Protein Chemistry

Day 1 part 2

Quiz ChatGPT conversation

1. Genetic Code & Amino-Acid Similarity 🧬

The genetic code consists of 64 codons:
- 61 encode amino acids
- 3 are stop codons
Amino acids with similar chemical properties cluster together in the codon table:
- Non-polar amino acids cluster together
- Polar, basic, and acidic amino acids also cluster

Why this matters

Single-nucleotide mutations often change an amino acid into another with similar chemistry
This buffers proteins against catastrophic functional loss
Example: a hydrophobic amino acid often mutates into another hydrophobic one

2. Canonical, Rare, and Non-Natural Amino Acids 🧪

20 standard amino acids are used in proteins
Two rare natural amino acids:
- Selenocysteine
- Pyrrolysine
These are not expected to be memorized, but they demonstrate that biology can expand the genetic code

Non-natural amino acids

Can be engineered into proteins
Retain amino + carboxyl groups but have novel side chains
Require genetic engineering
Used to introduce:
- Fluorescent probes
- Crosslinkers
- Novel chemical reactivity

3. Naming Systems & Mass Spectrometry ⚖️

Amino acids have:
- Full name
- 3-letter code
- 1-letter code
Ambiguous notation:
- Asx = Asparagine or Aspartate
- Glx = Glutamine or Glutamate

Mass spectrometry problem

Leucine and isoleucine have:
- Identical molecular weight
- Indistinguishable by MS alone
Result: ambiguity in protein sequencing unless supported by DNA data

4. Hydrophobicity & Transfer Free Energy 🌊

Hydrophobicity is quantified by:

ΔG of transferring an amino-acid side chain from a hydrophobic to a hydrophilic environment

Key points

Absolute ΔG values depend on experimental setup
Relative order is conserved

Hydrophobic extremes

Very hydrophobic: Phenylalanine, Leucine, Isoleucine
Borderline: Glycine (tiny side chain)
Strongly hydrophilic: Charged amino acids

Does hydrophobicity depend on pH?

Generally no
Exception: amino acids with titratable side chains (e.g. Lys, Asp, Glu)
Protonation state changes → charge changes → hydrophobicity changes

5. Ionization, pKa, and pI 🔋

Example: Glycine

Two ionizable groups:
- α-carboxyl (~pKa ≈ 2)
- α-amino (~pKa ≈ 9.6)
pI ≈ 6 (net charge = 0)

Key definitions

pKa: pH where 50% protonated
pI: pH where net charge = 0

Amino acids with side-chain pKa

Basic: Lys, Arg, His
Acidic: Asp, Glu
Special: Cys (~8.3), Tyr (~10.9)

6. Why Cysteine Is Exceptionally Reactive ⚡

This is a core concept.

pKa of cysteine ≈ 8.3

At physiological pH (~7):
- ~10% exists as thiolate (S⁻)

Consequences

Thiolate is a very strong nucleophile
Protonated thiol (–SH) is weak
Even partial deprotonation is enough for reactivity

Functional impact

Cysteine participates in:
- Catalysis
- Redox chemistry
- Disulfide bond formation

Why tyrosine does not behave similarly

pKa ≈ 10.9
Almost no deprotonation at pH 7
Negligible nucleophilicity in biology

7. Why Serine (or Cysteine) Is Placed Near Histidine 🧠

This explains catalytic triads.

Serine and cysteine alone are weak nucleophiles
Histidine acts as a general base
Histidine:
- Accepts a proton
- Activates Ser-O⁻ or Cys-S⁻

Result

Formation of a powerful nucleophile
Enables peptide bond cleavage
Core principle of:
- Serine proteases
- Cysteine proteases

8. pKa Is NOT Fixed in Proteins 🌡️

pKa values listed in tables are for free amino acids in water
In proteins, pKa shifts due to:
- Nearby charges
- Hydrophobic environments
- Hydrogen bonding

