amino group is in a ring, which creates a rigid structure that reduces its flexibility and ability to form hydrogen bonds. This can have a significant impact on protein structure and function.

Question 18

Glycine is not a stereoisomer because

Accepted Answer

Glycine is not a stereoisomer because it lacks a chiral carbon, meaning it cannot form enantiomers or exist in two non-superimposable mirror-image forms. All the given reasons (A, B, and C) collectively explain why glycine does not have stereoisomers.

Question 19

Tyrosine and tryptophan are less hydrophobic than phenylalanine because

Accepted Answer

Hydrophobicity is influenced by the presence of polar or charged groups in the amino acid side chain. Phenylalanine has no polar group in the side chain, making it more hydrophobic compared to tyrosine and tryptophan, which both have polar groups in their side chains. The indole group in tryptophan may actually increase its hydrophobicity, but this is countered by the presence of the polar side chain. The fact that phenylalanine is a phenol is not directly relevant to its hydrophobicity.

Question 20

Amino acids found in meteorites and near stars are

Accepted Answer

Both D and L isomers of amino acids have been found in meteorites and in the interstellar medium, suggesting that they were formed through non-biological processes before being incorporated into life on Earth.

Quiz 3: Amino Acids and the Primary Structures of Proteins