Chemical Structure of Amino Acids Quiz

Quiz Answer Key and Fun Facts

1. The simplest of all amino acids, which amino acid is represented by this R group (A=amino acid backbone): A-H ?

Answer: Glycine

Glycine is the simplest of the amino acids, with only a hydrogen atom serving as its R group. It is a non-essential amino acid as it is synthesized in the body from serine, another amino acid. Glycine is unique among the 20 standardized amino acids in that it does not have a chiral center - a central carbon atom containing four different attached groups.

This means glycine does not form enantiomers as the other 19 amino acids do. It can exist in both hydrophobic or hydrophilic conditions.

2. One of two sulfur-containing amino acids, which amino acid has the following R group (A=amino acid backbone): A-CH2-SH ?

Answer: Cysteine

Cysteine is one of two amino acids which contain sulfur - the other is methionine. However, unlike in methionine, the sulfur atom in cysteine is very chemically reactive, and can be oxidized to form disulfide bonds which are important in protein structures, particularly tertiary structures.

Cysteine is nonpolar, uncharged, and hydrophobic. It is non-essential, however may be essential in rare cases, for example in infants, the elderly, and people with absorption problems. Cysteine is potentially toxic, thus is oxidized to a dimer of two cysteine atoms called cystine, which is non-toxic.

3. Which amino acid's R group is this (A=amino acid backbone, neg=negative charge): A-CH2-CH2-C(=O)-O(neg) ?

Answer: Glutamic acid

Glutamic acid is one of two acidic amino acids (the other is aspartic acid). The carboxylated form (shown in the question, which is the form glutamic acid takes at physiological pH) is very important in taste sensation, contributing to the taste known as umami. MSG, found in many foods, is monosodium glutamate. Glutamic acid is non-essential.

4. Name the amino acid, one of three alcohol (-OH) containing amino acids, that this R group is part of (A=amino acid backbone): A-CH(CH3)-OH

Answer: Threonine

Threonine is a polar, uncharged amino acid, and is hydrophilic - it can form hydrogen bonds with water, and is thus found mostly on the protein surface. It is one of three amino acids which contain an alcohol (-OH) group: the others are serine and tyrosine. Threonine is an essential amino acid, meaning it cannot be synthesized in the body, and must be contained in the diet. Poultry, meat, and lentils are just a few of the foodstuffs containing threonine.

5. Which unusual amino acid does this R group belong to (pos=positive charge): A closed ring structure forming from alpha C-CH2-CH2-CH2-H2N(pos)-alpha C. [NB: only one alpha C is present, this closes the ring. Alpha C and H2N(pos) are part of amino acid backbone.]

Answer: Proline

Proline is an unusual amino acid, in that the backbone is involved in the ring structure. It is a non-essential, hydrophobic amino acid. The ring structure frequently causes disruptions in otherwise regular secondary structures - alpha-helices and beta-sheets. Proline can be oxidized to a non-standard amino acid called hydroxyproline by an enzyme named prolyl hydroxylase; hydroxyproline can go on to form polyproline helices with other hydroxyprolines or prolines. This is the main substituent of the fibrous protein collagen, found in skin. Vitamin C is a co-factor of this enzyme - symptoms of scurvy (a disease caused by lack of vitamin C) result from lack of hydroxyproline, and thus less effective collagen is formed.

Proline is one of six aliphatic amino acids.

I apologize for the poor representation of ring structures in this quiz - unfortunately non-standard characters are not supported by the system, so I couldn't enter vertical bonds.

6. This structure represents the R group of which amino acid, which is known for its role in mutations causing sickle-cell anemia [A=amino acid backbone]: A-CH(CH3)-CH3 ?

Answer: Valine

Valine is a non-polar amino acid. It is essential, so it must be eaten in foods such as fish, lentils and peanuts.

Valine is well known because of its role in sickle-cell anemia - the gene for the beta chain of haemoglobin contains a glutamic acid residue at point 6. In the sickle-cell mutation, this glutamic acid is replaced by valine. Because valine is hydrophobic, it turns in to the center of the protein, and the beta chain does not fold properly, causing the haemoglobin to be defective.

