Serine (Ser, S): side chain and class?

Side chain: -CH2-OH. Class: polar uncharged (alcohol). Can donate and accept hydrogen bonds; common on protein surfaces.

Threonine (Thr, T): key structural feature?

Side chain: -CH(OH)-CH3. Polar uncharged alcohol with a chiral beta carbon. Forms hydrogen bonds and is often involved in recognition and phosphorylation.

Asparagine (Asn, N) vs Glutamine (Gln, Q): main difference?

Both are polar uncharged amides. Asn: -CH2-CONH2 (shorter). Gln: -CH2-CH2-CONH2 (one extra CH2).

Cysteine (Cys, C): special property?

Side chain: -CH2-SH (thiol). Can form disulfide bonds (-S-S-) with another Cys under oxidizing conditions, stabilizing protein structure.

Aspartate (Asp, D): charge and side chain at pH 7?

Side chain: -CH2-COO−. Typically deprotonated and negatively charged at physiological pH; acidic amino acid.

The pH at which half of a given ionizable group is protonated and half is deprotonated; it indicates the acidity/basicity of that group.

A functional group consisting of a sulfur atom bonded to a hydrogen atom (-SH), as in cysteine.

A five-membered aromatic ring containing two nitrogen atoms; the side chain of histidine is an imidazole.

A noncovalent interaction between oppositely charged side chains (typically acidic and basic residues), combining ionic attraction and hydrogen bonding.

Side Chains in Focus II: Polar, Acidic, and Basic Amino Acids — BCH 204 Essentials: Structure, Properties, and Functions of the 20 Amino Acids

Q: Which pair of amino acid side chains is most likely to form a strong salt bridge at physiological pH (~7.4)?

Aspartate and Lysine. Aspartate is usually negatively charged (carboxylate), and Lysine is usually positively charged (protonated amino group) at physiological pH. Their opposite charges favor a strong ionic interaction (salt bridge). Ser/Thr and Gln/Asn are polar but uncharged, and Cys/His are often neutral under these conditions, so they do not form classic salt bridges as reliably.

From Core to Surface: Why Polar and Charged Side Chains Matter

Surface Chemistry of Proteins

Nonpolar and aromatic side chains like to hide in the protein core. In contrast, polar, acidic, and basic side chains cluster on the surface, where they control solubility and interactions with the environment.

What This Module Covers

You will learn the structures of polar uncharged (Ser, Thr, Asn, Gln, Cys), acidic (Asp, Glu), and basic (Lys, Arg, His) amino acids, and how they form hydrogen bonds and ionic interactions.

Physiological pH Context

At pH ~7.2–7.4, Asp and Glu are usually negatively charged, Lys and Arg are usually positively charged, while Ser, Thr, Asn, Gln, and Cys are polar but neutral. Histidine is special and can switch charge state near this pH.

Polar Uncharged Side Chains: Ser, Thr, Asn, Gln, Cys

Serine and Threonine

Ser: -CH2-OH (small alcohol). Thr: -CH(OH)-CH3 (alcohol plus a methyl, chiral beta carbon). Both form strong hydrogen bonds and are common on protein surfaces.

Asparagine and Glutamine

Asn: -CH2-CONH2. Gln: -CH2-CH2-CONH2. Both are amides. Their carbonyl oxygens accept H-bonds, and their NH2 groups donate H-bonds, making them key in binding and recognition.

Cysteine

Cys: -CH2-SH, a thiol. Less polar than Ser but chemically reactive. Two Cys residues can oxidize to form a disulfide bond (-S-S-), stabilizing protein structure, especially outside the cell.

Acidic Side Chains: Aspartate and Glutamate

Asp and Glu Structures

Asp: -CH2-COO−. Glu: -CH2-CH2-COO−. Both carry a negative charge at physiological pH due to their carboxylate groups.

Acidic Side-Chain Properties

Side-chain pKa around 4–4.5 means Asp and Glu are usually deprotonated and negatively charged at pH ~7. They are strong hydrogen bond acceptors and key for ionic interactions.

