Protein Study Guide

📖 Core Concepts Protein – a large macromolecule made of one or more polypeptide chains (linear sequences of amino‑acid residues joined by peptide bonds). Polypeptide vs Peptide – < 20–30 residues → peptide (usually not a full‑size protein); longer chains → polypeptide/protein. Primary structure – the exact linear order of residues; determines all higher‑order structures. Secondary structure – local regular motifs stabilized by H‑bonds: α‑helix, β‑sheet, turns. Tertiary structure – the overall 3‑D fold of a single chain (hydrophobic core, salt bridges, disulfide bonds, etc.). Quaternary structure – assembly of multiple folded subunits into a functional complex. Domains – compact, independently folding units that often carry a specific function (e.g., SH3 domain). Post‑translational modification (PTM) – chemical changes after synthesis (phosphorylation, glycosylation, prosthetic‑group attachment) that modulate activity, stability, localization. Protein turnover – continuous degradation and synthesis; half‑life ranges from minutes to years. --- 📌 Must Remember Sequence direction: synthesis N‑terminus → C‑terminus on ribosomes; chemical synthesis runs C‑terminus → N‑terminus. Peptide bond: partial double‑bond character → planar, restricted rotation. Proline: side‑chain loops back to the backbone N, reducing flexibility; often introduces kinks. Enzyme classification: EC numbers categorize enzymes by reaction type. Typical protein size: archaea 283 aa (30 kDa), bacteria 311 aa (34 kDa), eukaryotes 438 aa (49 kDa); titin ≈27 000 aa (3000 kDa). Key structural techniques: X‑ray crystallography (high‑resolution, needs crystals), cryo‑EM (large complexes, lower resolution), NMR (solution structures ≤ 30 kDa). Young’s modulus: \(E = \sigma / \varepsilon\) – stiffness; collagen/keratin ≫ elastin, globular enzymes ≪ fibrous proteins. Protein families of functional importance: enzymes, structural (actin, collagen), signalling (receptors, hormones), transport (haemoglobin), immune (antibodies). --- 🔄 Key Processes Ribosomal Protein Synthesis Transcription → pre‑mRNA → splicing → mature mRNA. mRNA bound to ribosome; each codon (3 nt) recruits matching aminoacyl‑tRNA (charged by aminoacyl‑tRNA synthetase). Peptide bond formation adds the amino acid to the growing chain (N‑terminus to C‑terminus). Elongation up to 20 aa s⁻¹ in prokaryotes; termination releases the polypeptide. Chemical Peptide Synthesis (Solid‑Phase) Anchor C‑terminal residue to resin. Sequentially couple protected amino acids from C → N direction. Deprotect, cleave, and purify; efficient up to 300 aa. Protein Purification Workflow Cell lysis → crude lysate. Ultracentrifugation to remove debris. Salting‑out or affinity chromatography (e.g., His‑tag + Ni²⁺). Ion‑exchange / size‑exclusion chromatography for polishing. Analytical checks – SDS‑PAGE, activity assay, isoelectric focusing. Structural Determination (X‑ray Crystallography) Purify protein → crystallize. Collect diffraction pattern. Solve phase problem → generate electron density map. Build atomic model, deposit to PDB. Proteolytic Digestion (in Digestion) Pepsin (stomach, acidic) → endopeptidase. Trypsin/Chymotrypsin (small intestine) → cleave after Lys/Arg (trypsin) or aromatic residues (chymotrypsin). --- 🔍 Key Comparisons Protein vs Polypeptide vs Peptide Protein: folded, functional macromolecule (often > 30 kDa). Polypeptide: linear chain of residues (no requirement to be folded). Peptide: short chain (< 20–30 residues). Ribosomal vs Chemical Synthesis Ribosomal: N→C direction, uses tRNAs, co‑translational folding, limited to cellular context. Chemical: C→N direction, can incorporate non‑natural residues, limited length (300 aa). Globular vs Fibrous vs Membrane Proteins Globular: soluble, compact, enzymatic or regulatory. Fibrous: structural, elongated, e.g., collagen, keratin. Membrane: span lipid bilayer, receptors, channels. Exopeptidase vs Endopeptidase Exopeptidase: cleaves terminal peptide bonds (amino‑ or carboxypeptidases). Endopeptidase: cuts internal bonds (pepsin, trypsin, chymotrypsin). --- ⚠️ Common Misunderstandings “All proteins are globular.” → Fibrous (collagen) and membrane proteins exist; they dominate certain functional categories. “Disulfide bonds are present in every protein.” → Only extracellular or oxidizing‑environment proteins often contain them. “Protein mass = # residues × 110 