Examples

Acidic residue near negative charges → higher pKa → prefers protonation
Acidic residue near positive charges → lower pKa → prefers deprotonation
Hydrophobic environment → neutral state favored

Implication

Protein pI cannot be predicted perfectly
Must be measured experimentally
Critical for purification (e.g. ion-exchange chromatography)

9. UV Absorption & Protein Concentration 📈

280 nm absorption

Dominated by tryptophan
Tyrosine contributes weakly
Phenylalanine contributes minimally

210–220 nm absorption

All peptide bonds absorb
Allows concentration measurement even without aromatics
Problem: many contaminants also absorb

10. Amino-Acid Analysis (Composition, Not Sequence) 🧪

Proteins hydrolyzed:
- 110 °C
- 6 M acid
- ~16 h
Peptide bonds fully broken
Amino acids derivatized with fluorescent tags
Separated by chromatography
Output:
- Which amino acids
- Their relative amounts
Does not give sequence information

11. Amino-Acid Frequencies in Proteins 📊

If random: each amino acid ≈ 5%
Observed deviations:
- Lysine: high frequency → flexible, easy to accommodate
- Cysteine: low frequency → reactive, disulfide risk
- Tryptophan: low frequency → bulky, rigid

12. Levels of Protein Structure 🏗️

Primary: amino-acid sequence
Secondary: α-helices, β-sheets, turns
Tertiary: 3D fold of one chain
Quaternary: multi-subunit assemblies

13. Peptide Bond Chemistry 🔗

Formed by:
- Nucleophilic attack of amino group on carboxyl carbon
- Release of water
Peptide bond has:
- Partial double-bond character
- Planarity
- Dipole moment
No free rotation around peptide bond

14. Cis vs Trans Peptide Bonds ⚠️

Trans favored due to sterics
Exceptions:
- Glycine (tiny)
- Proline (ring locks geometry)
Cis–trans isomerization of proline:
- Slow
- Requires peptidyl-prolyl isomerases

15. Backbone Angles & Ramachandran Plot 📐

Two rotatable angles:
- φ (phi): N–Cα
- ψ (psi): Cα–C
Steric hindrance restricts allowed combinations
Ramachandran plot shows:
- α-helix regions
- β-sheet regions
- Disallowed zones

16. Secondary Structure Elements 🌀

α-Helix

3.6 residues per turn
Hydrogen bond: i → i+4
Rise: 1.5 Å per residue
One turn: 5.4 Å
Can be amphipathic
- One hydrophobic face
- One hydrophilic face
Crucial for membrane proteins

β-Sheets

Parallel or antiparallel
Hydrogen bonds between strands
Longer rise per residue (~3.5 Å)
Connected by turns and loops

17. Amino-Acid Preferences for Secondary Structure 🧩

α-Helix lovers: Ala, Leu, Met
β-Sheet lovers: Val, Ile, Phe
Turn/coil: Gly, Pro
Proline:
- Breaks helices
- Promotes turns
Collagen:
- Extremely proline-rich
- Forms specialized triple helices

Final takeaway 🎯

This lecture builds the chemical foundation of proteins:

Why amino acids behave differently
How environment controls charge and reactivity
Why cysteine is rare but powerful
How structure emerges from chemistry
Why enzymes position residues precisely

Quiz

Score: 0/30 (0%)

Q0. Why are amino acids with similar chemical properties clustered together in the genetic code table?

To minimize translation errors during ribosome binding

To ensure single-nucleotide mutations often substitute chemically similar amino acids

To reduce the total number of codons needed

To increase the rate of protein synthesis

Q1. Which statement best explains why leucine and isoleucine cannot be distinguished by mass spectrometry alone?

They have identical pKa values

They have identical side-chain polarity

They have identical molecular weights

They ionize at the same pH

Q2. What does the hydrophobicity scale of amino acids primarily measure?

The solubility of amino acids in water

The free energy change of transferring a side chain from hydrophobic to hydrophilic environments

The tendency of amino acids to form secondary structures

The pKa values of side chains

Q3. Why is glycine considered borderline hydrophobic despite being classified as non-polar?

