Study guides › B/B › Biochemistry
Amino Acids & Peptides
Proteins are built from 20 amino acids. You will learn to recognize each by class from its structure, predict charge and isoelectric point from pKa values, understand the peptide bond, and reason through protein sequencing experiments.
Study this chapter in PrepMed (free)
What this chapter covers
- The 20 Amino Acids by Class. Every protein question starts with the side chain: its polarity, charge, and shape decide where a residue sits and what it does.
- Zwitterions, pKa & Isoelectric Point. Charge versus pH questions are everywhere on the MCAT, and they all reduce to Henderson-Hasselbalch and the pI.
- The Peptide Bond & Disulfides. The peptide bond is planar and rigid because of resonance, which is the reason proteins fold into regular shapes.
- Peptide Sequencing & Analysis. Sequencing questions give fragments from different enzymes, and you must assemble one consistent sequence.
Key points to know
- All chiral natural amino acids are L; L is (S) except Cys (R)
- Glycine is achiral; proline is a cyclic secondary amine that breaks helices
- Zwitterion: –NH3⁺ and –COO⁻ together, net charge 0
- At pH = pKa, a group is 50% protonated
- Peptide bond = amide, formed by condensation, written N→C
- Partial double-bond character makes the unit planar and mostly trans
- Edman: N-terminal, stepwise, about 50 residues
- Trypsin: after Lys/Arg, not before Pro
Try a practice question
Which amino acid is achiral?
- Alanine
- Glycine
- Proline
- Serine
Show the answer
B. Glycine's R group is hydrogen, so the alpha carbon carries two identical substituents and is not a stereocenter. All the others have four different groups.
Common questions
What does this chapter cover?
Amino Acids & Peptides covers The 20 Amino Acids by Class, Zwitterions, pKa & Isoelectric Point, The Peptide Bond & Disulfides, Peptide Sequencing & Analysis.
What should I know first?
All chiral natural amino acids are L; L is (S) except Cys (R) Glycine is achiral; proline is a cyclic secondary amine that breaks helices Zwitterion: –NH3⁺ and –COO⁻ together, net charge 0
21 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.