7. Which amino acid, first obtained from silk, has this R group [A=amino acid backbone]: A-CH2-OH ?

Answer: Serine

Serine is a non-essential, hydrophilic amino acid, which plays an important role in the synthesis of purines and pyrimidines (the two types of nitrogenous bases in DNA) in the body. It is also important as a component of the active site in many enzymes, such as chymotrypsin.

Serine's name is derived from the Latin for silk (as an adjective), which is serica, as serine was first obtained from silk proteins.

8. Which amino acid, the second most commonly integrated into proteins, has this R group [A=amino acid backbone]: A-CH3 ?

Answer: Alanine

Alanine is a hydrophobic, non-essential amino acid. It is the second most commonly integrated amino acid in proteins, the first being leucine. Although alanine is non-essential, it can be found in various foodstuffs, particularly in meats. It has been suggested that high concentrations of alanine in the body are linked to hypertension.

9. Which aromatic amino acid is represented by this R group (A=amino acid backbone): A-CH2-pentagonal ring containing an NH group and double bond-hexagonal phenyl group ? (Or A-CH2-indole ring) [Sorry for the poor representation!]

Answer: Tryptophan

Tryptophan is one of three aromatic amino acids, along with tyrosine and phenylalanine. It is moderately hydrophobic but can form hydrogen bonds with water, so can be found on both the surface and inside of a protein. As stated in the question, tryptophan has a double ring structure called an indole.

It is an essential amino acid. Tryptophan is important as a precursor of compounds such as serotonin (a neurotransmitter) and auxin (a phytohormone).

10. Which amino acid has this R group and plays an important role in secondary structures of proteins [A=amino acid backbone]: A-CH2-C(=O)-NH2 ?

Answer: Asparagine

The answer is asparagine, which is a non-essential, hydrophilic amino acid. Asparagine is found in asparagus, and it is metabolic products of it that give urine its distinct smell after one has consumed asparagus. Asparagine plays an important role in secondary protein structures, found often at each end of alpha helices, and in turns of beta sheets.

11. Which aliphatic amino acid does this represent [A=amino acid backbone]: A-CH2-CH(CH3)-CH3 ?

Answer: Leucine

This may have been a bit more difficult, as each answer was an amino acid with a solely hydrocarbon R group - altogether these are known as the aliphatic amino acids.

The correct answer is in fact leucine, a hydrophobic, essential amino acid. In the body, leucine is mainly used in the adipose tissue and muscle, and to a much lesser extent in the liver. Although it cannot be synthesized by humans or animals, plants can make leucine using pyruvic acid.

Leucine is added to food as a flavor enhancer, and is coded E641.

12. Which amino acid, one of three basic amino acids, does this R group represent [A=amino acid backbone, (pos)=positive charge]: A-CH2-CH2-CH2-CH2-NH3(pos) ?

Answer: Lysine

The correct answer is lysine, a hydrophilic, essential amino acid. Lysine is a basic amino acid, and can form hydrogen bonds from its amino group. It can be synthesized in plants. Lysine is extremely important in production of collagen, which contains derivatives of it. Lysine is necessary for synthesizing every protein in the body.

13. Which amino acid is represented here by this R group [A=amino acid backbone]: A-CH(CH3)-CH2-CH3 ?

Answer: Isoleucine

The correct answer is isoleucine, which, as the name suggests, is an isomeric form of leucine. Isoleucine is a hydrophobic, essential amino acid. As with many essential amino acids, it can be synthesized in plants. It can exist in four stereoisomeric forms, but in the body, it only exists in one form, as is the case with all amino acids - only the L-form is present in nature.

14. Which amino acid has the following R group (A=amino acid backbone): A-CH2-Ph [Hint: Ph is C6H5, a phenyl group.]

Answer: phenylalanine

Phenylalanine is an essential, non-polar, hydrophobic amino acid. In the body, it can be converted through various reactions to dopamin, norepinephrine (noradrenalin), and epinephrine (adrenalin).

