Which pair of one-letter codes belongs to glutamine and glutamic acid (glutamate), in that order?
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This free amino acid quiz has 15 questions written by the StudyPDF team. It covers the one-letter codes, the side chain classes, charge at physiological pH, essential amino acids, chirality, the isoelectric point and the special cases that show up on biochemistry and MCAT-style exams, such as proline, cysteine and histidine. Answer all 15, then check your score and read the explanation for every question.
All 15 questions at a glance
Prefer to read before you play? Every question in the sampler is listed below. Answers and explanations stay collapsed until you open them.
1.Which pair of one-letter codes belongs to glutamine and glutamic acid (glutamate), in that order?
- A. E and Q
- B. Q and E
- C. N and D
- D. D and N
Answer: B. Q and E
Glutamine is Q and glutamic acid is E. Asparagine (N) and aspartic acid (D) are the same amide and acid pair with a side chain one carbon shorter, so N and D belong to a different pair of amino acids. The trap here is the order: E and Q has the right letters in the wrong order, and D and N is the shorter pair in reverse.
2.Which pair of amino acids has side chains that are positively charged at physiological pH?
- A. Aspartate and glutamate
- B. Serine and threonine
- C. Valine and leucine
- D. Lysine and arginine
Answer: D. Lysine and arginine
The lysine side chain ends in an amino group with a pKa near 10.5, and the arginine side chain ends in a guanidinium group with a pKa near 12.5. Both are far above 7.4, so both are protonated and positive in the body. Aspartate and glutamate carry negative carboxylates, serine and threonine are polar but uncharged, and valine and leucine are nonpolar. Histidine is mostly neutral at 7.4 because its pKa is near 6, so it does not belong in this pair even though it is classed as basic.
3.Which of these amino acids have nonpolar side chains?
Select all that apply.
- A. Valine
- B. Serine
- C. Leucine
- D. Asparagine
- E. Methionine
Answer: A. Valine, C. Leucine, E. Methionine
Valine and leucine have branched hydrocarbon side chains with nothing to form hydrogen bonds. Methionine has a thioether, with sulfur bonded to two carbons, and that sulfur is a poor hydrogen-bond partner, so methionine counts as nonpolar too. Serine has a hydroxyl group and asparagine has an amide, so both are polar and uncharged. Nonpolar residues tend to sit in the interior of a folded protein, away from water.
4.Complete the sentence.
The one-letter code for lysine is _____, and lysine has a _____ side chain.
Options for blank 1: L, K, Y, R
Options for blank 2: basic, acidic, nonpolar, polar but uncharged
Answer: K, basic
Lysine is K. L is already leucine, Y is tyrosine and R is arginine. A common memory aid is that K sits right next to L in the alphabet, and L already belongs to leucine. The side chain ends in an amino group that picks up a proton at physiological pH, which makes it basic and positively charged. Lysine, arginine and histidine are the basic three.
5.Which of these side chains has a pKa closest to physiological pH (about 7.4), so small pH shifts can switch it between protonated and deprotonated?
- A. Aspartate
- B. Lysine
- C. Arginine
- D. Histidine
Answer: D. Histidine
The imidazole ring of histidine has a pKa near 6, the closest of these four to the pH of the body and the one textbooks single out as a physiological buffer. Near that pH it can pick up and give back protons, which is why histidine is common in enzyme active sites and in buffering proteins such as hemoglobin. Aspartate (about 3.9) is deprotonated and negative well below 7.4, while lysine (about 10.5) and arginine (about 12.5) stay protonated and positive well above it.
6.Glycine has an alpha-carboxyl pKa of about 2.3 and an alpha-amino pKa of about 9.6, and its side chain has no ionizable group. What is its approximate isoelectric point (pI)?
- A. 5.95
- B. 2.3
- C. 7.0
- D. 9.6
Answer: A. 5.95
The pI is the pH at which the net charge is zero. With no ionizable side chain, it sits halfway between the two pKa values: (2.3 + 9.6) / 2 = 5.95. Around that pH the molecule is mostly the neutral zwitterion, with a positive amino group and a negative carboxylate. For an acidic amino acid, average the two lowest pKa values instead, and for a basic one the two highest. The answers 2.3 and 9.6 are single pKa values, and 7.0 is not special for an amino acid.
7.Complete the sentence.
In a solution whose pH is below an amino acid's pI, the molecule carries a net _____ charge, so in an electric field it moves toward the _____.
Options for blank 1: negative, zero, positive, variable
Options for blank 2: positive electrode, negative electrode, neither electrode, both electrodes at once
Answer: positive, negative electrode
Lowering the pH below the pI means more protons are around, so the groups hold on to them: the amino group stays positive and the carboxyl group loses its negative charge. The net charge is positive, and a positive molecule migrates toward the negative electrode, the cathode. Above the pI the opposite happens, the net charge is negative and the molecule moves toward the anode. At the pI itself it has no net charge and does not move.