Functional Roles of Asp and Glu

Asp and Glu frequently appear in active sites as general acids/bases, in metal binding sites, and in salt bridges with Lys or Arg that stabilize protein structure.

Basic Side Chains: Lysine, Arginine, Histidine

Lysine and Arginine

Lys: -(CH2)4-NH3+, a long chain ending in a primary amino group (pKa ~10.5). Arg: -(CH2)3-guanidinium, a resonance-stabilized, strongly basic group (pKa ~12.5), almost always positively charged.

Histidine’s Imidazole

Histidine has an imidazole ring with two nitrogens and a side-chain pKa near 6. It can be neutral or positively charged near physiological pH, making it ideal for proton transfer in catalysis.

Functional Roles of Basic Residues

Lys and Arg often bind negatively charged partners like DNA or Asp/Glu. His commonly appears in catalytic triads and metal-binding sites, where its variable protonation is crucial.

Hydrogen Bonding: Who Donates, Who Accepts?

Donors vs Acceptors

Donors have X-H (e.g., O-H, N-H). Acceptors have lone pairs (O, N, sometimes S). Side chains differ in whether they donate, accept, or do both.

Polar Uncharged Side Chains

Ser/Thr: OH is both donor and acceptor. Asn/Gln: side-chain NH2 donates, carbonyl O accepts. Cys: SH is a weak donor and S is a weak acceptor.

Charged Side Chains and His

Asp/Glu (COO−) are strong acceptors. Lys/Arg (protonated) are strong donors. Histidine’s imidazole can act as both donor and acceptor, depending on its protonation state.

Predict the Interaction: Quick Thought Exercise

Use this step to mentally practice predicting hydrogen bonds and ionic interactions.

Scenario 1: A lysine side chain (Lys) is near an aspartate (Asp) in a folded protein.

Question: What type of interaction is most likely between their side chains at pH 7?
Think: Lys is positively charged; Asp is negatively charged.
Likely answer: A salt bridge (ionic interaction), possibly with additional hydrogen bonds.

Scenario 2: A serine (Ser) is positioned next to the carbonyl oxygen of a bound substrate.

Question: Is Ser acting as an H-bond donor or acceptor here?
If the OH hydrogen of Ser points toward the substrate carbonyl O, Ser is acting as a donor.
If the substrate has an N-H pointing to Ser O, Ser is acting as an acceptor.

Scenario 3: A histidine (His) in an enzyme active site alternates between protonated and deprotonated forms during catalysis.

Question: Why is His especially suited for this role?
Because its side-chain pKa is near physiological pH, His can gain or lose a proton easily, acting as a general acid or base.

Scenario 4: An arginine (Arg) side chain is found bound to the phosphate backbone of DNA.

Question: What interactions stabilize this binding?
The guanidinium group of Arg forms strong electrostatic interactions and hydrogen bonds with the negatively charged phosphate oxygens.

Pause after each scenario and try to answer before reading the explanation. This mirrors how you will reason through exam questions and structural problems.

Real-World Example: Catalytic Triad in Serine Proteases

The Catalytic Triad Components

Serine proteases use a catalytic triad: Ser, His, and Asp. These three residues are positioned precisely in 3D space to work together in peptide bond hydrolysis.

Division of Labor in the Triad

Asp stabilizes and orients His. His acts as a general base and acid, shuttling protons. Ser, activated by His, becomes a strong nucleophile that attacks the peptide bond.

Why This Matters

This example shows how polar, acidic, and basic side chains combine their properties to achieve efficient catalysis. Recognizing such clusters helps you infer function from structure.

Check Understanding: Side-Chain Properties

Answer this question to test your understanding of side-chain behavior at physiological pH.

Which pair of amino acid side chains is most likely to form a strong salt bridge at physiological pH (~7.4)?

