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Quizzes > Science

Amino Acid Quiz: Test Your Knowledge of All 20

Quick amino acids structure quiz to check names, codes, and properties. Instant results.

Editorial: Review CompletedCreated By: Macarena RodriguezUpdated Aug 28, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for amino acids quiz on a teal background

This amino acid quiz helps you practice the 20 amino acids, from structures and side chains to one-letter codes and properties. Keep learning with the amino acid abbreviations quiz, explore biochemistry quiz questions, or widen your practice with chemistry quiz questions.

How many standard amino acids are incorporated into eukaryotic proteins?
18
22
26
20
There are 20 proteinogenic amino acids that are genetically encoded and incorporated into proteins during translation. These amino acids are termed the standard amino acids. Occasionally other residues like selenocysteine are inserted, but the core set is 20.
Which amino acid has a thiol ( - SH) side chain?
Methionine
Threonine
Cysteine
Serine
Cysteine contains a sulfhydryl (thiol) group that can form disulfide bonds, stabilizing tertiary and quaternary protein structures. Methionine also contains sulfur but in a thioether linkage rather than a free thiol. Serine and threonine contain hydroxyl groups, not thiols.
Which amino acid is achiral?
Proline
Valine
Glycine
Alanine
Glycine has two hydrogen atoms attached to its ?-carbon, making it achiral. All other standard amino acids (except glycine) have four different substituents on the ?-carbon and are chiral. Proline is cyclic but still chiral.
At physiological pH (~7.4), what is the predominant form of amino acids?
Neutral
Zwitterion
Cationic
Anionic
At physiological pH, the amino group is protonated ( - NH3+) and the carboxyl group is deprotonated ( - COO - ), resulting in a zwitterion. This dipolar form is common in amino acid solutions. Neither the fully cationic nor fully anionic forms predominate.
Which amino acid is aromatic?
Phenylalanine
Lysine
Leucine
Aspartic acid
Phenylalanine contains a benzyl side chain, making it aromatic. Other aromatic amino acids include tyrosine and tryptophan. Leucine is aliphatic, lysine is basic, and aspartic acid is acidic.
Which amino acid has a positively charged side chain at physiological pH?
Asparagine
Glutamate
Lysine
Cysteine
Lysine has an ?-amino group that is protonated at physiological pH, giving it a positive charge. Glutamate is negatively charged at pH 7.4, asparagine is uncharged polar, and cysteine is neutral unless oxidized.
Which amino acid is classified as acidic at physiological pH?
Glutamic acid
Valine
Lysine
Tyrosine
Glutamic acid has a carboxylate side chain that is deprotonated at physiological pH, giving it a negative charge. Aspartic acid is also acidic, but glutamic acid has one more methylene group. Lysine is basic, valine is nonpolar, and tyrosine is polar aromatic.
Which amino acid has a cyclic secondary amine in its side chain?
Phenylalanine
Proline
Alanine
Histidine
Proline's side chain forms a five-membered ring with the amino nitrogen, making it a secondary amine. This ring imposes conformational constraints on peptide chains. Alanine and phenylalanine have simple side chains, and histidine contains an imidazole ring.
What type of bond links amino acids in a protein?
Phosphodiester bond
Ester bond
Peptide bond
Glycosidic bond
A peptide bond is an amide linkage formed between the carboxyl group of one amino acid and the amino group of another. It is created during ribosomal protein synthesis. Ester bonds link alcohol and acid groups, not amino acids.
Which amino acid contains an imidazole side chain?
Lysine
Histamine
Histidine
Hydroxylysine
Histidine's side chain is an imidazole ring, which can be protonated or deprotonated near physiological pH and is important in enzyme active sites. Histamine is a decarboxylation product of histidine, not a standard amino acid.
How many chiral centers does threonine have?
One
Three
Zero
Two
Threonine has two chiral centers: one at the ?-carbon and one at the ?-carbon bearing the hydroxyl group. This gives rise to stereoisomers. Most amino acids have only one chiral center at the ?-carbon.
Which amino acid is essential in humans?
Tyrosine
Alanine
Asparagine
Valine
Valine is one of the nine essential amino acids that cannot be synthesized by humans and must be obtained from the diet. Alanine and asparagine are nonessential, and tyrosine is conditionally essential.
What is the approximate pKa of the ?-carboxyl group of amino acids?
9.0
2.0
7.4
4.5
The ?-carboxyl group of most amino acids has a pKa around 2.0, meaning it's mostly deprotonated at physiological pH. The ?-amino group has a much higher pKa (around 9.0). This difference leads to the zwitterionic state.
What is the three-letter code for tryptophan?
Tri
Tyr
Trp
Tpt
Tryptophan is abbreviated as Trp in the three-letter code system. Tyr is tyrosine, and the others are not standard codes. Trp contains an indole aromatic ring.
Which amino acid side chain can form hydrogen bonds via a hydroxyl group?
Leucine
Serine
Methionine
Phenylalanine
Serine has a hydroxyl group in its side chain that can form hydrogen bonds, contributing to enzyme active sites and structural stabilization. Threonine and tyrosine also have hydroxyl groups. Leucine and phenylalanine are nonpolar.
Which amino acid has a guanidinium group in its side chain?
Lysine
Arginine
Ornithine
Histidine
Arginine's side chain contains a guanidinium group, which is positively charged at physiological pH and engages in strong hydrogen bonding. Lysine has an amino group, histidine has an imidazole, and ornithine is non-proteinogenic.
Which amino acid is exclusively ketogenic?
Isoleucine
Leucine
Lysine
Threonine
Leucine is solely ketogenic, meaning it can be converted into acetyl-CoA or acetoacetate but not glucose. Lysine is also exclusively ketogenic but not a standard answer in some classifications. Isoleucine and threonine are both glucogenic and ketogenic.
Which post-translational modification is required for stabilizing collagen's triple helix?
Methylation of lysine
Hydroxylation of proline
Phosphorylation of serine
Glycosylation of asparagine
Prolyl hydroxylase adds hydroxyl groups to proline residues in collagen, which is critical for hydrogen bonding and triple-helix stability. Without this modification, collagen fibers are weak. Other modifications play roles in different proteins.
Which amino acid is the precursor for gamma-aminobutyric acid (GABA)?
Glutamate
Glycine
Glutamine
Aspartate
Glutamate is decarboxylated by glutamate decarboxylase, using pyridoxal phosphate, to form GABA, the main inhibitory neurotransmitter in the central nervous system. Glutamine is a precursor to glutamate.
Which amino acid side chain has a pKa closest to physiological pH, allowing it to act as a general acid/base in enzymes?
Arginine
Histidine
Lysine
Aspartate
Histidine's imidazole side chain has a pKa around 6.0, close to physiological pH, enabling it to gain or lose a proton during enzyme catalysis. Aspartate and glutamate have lower pKa values, and lysine and arginine have higher ones.
What is the isoelectric point (pI) of glycine given pKa1 = 2.35 and pKa2 = 9.78?
10.25
8.50
4.45
6.06
The pI of a neutral amino acid is the average of its two pKa values: (2.35 + 9.78)/2 ? 6.06. At this pH, the net charge of glycine is zero. Other values correspond to calculations for acidic or basic residues.
What stereoisomer are standard proteinogenic amino acids found in eukaryotes?
D-configuration
L-configuration
D,L mixture
Racemic mixture
Eukaryotic proteins are synthesized from L-amino acids, which correspond to the S-configuration in Fischer projections (except cysteine, which is R). D-amino acids are rare in proteins. Racemic mixtures are synthetic.
In N-linked glycosylation, which amino acid residue is glycosylated?
Threonine
Serine
Tyrosine
Asparagine
N-linked glycosylation occurs at the consensus sequence Asn-X-Ser/Thr, where the amide nitrogen of asparagine is glycosylated in the endoplasmic reticulum. Serine and threonine hydroxyl groups are targets for O-linked glycosylation, not N-linked.
Which amino acid is the precursor for nitric oxide synthesis via nitric oxide synthase?
Arginine
Lysine
Citrulline
Ornithine
Nitric oxide synthase converts L-arginine to L-citrulline and nitric oxide. Citrulline is the coproduct, not the substrate. Ornithine and lysine are not used in NO synthesis.
Which amino acid donates one-carbon units to the folate cycle through its conversion to glycine?
Serine
Methionine
Threonine
Alanine
Serine hydroxymethyltransferase converts serine to glycine while transferring a one-carbon unit to tetrahydrofolate, fueling the folate cycle. Threonine and methionine donate carbons differently, and alanine does not participate.
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Study Outcomes

