Proteins — Structure, Bonds, Levels & Functional Overview

Ice-Breaker: “Let’s Play Rebus – Guess the Word!”

• Instructor opened the session with a series of visual rebus puzzles, each illustrating a protein-rich food; serves as a memory hook for the succeeding biochemical discussion.
• Puzzles & answers (chronological order):
– whale – a = o, Q + s, egg ⟹ Whole Eggs
– sax, x = 1 + –le, lemon, √*x = 1 +, –le ⟹ Salmon
– man, –sk + | + –ey, ski, key ⟹ Milk
– –ide + r = s + t, chicken, bride, ear ⟹ Chicken Breast
– alien –ien +, lemon –le +, cd –C + S ⟹ Almonds
• Pedagogical purpose: transitions the class from everyday foods → underlying macromolecule (Proteins).

Learning Objectives

• Define the basic structure and function of proteins.
• Classify proteins according to their biological role (structural, enzymatic, transport, etc.).
• Explain each of the four hierarchical levels of protein structure (primary → quaternary).

Amino Acids – The Fundamental Monomers

• Proteins are polymers of amino acids (AAs) linked by peptide bonds.
• Core components of every AA:
– Amine group $\left(\mathrm{-NH_2}\right)$
– Carboxyl group $\left(\mathrm{-COOH}\right)$
– Central (\alpha) carbon with attached hydrogen + variable side chain (R-group).
• Glycine is highlighted as the simplest AA – R-group = H.
• Existence of 20 genetically encoded AAs distinguishes primary structures.

Peptide Bonds & Peptide Nomenclature

• Peptide bond formation is a dehydration (condensation) reaction:
$\text{AA}1 + \text{AA}2 \;\xrightarrow{\;\Delta,\, -H2O\;}\; \text{Dipeptide} + H2O$
• Terminology by size:
– Dipeptide : exactly 2 AAs.
– Oligopeptide : 3 – 10 AAs.
– Polypeptide : > 10 AAs; a single-chain protein is typically a long polypeptide.
• Directionality: N-terminus (free $\mathrm{-NH_3^+}$ ) ➝ C-terminus (free $\mathrm{-COO^-}$ ) establishes biosynthetic & sequence reference.

Hierarchical Levels of Protein Structure

1. Primary Structure

• Linear order of AAs joined by covalent peptide bonds.
• Encoded directly by mRNA template; variations (mutations) here cascade upward.

2. Secondary Structure

• Local folding patterns stabilized by backbone hydrogen bonding.
• Two canonical motifs:
– $\alpha$ -Helix (coiled, right-handed).
– $\beta$ -Pleated Sheet (parallel or antiparallel).

3. Tertiary Structure

• Overall 3-D conformation of a single polypeptide chain.
• Stabilizing forces:
– Hydrophobic interactions (non-polar R groups cluster internally).
– Hydrogen bonds, ionic (salt bridges), disulfide bridges, van der Waals.
• Environmental pH effect on charged residues:
– Low pH (acidic): $\mathrm{AA + H^+ \rightarrow AA^{+}}$ → more positive.
– High pH (basic): $\mathrm{AA \rightarrow AA^- + H^+}$ → more negative.
– Isoelectric point (pI): pH where net charge = 0.

4. Quaternary Structure

• Assembly of two or more tertiary subunits into a functional multimer (e.g., hemoglobin = $\alpha2\beta2$ ).

Denaturation of Proteins

• Defined as loss of native (functional) conformation → loss of biological activity.
• Inducing agents: heat, pH extremes, organic solvents, heavy metals, detergents.
• Usually irreversible because primary structure remains intact yet refolding is kinetically unfavorable.

(Brief) Classification of Proteins by Function

• Although slides titled “Classification of Protein” were blank, standard categories to anticipate:
– Structural: collagen, keratin.
– Enzymatic: DNA polymerase, trypsin.
– Transport: hemoglobin, albumin.
– Signaling: insulin, growth hormone.
– Defensive: antibodies (immunoglobulins).
– Motor: myosin, kinesin.

Flowchart – From Sequence to Function

1️⃣ Gene (DNA) → mRNA → ribosomal translation → Primary structure.
2️⃣ Spontaneous or chaperone-assisted folding → Secondary/Tertiary.
3️⃣ Subunit association → Quaternary.
4️⃣ Post-translational mods → mature, functional protein.

Key Numerical & Chemical Facts to Remember

• 20 canonical amino acids.
• Peptide bond length ≈ $1.32\,\text{Å}$ , planarity restricts rotation.
• $pK_a$ ranges: carboxyl ≈ 2, amine ≈ 9–10, side-chain dependent.
• Typical protein size: 50 – 1000 AAs (≈ 5.5 kDa to 110 kDa, using average 110 Da per AA).

Practical / Real-World Connections

• Nutritional aspect: foods from rebus game (eggs, salmon, milk, chicken breast, almonds) supply all essential amino acids (complete proteins).
• Clinical: denaturation under fever can impair enzymes; pI exploited in isoelectric focusing during lab diagnostics.
• Pharmaceutical: protein folding diseases (Alzheimer’s, CJD) underscore tertiary/quaternary integrity.

Ethical & Philosophical Notes

• Genetic engineering of proteins (e.g., insulin) raises bioethical debates on GMO foods & biotech patents.
• Protein supplementation industry: need for evidence-based claims vs. marketing.

Study Tips & Mnemonics

• "LAMP" : Levels = Linear, Alpha/beta, 3-D, Multimeric → Primary→Secondary→Tertiary→Quaternary.
• Remember peptide bond formation Loses H₂O (L for Loss, L for Linkage).
• Essential AAs phrase: “PVT TIM HALL” (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine).

Quick Self-Check Questions

Which level of structure is disrupted first during mild heat denaturation?
Why can changing a single nucleotide in DNA radically alter tertiary structure?
Explain why proteins precipitate at their pI during lab purification.

Practice writing the reaction for tripeptide formation and label N/C termini!