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<h1>Protein Structure: Classification and Levels of Organization</h1>
<div class="section" id="introduction">
<h2>Introduction to Proteins</h2>
<p><strong>Definition:</strong> Proteins are polymers of amino acids joined together by peptide bonds. They are essential biological macromolecules that perform diverse functions in living organisms.</p>
<ul>
<li>Proteins consist of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur</li>
<li>A protein typically contains more than 100 amino acids (molecular weight > 10,000 Da)</li>
<li>The amino acid sequence determines the protein's structure and function</li>
<li>Protein structure is determined by the genetic code in DNA</li>
</ul>
<h3>Basic Structure of Amino Acids:</h3>
<p>Each amino acid has four groups attached to a central carbon atom (ฮฑ-carbon):</p>
<ol>
<li><strong>Amino group (-NH₂)</strong> - basic group</li>
<li><strong>Carboxyl group (-COOH)</strong> - acidic group</li>
<li>Hydrogen atom (-H)</li>
<li>R group (side chain) - varies among amino acids</li>
</ol>
</div>
<div class="section" id="classification">
<h2>Classification of Proteins</h2>
<h3>Based on Molecular Shape</h3>
<p>Proteins are primarily classified into two main types based on their three-dimensional structure:</p>
<h4>1. Fibrous Proteins</h4>
<ul>
<li>Elongated, fiber-like structure</li>
<li>Generally <strong>insoluble in water</strong> (hydrophobic)</li>
<li>Structural in nature - provide support and strength</li>
<li>Long, narrow, or thin thread-like appearance</li>
<li>Repetitive amino acid sequences</li>
<li>Less complex tertiary structure</li>
<li>Stabilized primarily by hydrogen and disulfide bonds</li>
</ul>
<p><strong>Examples:</strong> Collagen, Keratin, Myosin, Elastin, Fibrin, Fibroin</p>
<p><strong>Functions:</strong> Structural support, providing mechanical strength, maintaining cell shape</p>
<h4>2. Globular Proteins</h4>
<ul>
<li>Spherical or roughly round shape</li>
<li>Generally <strong>soluble in water</strong> (hydrophilic)</li>
<li>Functional in nature - catalyze reactions and transport molecules</li>
<li>Irregular amino acid sequences</li>
<li>Complex, highly folded tertiary structure</li>
<li>Compact structure with hydrophobic core</li>
<li>More sensitive to changes in temperature and pH</li>
</ul>
<p><strong>Examples:</strong> Hemoglobin, Myoglobin, Insulin, Albumin, Enzymes, Immunoglobulins (Antibodies), Protein kinase A</p>
<p><strong>Functions:</strong> Enzymatic catalysis, transport and storage, regulation and control, defense and immunity, motion and muscle contraction, cell signaling</p>
<h4>Comparison Table</h4>
<table>
<thead>
<tr>
<th>Feature</th>
<th>Fibrous Proteins</th>
<th>Globular Proteins</th>
</tr>
</thead>
<tbody>
<tr><td>Shape</td><td>Long and narrow</td><td>Spherical/rounded</td></tr>
<tr><td>Solubility</td><td>Insoluble in water</td><td>Soluble in water</td></tr>
<tr><td>Function</td><td>Structural</td><td>Functional/catalytic</td></tr>
<tr><td>Amino acid sequence</td><td>Repetitive</td><td>Irregular</td></tr>
<tr><td>Sensitivity to environment</td><td>Low</td><td>High</td></tr>
<tr><td>pH and temperature stability</td><td>Stable</td><td>Less stable</td></tr>
</tbody>
</table>
</div>
<div class="section" id="amino-acids">
<h2>Amino Acids and Peptide Bonds</h2>
<h3>Peptide Bond Formation</h3>
<p><strong>Definition:</strong> A peptide bond is a covalent amide bond formed between the carboxyl group of one amino acid and the amino group of another amino acid.</p>
<h4>Formation Process (Dehydration Synthesis):</h4>
<ol>
<li>Two amino acids approach each other</li>
<li>The <strong>carboxyl group (-COOH)</strong> of amino acid 1 reacts with the <strong>amino group (-NH2)</strong> of amino acid 2</li>