8.In nearly every L-amino acid the alpha carbon has the S configuration. Which of these is the exception, with an R alpha carbon?
- A. Serine
- B. Alanine
- C. Cysteine
- D. Methionine
Answer: C. Cysteine
R and S come from the Cahn-Ingold-Prelog priority rules, not from the L or D family. In cysteine, the CH2SH group outranks the carboxyl group because sulfur has a higher atomic number than oxygen, so the same spatial arrangement that is labeled S in serine, alanine or methionine is labeled R. L-cysteine and its neighbors have the same three-dimensional layout. Only the priority order changes, which is why L does not simply mean S.
9.Complete the sentence.
_____ is the only one of the 20 standard amino acids with no chiral center, and the amino acids built into proteins are the _____ stereoisomers.
Options for blank 1: Alanine, Glycine, Proline, Serine
Options for blank 2: D, racemic, L, meso
Answer: Glycine, L
The side chain of glycine is just a hydrogen, so its alpha carbon has two identical hydrogens and no mirror-image form. Every other standard amino acid has four different groups on the alpha carbon, so each can exist as an L or a D form. Ribosomes build proteins from the L form. D-amino acids do exist in nature, for example in bacterial cell walls, but they are not what the genetic code builds into proteins.
10.What makes proline structurally unusual among the 20 standard amino acids?
- A. Its side chain loops back onto the backbone nitrogen, forming a ring
- B. It is the only one with a sulfur atom in its side chain
- C. It is the only one with an aromatic ring
- D. It is the only one without a chiral alpha carbon
Answer: A. Its side chain loops back onto the backbone nitrogen, forming a ring
The three-carbon side chain of proline bonds to the alpha carbon and to the backbone nitrogen, which closes a five-membered ring. That makes its nitrogen a secondary amine with no N-H to donate to a backbone hydrogen bond, and the ring limits rotation. So proline often starts turns and interrupts alpha helices. Sulfur is in cysteine and methionine, aromatic rings are in phenylalanine, tyrosine and tryptophan, and glycine is the one without a chiral center. Proline's alpha carbon is chiral.
11.Two cysteine residues in a folded protein are oxidized and joined. What is the covalent link called?
- A. A peptide bond
- B. A hydrogen bond
- C. A disulfide bond
- D. A salt bridge
Answer: C. A disulfide bond
The thiol groups (-SH) of two cysteines are oxidized to form an S-S bond, and the oxidized pair is called cystine. Disulfide bonds help stabilize the tertiary and quaternary structure of many secreted proteins, such as antibodies and insulin. The inside of a cell is a reducing environment, so most cysteines there stay as free thiols. A peptide bond joins neighboring backbone atoms, and a hydrogen bond and a salt bridge are noncovalent.
12.Which amino acids have a side chain that a protein kinase can phosphorylate?
Select all that apply.
- A. Serine
- B. Valine
- C. Threonine
- D. Alanine
- E. Tyrosine
Answer: A. Serine, C. Threonine, E. Tyrosine
Serine, threonine and tyrosine each carry a hydroxyl group on the side chain, and a kinase transfers a phosphate from ATP onto it. Serine and threonine kinases act on the first two, and tyrosine kinases on the third. Adding a phosphate adds negative charge and can switch an enzyme or a signaling protein on or off. Valine and alanine have hydrocarbon side chains with nothing to accept the phosphate.
13.Which of these amino acids are essential in the adult diet?
Select all that apply.
- A. Leucine
- B. Alanine
- C. Lysine
- D. Serine
- E. Tryptophan
Answer: A. Leucine, C. Lysine, E. Tryptophan
Leucine, lysine and tryptophan are three of the nine on the usual adult list, which also has histidine, isoleucine, methionine, phenylalanine, threonine and valine. Essential means the body cannot make enough of it. Alanine and serine are made from common metabolic intermediates (alanine from pyruvate, serine from 3-phosphoglycerate), so they are nonessential. Arginine, cysteine, glutamine, glycine, proline and tyrosine are the conditionally essential ones. Course lists vary a little, so use yours. This is exam content, not dietary advice.
14.Which of these is NOT one of the 20 standard amino acids that the genetic code specifies for proteins?
- A. Arginine
- B. Ornithine
- C. Asparagine
- D. Glutamine
Answer: B. Ornithine
Ornithine is an intermediate of the urea cycle, made from arginine by the enzyme arginase, and no codon specifies it. Arginine, asparagine and glutamine are all standard. Selenocysteine and pyrrolysine are found in the proteins of some organisms through special recoding, but they are not among the 20 of the standard codon table either.