Serine and Threonine
Aspartate and Lysine
Glutamine and Asparagine
Cysteine and Histidine (both neutral)

Show Answer

Answer: B) Aspartate and Lysine

Aspartate is usually negatively charged (carboxylate), and Lysine is usually positively charged (protonated amino group) at physiological pH. Their opposite charges favor a strong ionic interaction (salt bridge). Ser/Thr and Gln/Asn are polar but uncharged, and Cys/His are often neutral under these conditions, so they do not form classic salt bridges as reliably.

Flashcards: Recognize and Classify Side Chains

Use these flashcards to reinforce structures, codes, and key properties of polar, acidic, and basic amino acids.

Serine (Ser, S): side chain and class?: Side chain: -CH2-OH. Class: polar uncharged (alcohol). Can donate and accept hydrogen bonds; common on protein surfaces.
Threonine (Thr, T): key structural feature?: Side chain: -CH(OH)-CH3. Polar uncharged alcohol with a chiral beta carbon. Forms hydrogen bonds and is often involved in recognition and phosphorylation.
Asparagine (Asn, N) vs Glutamine (Gln, Q): main difference?: Both are polar uncharged amides. Asn: -CH2-CONH2 (shorter). Gln: -CH2-CH2-CONH2 (one extra CH2).
Cysteine (Cys, C): special property?: Side chain: -CH2-SH (thiol). Can form disulfide bonds (-S-S-) with another Cys under oxidizing conditions, stabilizing protein structure.
Aspartate (Asp, D): charge and side chain at pH 7?: Side chain: -CH2-COO−. Typically deprotonated and negatively charged at physiological pH; acidic amino acid.
Glutamate (Glu, E): mnemonic vs Asp?: Side chain: -CH2-CH2-COO− (longer than Asp). Mnemonic: "Glu is Greater" (one extra CH2) than Asp.
Lysine (Lys, K): side chain and typical charge?: Side chain: -(CH2)4-NH3+. Side-chain pKa ~10.5; usually protonated and positively charged at physiological pH; basic amino acid.
Arginine (Arg, R): distinctive group and role?: Side chain ends in a guanidinium group, which is resonance-stabilized and strongly basic (pKa ~12.5). Almost always positively charged; often binds DNA/RNA and negative side chains.
Histidine (His, H): why is it catalytically important?: Imidazole side chain with pKa near physiological pH. Can easily gain or lose a proton, acting as a general acid or base in many enzyme active sites.
Which side chains commonly form salt bridges?: Negatively charged: Asp, Glu. Positively charged: Lys, Arg, and protonated His. Salt bridges usually pair one negative with one positive side chain.

Key Terms

pKa: The pH at which half of a given ionizable group is protonated and half is deprotonated; it indicates the acidity/basicity of that group.
thiol: A functional group consisting of a sulfur atom bonded to a hydrogen atom (-SH), as in cysteine.
imidazole: A five-membered aromatic ring containing two nitrogen atoms; the side chain of histidine is an imidazole.
salt bridge: A noncovalent interaction between oppositely charged side chains (typically acidic and basic residues), combining ionic attraction and hydrogen bonding.
hydrogen bond: A noncovalent interaction between a hydrogen atom attached to an electronegative atom (donor) and another electronegative atom with a lone pair (acceptor), such as O or N.
disulfide bond: A covalent bond formed between the sulfur atoms of two cysteine side chains (-S-S-), important for stabilizing protein structure.
basic amino acid: An amino acid with a side chain that is usually protonated and positively charged at or near physiological pH (Lys, Arg, partially His).
acidic amino acid: An amino acid with a side chain that is usually deprotonated and negatively charged at physiological pH (Asp, Glu).
guanidinium group: A strongly basic, resonance-stabilized cationic group found in arginine’s side chain, with one carbon bonded to three nitrogens.
polar uncharged amino acid: An amino acid whose side chain can form hydrogen bonds but is not ionized (not charged) at physiological pH (e.g., Ser, Thr, Asn, Gln, often Cys).