<li>The carboxyl group loses a hydroxyl group (<strong>-OH</strong>)</li>
<li>The amino group loses a hydrogen atom (<strong>-H</strong>)</li>
<li>These combine to form a <strong>water molecule (H2O)</strong></li>
<li>A <strong>C-N covalent bond</strong> forms between the two amino acids</li>
</ol>
<h4>Chemical Equation:</h4>
<pre><code>
Amino acid 1 (-COOH) + Amino acid 2 (-NH2) →
Amino acid 1-CO-NH-Amino acid 2 + H2O
</code></pre>
<p><strong>Result:</strong> The resulting molecule is called a <strong>dipeptide</strong> (two amino acids) or <strong>polypeptide</strong> (many amino acids).</p>
<h4>Characteristics of Peptide Bonds:</h4>
<ul>
<li><strong>Strong covalent bonds</strong> with partial double-bond character</li>
<li><strong>Rigid and planar</strong> in nature</li>
<li>Not broken by heating or high salt concentration</li>
<li>Broken by strong acids or bases (prolonged heat) and specific enzymes (proteases)</li>
<li>Formation is <strong>endergonic</strong> — requires ATP energy</li>
<li>Partial charge distribution: partial positive on amino hydrogen, partial negative on carboxyl oxygen</li>
</ul>
<h4>Peptide Bond Degradation</h4>
<ul>
<li>Occurs through <strong>hydrolysis</strong> — requires water molecules</li>
<li>Slow due to partial double-bond character</li>
<li>Important in protein digestion by proteases</li>
</ul>
</div>
<div class="section" id="levels">
<h2>Levels of Protein Structure</h2>
<p>Protein structure is organized hierarchically into <strong>four levels:</strong></p>
<ol>
<li><strong>Primary Structure</strong> — Linear sequence of amino acids</li>
<li><strong>Secondary Structure</strong> — Local folding patterns</li>
<li><strong>Tertiary Structure</strong> — Complete 3D shape of single polypeptide</li>
<li><strong>Quaternary Structure</strong> — Multiple polypeptide subunits</li>
</ol>
</div>
<div class="section" id="primary">
<h2>Primary Structure</h2>
<h3>Definition</h3>
<p>The primary structure is the linear sequence of amino acids joined by peptide bonds.</p>
<h3>Key Features</h3>
<ul>
<li>Determined by genetic code in DNA</li>
<li>Defines all higher levels of protein structure</li>
<li>Peptide and disulfide bonds stabilize sequence</li>
</ul>
<h3>Significance</h3>
<p>Single amino acid changes can drastically affect function — e.g., sickle-cell anemia caused by glutamic acid to valine substitution in hemoglobin. The sequence dictates folding and function.</p>
</div>
<div class="section" id="secondary">
<h2>Secondary Structure</h2>
<h3>Definition</h3>
<p>Local spatial arrangements stabilized by hydrogen bonds between backbone atoms (C=O and N-H).</p>
<h3>Main Types</h3>
<h4>1. Alpha-Helix (ฮฑ-helix)</h4>
<ul>
<li>Right-handed coil stabilized by hydrogen bonds between carbonyl oxygen of amino acid n and amide hydrogen of amino acid n+4</li>
<li>3.6 residues per turn, rise 5.4 โซ per turn</li>
<li>Found in keratin, myosin, myoglobin, hemoglobin</li>
<li>Amino acids like alanine promote formation; proline disrupts</li>
</ul>
<h4>2. Beta-Pleated Sheet (ฮฒ-sheet)</h4>
<ul>
<li>Multiple polypeptide strands aligned side-by-side, stabilized by intermolecular hydrogen bonds</li>
<li>Parallel and antiparallel types (antiparallel more stable)</li>
<li>Common in fibrous proteins and enzymes</li>
</ul>
<h4>Other Structures</h4>
<ul>
<li>Beta turns and loops help fold and reverse the chain direction</li>
<li>Important for protein flexibility</li>
</ul>
</div>
<div class="section" id="tertiary">
<h2>Tertiary Structure</h2>
<p>The full 3D conformation of a single polypeptide chain determined by R-group interactions.</p>
<h3>Stabilizing Interactions</h3>
<ul>