Da.” – Approximate only; actual mass varies with side‑chain composition and PTMs. “PTMs always increase activity.” – PTMs can activate, inhibit, or target proteins for degradation. “A peptide bond can rotate freely.” – Resonance makes it planar; φ/ψ angles are constrained. --- 🧠 Mental Models / Intuition “Backbone as a train track”: peptide bonds are the rigid rails (planar), while φ (phi) and ψ (psi) angles are the switches that determine whether the track coils into an α‑helix or folds into a β‑sheet. “Protein as LEGO®”: domains are interchangeable bricks; swapping or shuffling domains creates multi‑functional proteins. “Folding funnel”: the polypeptide slides down a free‑energy landscape toward the lowest‑energy (native) conformation (Anfinsen’s thermodynamic hypothesis). --- 🚩 Exceptions & Edge Cases Selenocysteine & Pyrrolysine – 21st/22nd amino acids incorporated via recoding mechanisms. Proline’s restricted φ angle – often found at helix‑turning points. Intrinsically Disordered Proteins (IDPs) – 33 % of eukaryotic proteins contain long disordered segments that function without a fixed structure. Membrane proteins – poor crystallization; cryo‑EM or electron crystallography preferred. Chemical synthesis inefficiency > 300 aa – yields drop dramatically; recombinant expression is preferred. --- 📍 When to Use Which | Situation | Preferred Method / Tool | Reason | |-----------|--------------------------|--------| | High‑resolution structure of a soluble protein | X‑ray crystallography | Gives atomic detail if crystals can be grown. | | Structure of a large complex (> 200 kDa) or membrane protein | Cryo‑EM | Works without crystals; tolerates heterogeneity. | | Small (< 30 kDa) protein in solution | NMR spectroscopy | Provides dynamics and solution‑state details. | | Rapid identification of unknown protein | Mass spectrometry (MS) | Sensitive, can detect PTMs, works on complex mixtures. | | Need to introduce a single amino‑acid change | Site‑directed mutagenesis + expression | Precise functional probing. | | Protein > 300 aa and needs isotopic labeling | Recombinant expression in E. coli (or eukaryotic host) | Efficient, allows isotope incorporation for NMR. | | Design a mutant based on a known homolog | Homology modeling | Use a template with ≥ 30 % sequence identity for reliable model. | | Purify a recombinant protein quickly | His‑tag + Ni‑NTA affinity chromatography | Strong, specific binding, minimal impact on activity. | | Detect cellular location | GFP‑fusion microscopy or indirect immunofluorescence | Direct visual read‑out in live or fixed cells. | --- 👀 Patterns to Recognize α‑Helix: i → i+4 H‑bond pattern; side chains project outward, giving a helical wheel of hydrophobic residues on one face for membrane‑spanning helices. β‑Sheet: alternating side‑chain orientation (hydrophobic–hydrophilic) and inter‑strand H‑bonds; often paired with “β‑turn” motifs. PxxP motif → binding site for SH3 domains (proline‑rich). His‑tag purification → single band on SDS‑PAGE after Ni‑NTA column, especially if the tag is at the C‑terminus (less likely to interfere with N‑terminal signal peptides). Protease specificity: trypsin cleaves after Lys/Arg (unless followed by Pro); chymotrypsin after Phe/Tyr/Trp. Disorder prediction: long stretches lacking hydrophobic clusters and with many charged residues → likely IDP. --- 🗂️ Exam Traps Direction of synthesis: A common distractor will state “proteins are synthesized C‑terminus to N‑terminus.” Remember the ribosome adds residues to the C‑terminal end, building N→C. Mass calculation: Assuming every residue = 110 Da ignores the actual side‑chain mass and PTMs; answer choices that match the exact mass (e.g., calculated from sequence) are correct. Domain vs motif: A question may label a short PxxP as a “domain.” It is a motif (recognition site), not a full domain. Cryo‑EM resolution: Some think cryo‑EM always provides atomic resolution; in reality, it often yields 3–5 Å for large complexes, enough for overall shape but not side‑chain detail. Protein half‑life: “All proteins have a half‑life of 1 day.” The correct answer acknowledges a broad range (minutes to years) with an average of 1–2 days in mammalian cells. Disulfide bonds: “Cytosolic proteins always contain disulfide bonds.” The reducing environment of the cytosol prevents disulfide formation; they are common in secreted/extracellular proteins. ---