  1. Identify the 20 Standard Amino Acids -

    Recognize each amino acid by its full name, three-letter code, and one-letter code to solidify your foundational knowledge.

  2. Analyze Amino Acid Structures -

    Examine side chain compositions and backbone arrangements to distinguish between different amino acid structures.

  3. Classify by Polarity and Charge -

    Categorize amino acids as polar, nonpolar, acidic, or basic based on their R-group properties.

  4. Match Structures to Codes -

    Apply your understanding to link each amino acid structure with its correct three-letter and one-letter abbreviation.

  5. Apply Knowledge in a Scored Quiz -

    Engage with our amino acids quiz to test your recall and structural identification skills in a timed, scored format.

  6. Assess Your Biochemistry Proficiency -

    Receive instant feedback on your performance to pinpoint areas for review and track your progress over multiple quiz attempts.

Cheat Sheet

  1. Side Chain Classification & Properties -

    Understand the five main categories of side chains (nonpolar aliphatic, aromatic, polar uncharged, acidic, basic) to predict solubility and reactivity. A handy mnemonic is "GAVLIMP + FY W" for nonpolar aliphatic and aromatic amino acids. This foundational knowledge will boost your confidence during an amino acid practice session.

  2. Backbone Structure & Stereochemistry -

    Master the general amino acid backbone (amino, carboxyl, Ī±-carbon, and R group) and L-configuration common in proteins. Visualize Fischer projections and wedge-dash drawings to ace any amino acid structure quiz question. Regular drills with a quiz amino acids tool can solidify your stereochemical recognition.

  3. Essential vs. Nonessential Amino Acids -

    Distinguish essential amino acids your body can't synthesize (like Lys, Thr, Met) from nonessential ones synthesized endogenously. The phrase "PVT TIM HaLL" helps recall the nine essentials quickly. Test yourself with a 20 amino acids quiz to ensure you've got each one memorized.

  4. Zwitterions & pKa Values -

    Know that at physiological pH (~7.4), amino acids exist as zwitterions, and their side chains ionize based on characteristic pKa values. Apply the Hendersonā€Hasselbalch equation to predict protonation states during titrations or buffer calculations. Practicing these calculations in an amino acid practice setting can improve your accuracy under exam conditions.

  5. Active Revision with Interactive Quizzes -

    Use focused flashcards and online quizzes like amino acids quiz modules to reinforce identification and recall. Regular timed sessions with an amino acid structure quiz platform build speed and retention through spaced repetition. Tracking your progress in a 20 amino acids quiz format motivates continued improvement and exam readiness.

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