<li><strong>Hydrogen bonds</strong> between polar R groups</li>
<li><strong>Ionic bonds (salt bridges)</strong> between oppositely charged side chains</li>
<li><strong>Disulfide bonds</strong> between cysteine residues (covalent)</li>
<li><strong>Hydrophobic interactions</strong> among nonpolar side chains cluster internally</li>
<li><strong>Van der Waals forces</strong> weak attractions optimizing packed structure</li>
</ul>
<h3>Functional domains</h3>
<p>Specific 3D regions with distinct functions. Proteins fold into their native active forms at this level.</p>
</div>
<div class="section" id="quaternary">
<h2>Quaternary Structure</h2>
<p>Arrangement and interaction of multiple polypeptide chains (subunits) in multi-subunit proteins.</p>
<h3>Stabilizing Forces</h3>
<ul>
<li>Non-covalent interactions: hydrogen bonds, ionic bonds, hydrophobic, and Van der Waals forces</li>
</ul>
<h3>Examples</h3>
<ul>
<li>Hemoglobin: 4 subunits (2 ฮฑ, 2 ฮฒ)</li>
<li>Immunoglobulins, enzyme complexes</li>
</ul>
<h3>Significance</h3>
<ul>
<li>Essential for cooperative binding, allosteric regulation, and overall protein function</li>
</ul>
</div>
<div class="section" id="forces">
<h2>Stabilizing Forces Summary Table</h2>
<table>
<thead>
<tr>
<th>Force</th>
<th>Type</th>
<th>Strength</th>
<th>Location in Structure</th>
<th>Characteristics</th>
</tr>
</thead>
<tbody>
<tr><td>Peptide Bond</td><td>Covalent</td><td>Strong (400 kJ/mol)</td><td>Primary</td><td>Rigid, planar, forms backbone</td></tr>
<tr><td>Hydrogen Bonds</td><td>Noncovalent</td><td>Moderate (26 kJ/mol)</td><td>Secondary, Tertiary</td><td>Easily disrupted by heat/pH</td></tr>
<tr><td>Ionic Bonds</td><td>Noncovalent</td><td>Weak-Moderate</td><td>Tertiary, Quaternary</td><td>pH and salt sensitive</td></tr>
<tr><td>Disulfide Bonds</td><td>Covalent</td><td>Very Strong (400 kJ/mol)</td><td>Primary, Tertiary</td><td>Not easily broken</td></tr>
<tr><td>Hydrophobic Interactions</td><td>Noncovalent</td><td>Cumulative (~5 kJ/mol each)</td><td>Tertiary</td><td>Entropy-driven</td></tr>
<tr><td>Van der Waals Forces</td><td>Noncovalent</td><td>Very Weak (0.1-1 kJ/mol)</td><td>Tertiary</td><td>Significant in numbers</td></tr>
</tbody>
</table>
</div>
<div class="section" id="denaturation">
<h2>Protein Denaturation</h2>
<h3>Definition</h3>
<p>Denaturation disrupts a protein's native 3D structure, usually causing loss of biological activity.</p>
<h3>Types of Denaturation</h3>
<h4>Reversible Denaturation</h4>
<ul>
<li>Protein can refold when denaturing agent removed</li>
<li>Structure and function restored</li>
<li>Example: Mild heating, mild pH changes</li>
</ul>
<h4>Irreversible Denaturation</h4>
<ul>
<li>Protein cannot refold, permanent loss of function</li>
<li>Caused by prolonged heat, extreme pH, covalent modifications, or aggregation</li>
</ul>
<h3>Major Denaturing Agents</h3>
<h4>Temperature</h4>
<ul>
<li>Breaks hydrogen and hydrophobic bonds by increased kinetic energy</li>
<li>Denaturation temperature often 60-80°C</li>
<li>Example: Egg white albumin denatures at ~60°C during frying</li>
</ul>
<h4>pH (Acid/Base)</h4>
<ul>
<li>Disrupts ionic bonds by altering protonation states</li>
<li>Extreme acidic or basic pH causes unfolding</li>
<li>Example: Pepsin stable at acidic pH; most proteins denature outside pH 3-11</li>
</ul>
<h4>Chemical Agents</h4>
<ul>
<li>Urea and guanidine hydrochloride disrupt hydrogen bonds</li>
<li>Detergents disrupt hydrophobic interactions</li>
<li>Reducing agents (DTT, ฮฒ-mercaptoethanol) reduce disulfide bonds</li>
</ul>
<h4>Mechanical Stress</h4>
<ul>
<li>Physical agitation disrupts non-covalent bonds; often leads to irreversible denaturation</li>
</ul>
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