There exists a disease called phenylketonuria (PKU), sufferers of which cannot metabolize phenylalanine, and must therefore restrict their intake of phenylalanine in the diet. Phenylalanine is present in many foods, including in fizzy drinks, and also in the artificial sweetener, aspartame. These foods contain warning labels, so that PKU-sufferers can avoid eating them, and thus avoid a build-up of phenylalanine in the body.

15. Which amino acid is represented by the following R group and is abundant in blood in its free form: [A=amino acid backbone] : A-CH2-CH2-C(=O)-NH2 ?

Answer: Glutamine

The correct answer is glutamine, which is a hydrophilic, non-essential amino acid. It is formed from glutamic acid - the hydroxyl group is replaced by an amine group, thus glutamine can be called the amide of glutamic acid. With a free concentration of 500-900 micromoles per liter, glutamine has the highest free concentration of the amino acids in human blood. One advantage of glutamine is that it reduces healing times if consumed after surgery.

16. Which amino acid is represented by this R group and whose charge varies according to environmental pH [A=amino acid backbone, (pos)=positive charge]: A-CH2-(ring form at yC=CH-NH-CH=NH(pos)-yC) ? [NB: yC = gamma carbon. There is only one gamma carbon; the ring begins and ends at this same atom.]

Answer: Histidine

The answer is histidine, a hydrophilic amino acid. In essentiality, it is unusual - it is an essential amino acid for infants, but at a few years of age, the body starts to make histidine, thus it is classed as a non-essential amino acid.

Histidine can have four different configurations depending on pH of its environment. These configurations differ in charges.

17. Which amino acid is represented by this R group, shown here in its carboxylated form [A=amino acid backbone, (neg)=negative charge]: A-CH2-C(=O)-O(neg) ?

Answer: Aspartic acid

The correct answer is aspartic acid - this form is technically the carboxylate ion of the amino acid, called aspartate. Aspartic acid is hydrophilic and non-essential. It is one of two acidic amino acids.

Aspartate is very important biologically, for example as a neurotransmitter.

18. Which amino acid's R group is this, which is integrated first into every polypeptide chain [A=amino acid backbone]: A-CH2-CH2-S-CH3 ?

Answer: Methionine

The answer is methionine. As stated in the hint, it is one of two sulfur-containing amino acids, the other being cysteine. Methionine is an essential, hydrophobic amino acid, involved in the synthesis of various compounds in the body. It has only one RNA codon, AUG, which is also the start codon for protein synthesis on mRNA, meaning methionine begins each translated polypeptide - however, it can be, and often is, removed by a process called post-translational modification.

19. Which amino acid, discovered from a cheese protein, contains this R group [A=amino acid backbone]: A-CH2-Ph-OH ? [Ph - phenyl group.]

Answer: Tyrosine

Tyrosine is a non-essential amino acid. It is polar. The name derives from the Greek for "cheese" - "tyros"; when tyrosine was discovered, it was found in a protein from cheese.

Tyrosine is an important precursor to many compounds in the body, including epinephrine (adrenalin), and the pigment melanin. It can be synthesized in the body from phenylalanine, and is found in the thyroid hormones triiodothyronine (called T3) and thyroxine (T4).

20. Which amino acid is represented by this R group and is unusual regarding essentiality [A=amino acid backbone, [pos]=positive charge]: A-CH2-CH2-CH2-NH-C(=NH2[pos])-NH2 ?

Answer: Arginine

Arginine is a hydrophilic amino acid - in essentiality, it is unusual as with histidine; it is essential for infants, but becomes non-essential as the body begins to synthesize it. Arginine has many benefits from oral ingestion, including to treat erectile dysfunction, as it is a precursor to NO (nitric oxide) which helps dilate blood vessels, which in the penis causes an erection.

However, arginine has some disadvantages associated with it - for example, a clinical trial suggested that patients who take it after a heart attack actually do not benefit at all, and in fact more people died after taking it, than those who took a placebo instead.

Source: Author reeshy

This quiz was reviewed by FunTrivia editor crisw before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

This quiz may seem rather dull to some people, but I intend it to be educational, and good practice for students of biochemistry. I will give the structure of an amino acid R group, and you select the correct name. Good luck! (A= backbone of amino acid.)

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