15.A peptide bond joins two amino acids. Which groups react, and what is released?
- A. The side chains of the two residues, releasing carbon dioxide
- B. The alpha carbon of one and the side chain of the next, releasing hydrogen
- C. The carboxyl group of one and the amino group of the next, releasing water
- D. The amino groups of both residues, releasing ammonia
Answer: C. The carboxyl group of one and the amino group of the next, releasing water
A peptide bond is an amide link formed in a condensation (dehydration) reaction: the carboxyl group of one amino acid joins the amino group of the next and a molecule of water leaves. Side chains stay out of the backbone. A chain has a free amino group at one end, the N-terminus, and a free carboxyl group at the other, the C-terminus, and peptides are written from the N-terminus to the C-terminus.
The 20 amino acids: classes, codes and the essential list
Every one of the 20 standard amino acids has the same backbone: a central alpha carbon bonded to an amino group, a carboxyl group, a hydrogen and a side chain, written R. Only the side chain differs, so it decides almost everything a quiz asks: the class, the charge at physiological pH, whether the residue sits inside a protein or on its surface, and what it can do in an enzyme. Textbooks usually sort the 20 into five groups. Nonpolar aliphatic: glycine, alanine, valine, leucine, isoleucine, methionine and proline. Aromatic: phenylalanine, tyrosine and tryptophan. Polar uncharged: serine, threonine, cysteine, asparagine and glutamine. Acidic: aspartate and glutamate. Basic: lysine, arginine and histidine. Sources disagree on a few edge cases, for example whether tyrosine counts as aromatic or polar, so use the grouping from your own course.
You also need two sets of letters. Three-letter codes and one-letter codes, in alphabetical order: Ala A, Arg R, Asn N, Asp D, Cys C, Gln Q, Glu E, Gly G, His H, Ile I, Leu L, Lys K, Met M, Phe F, Pro P, Ser S, Thr T, Trp W, Tyr Y, Val V. The pairs that trip people up are glutamine (Q) against glutamate (E) and asparagine (N) against aspartate (D).
Nine amino acids are listed as essential for adults, which means the body cannot make enough of them and they have to come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Arginine, cysteine, glutamine, glycine, proline and tyrosine are often called conditionally essential, because the body makes them but can fall short during illness, rapid growth or stress. Lists differ slightly between courses, especially for arginine and histidine, so match the one your instructor gave you.
How to use this quiz, and what to learn in what order
The 15 questions are split like this: three on names and codes, three on side chain class and charge, two on pI and net charge, two on chirality, one on the essential list, three on special residues (proline, cysteine and the amino acids kinases phosphorylate) and one on the peptide bond. Most of them ask why, not just which. Why does histidine buffer near neutral pH while lysine does not? Why is cysteine R when its neighbors are S? Why does proline break a helix? Those are the explanations that carry over to a question you have never seen.
Take it once without looking anything up, then read the explanation for every question, including the ones you got right. Treat a miss as a topic. If you missed the pI question and the net charge question, the gap is probably the idea that charge follows pH relative to each pKa, not two separate facts. The pKa values given in the questions are approximate and differ a little between textbooks, so the quiz always hands you the numbers it needs. It does not ask you to draw structures, so sketch each side chain on paper as you go, because drawing is what makes the classes stick.
If you would rather work on paper, use the Download test as PDF button above the questions. The file has all 15 questions with an answer key on the last page, and it needs a free StudyPDF account, while playing online is free. The questions are original and written to common first-semester biochemistry content. They are not taken from a textbook, an exam or a prep company's bank, and nothing here is dietary advice.
How StudyPDF builds amino acid practice tests from your own notes
Fifteen questions can show you which part of the topic is shaky. They cannot get you ready on their own, and a generic quiz rarely matches the structures, the pKa tables and the groupings your biochemistry course actually uses.
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Written by the StudyPDF team. Last updated August 19, 2026.
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The StudyPDF team wrote every question and explanation to common first-semester biochemistry content. They are original items, not taken from a textbook, an exam or a prep company's bank, and StudyPDF is not affiliated with the AAMC or any exam provider. The essential amino acid items are exam review, not dietary advice.
Does this quiz cover all 20 amino acids, and what about selenocysteine?
Fifteen questions cannot ask about every residue, so the quiz samples the ideas that decide most questions: codes, classes, charge, pI, chirality and the special cases. For full coverage of every name, code and structure, build flashcards from your own notes. Selenocysteine and pyrrolysine are built into proteins in some organisms by special recoding, but they are not among the 20 standard amino acids, so this quiz stays with the 20.
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Yes. Upload your textbook chapters, slides or lecture notes to StudyPDF and you can build practice tests and flashcards from those exact pages, with explanations tied to your sources. If you have nothing to upload, name a topic such as amino acid side chains or the isoelectric point instead.