Its side chain is aromatic

Its side chain is very small and contributes little hydrophobic surface

It frequently carries a positive charge

It strongly hydrogen-bonds with water

Q4. Which amino acid is most hydrophobic based on membrane-to-water transfer free energy?

Alanine

Valine

Phenylalanine

Glycine

Q5. What does the isoelectric point (pI) of an amino acid represent?

The pH where the amino acid is fully protonated

The pH where the amino acid is fully deprotonated

The pH where the amino acid has no net charge

The pH where the amino acid is least soluble

Q6. Why is cysteine unusually reactive compared to most other amino acids?

Its side chain is aromatic

Its side chain has a pKa near physiological pH, allowing formation of a thiolate

It is always negatively charged in proteins

It forms hydrogen bonds more strongly than other amino acids

Q7. Why does tyrosine generally not act as a strong nucleophile in proteins?

It is sterically hindered

Its side chain is hydrophobic

Its side-chain pKa is too high for deprotonation at physiological pH

It cannot lose a proton

Q8. What is the functional role of histidine when placed near serine or cysteine in enzyme active sites?

It stabilizes the protein fold

It donates a proton to activate the nucleophile

It accepts a proton, increasing nucleophilicity of serine or cysteine

It forms a covalent bond with the substrate

Q9. Why are pKa values of amino-acid side chains not fixed inside proteins?

They depend on temperature only

They depend on the amino-acid sequence length

They are influenced by the local electrostatic and hydrophobic environment

They are altered by peptide bond formation

Q10. Which amino acid is typically underrepresented in proteins due to high chemical reactivity?

Alanine

Leucine

Cysteine

Lysine

Q11. Why is tryptophan relatively rare in proteins despite being hydrophobic?

It is chemically unstable

It has a bulky and rigid side chain that is difficult to accommodate

It disrupts hydrogen bonding

It is easily oxidized

Q12. What gives the peptide bond its partial double-bond character?

Hydrogen bonding between residues

Resonance between the carbonyl and amide nitrogen

Steric hindrance of side chains

Electrostatic attraction between termini

Q13. Why is the trans configuration of the peptide bond generally favored over cis?

It allows hydrogen bonding

It minimizes steric clashes between side chains

It is required for peptide bond formation

It stabilizes charged residues

Q14. What do the allowed regions in a Ramachandran plot represent?

Energies of peptide bonds

Preferred pKa values of side chains

Sterically allowed combinations of backbone dihedral angles

Locations of alpha-carbons in space

Q15. Single-nucleotide mutations often lead to amino-acid substitutions with similar chemical properties.

True

False

Q16. Leucine and isoleucine can be distinguished by standard amino-acid analysis using mass spectrometry.

True

False

Q17. Hydrophobicity scales always give identical numerical values regardless of experimental setup.

True

False

Q18. The pI of an amino acid is the pH at which it carries no net charge.

True

False

Q19. At physiological pH, a significant fraction of cysteine side chains are deprotonated.

True

False

Q20. Tyrosine is as nucleophilic as cysteine at physiological pH.

True

False

Q21. Histidine can function as both a proton donor and acceptor near physiological pH.

True

False

Q22. pKa values of amino-acid side chains are identical in free amino acids and within folded proteins.

True

False

Q23. Cysteine is rare in proteins partly because it can form unwanted disulfide bonds.

True

False

Q24. Tryptophan strongly absorbs UV light at 280 nm.

True

False

Q25. Peptide bonds allow free rotation around the C–N bond.

True

False

Q26. Most peptide bonds in proteins adopt the cis configuration.

True

False

Q27. Proline is an exception where cis peptide bonds can be relatively common.

True

False

Q28. Alpha helices are stabilized primarily by hydrogen bonds between backbone atoms.

True

False

Q29. Glycine and proline are often found in turns rather than alpha helices or beta sheets.

True

False