Amino Acid Structure
and Classification
Master the 20 standard amino acids: their general structure, R-group properties, and how classification drives protein function.
Why Amino Acids Matter
Amino acids are the monomers of proteins: every enzyme, receptor, antibody, and structural fiber in your body is built from just 20 standard amino acids. Understanding their structure lets you predict how proteins fold, how mutations cause disease, and how drugs interact with biological targets.
The General Structure of an Amino Acid
The Zwitterion at Physiological pH
At physiological pH (~7.4), amino acids exist as zwitterions: the amino group is protonated (\(\text{NH}_3^+\)) and the carboxyl group is deprotonated (\(\text{COO}^-\)). The molecule carries both a positive and a negative charge, giving it a net charge of zero at its isoelectric point (\(pI\)).
Stereochemistry: L-Amino Acids
All 20 standard amino acids (except glycine) have a chiral alpha carbon with four different substituents. Biological systems use exclusively the L-configuration (S in most cases by Cahn-Ingold-Prelog rules).
The Peptide Bond: Linking Amino Acids
Classifying by R-Group Properties
The side chain (R group) determines each amino acid's chemical behavior. We classify the 20 standard amino acids into categories based on R-group polarity, charge, and chemical nature.
Nonpolar Amino Acids in Detail
Polar and Charged Amino Acids
| Category | Amino Acids | Key Feature |
|---|---|---|
| Polar Uncharged | Ser, Thr, Cys, Tyr, Asn, Gln | H-bond donors/acceptors |
| Positive at pH 7.4 | Lys (K), Arg (R), His (H) | Basic R groups |
| Negative at pH 7.4 | Asp (D), Glu (E) | Acidic R groups |
Acidic vs Basic Amino Acids
| Property | Acidic (Asp, Glu) | Basic (Lys, Arg, His) |
|---|---|---|
| R-group functional group | \(\text{-COOH}\) (extra carboxyl) | \(\text{-NH}_2\), guanidinium, imidazole |
| Charge at pH 7.4 | Negative (−1) | Positive (+1) |
| R-group \(pK_a\) | ~3.7 (Asp), ~4.1 (Glu) | ~10.5 (Lys), ~12.5 (Arg), ~6.0 (His) |
| \(pI\) calculation | \(pI = \frac{pK_{a1} + pK_{aR}}{2}\) | \(pI = \frac{pK_{a2} + pK_{aR}}{2}\) |
| Role in proteins | Salt bridges, catalysis | DNA binding, salt bridges |
How to Calculate \(pI\): Step by Step
Example 1: \(pI\) of Alanine
Problem: Calculate the isoelectric point of alanine. Given: \(pK_{a1} = 2.34\) (\(\alpha\)-COOH) and \(pK_{a2} = 9.69\) (\(\alpha\)-\(\text{NH}_3^+\)). Alanine has a nonpolar R group (no ionizable side chain).
Example 2: \(pI\) of Aspartate
Problem: Calculate the \(pI\) of aspartic acid. Given: \(pK_{a1} = 1.88\) (\(\alpha\)-COOH), \(pK_{aR} = 3.65\) (R-group COOH), \(pK_{a2} = 9.60\) (\(\alpha\)-\(\text{NH}_3^+\)).
Example 3: \(pI\) of Lysine
Problem: Calculate the \(pI\) of lysine. Given: \(pK_{a1} = 2.18\), \(pK_{a2} = 8.95\) (\(\alpha\)-\(\text{NH}_3^+\)), \(pK_{aR} = 10.53\) (R-group \(\text{NH}_3^+\)).
Common Mistakes to Avoid
Summary: The 20 Amino Acids
| Category | Members | Key Features | Charge at pH 7.4 |
|---|---|---|---|
| Nonpolar Aliphatic | G, A, V, L, I, P, M | Hydrophobic core | 0 |
| Aromatic | F, W | UV absorption (280 nm) | 0 |
| Polar Uncharged | S, T, C, Y, N, Q | H-bonds, Cys: \(\text{S-S}\) bonds | 0 |
| Positively Charged | K, R, H | DNA binding, salt bridges | +1 |
| Negatively Charged | D, E | Catalysis, salt bridges | −1 |
Question 1 of 5
Which amino acid has a side chain \(pK_a\) near physiological pH, allowing it to function as both a proton donor and acceptor in enzyme active sites?
Question 2 of 5
What is the \(pI\) of glutamic acid given the following \(pK_a\) values: \(pK_{a1} = 2.19\), \(pK_{aR} = 4.25\), \(pK_{a2} = 9.67\)?
Question 3 of 5
A mutation replaces a leucine residue in the interior of a globular protein with a glutamate. What is the most likely effect on protein structure?
Question 4 of 5
Which of the following amino acids can form disulfide bonds that stabilize tertiary and quaternary protein structure?
Question 5 of 5
At pH 1.0, what is the net charge on the amino acid alanine (\(pK_{a1} = 2.34\), \(pK_{a2} = 9.69\))?