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# Protein Structures This document describes the four levels of protein structure. ## 1. Primary Structure * The primary structure is the linear sequence of amino acids (aa) in a polypeptide chain. * This sequence is determined by the sequence of nucleotides in the gene (DNA). * The primary...

# Protein Structures This document describes the four levels of protein structure. ## 1. Primary Structure * The primary structure is the linear sequence of amino acids (aa) in a polypeptide chain. * This sequence is determined by the sequence of nucleotides in the gene (DNA). * The primary structure is unique to each protein species/type. * Alterations in the primary structure can impact the protein's function. * Insulin's primary structure is the same in all humans, though similar but not identical in different species. ## 2. Secondary Structure * Secondary structure arises from the folding of the polypeptide chain. * Two main forms are: * Helices (e.g., α-helix, π-helix, 310-helix). They differ based on their turns and pitch, where the α-helix is common. * β-pleated sheets (parallel or anti-parallel strands). * These structures are held together by hydrogen bonds between the carbonyl (C=O) and amino (N-H) groups of the polypeptide backbone, or side chains. * Less regular structures include turns and random coils which connect and link the helices and sheets. * The specific type of secondary structure a segment will adopt is determined by the local primary structure (amino acid sequence within that segment). ## 3. Tertiary Structure * Tertiary structure is the three-dimensional arrangement of the entire polypeptide chain. * Helical and nonhelical regions of the secondary structures fold back and forth. * The shape can be globular, cylindrical, spherical, ovoid, or fibrous. * Tertiary structure is determined by the interactions between the amino acid side chains. * Important interactions include: * Hydrophobic interactions (nonpolar amino acids move inwards away from water). * Ionic bonds/salt bridges * Hydrogen bonds * Disulfide bridges (covalent bonds between cysteine amino acids forming a di-S bridge). * Active protein domains are formed (e.g., in enzymes: binding sites and catalytic sites). * Certain proteins require multiple subunits to become active (tertiary structure not enough). ## 4. Quaternary Structure * Quaternary structure describes the arrangement of multiple polypeptide chains in a protein complex. * The subunits can be identical or different. * Stabilized by weak bonds (rarely covalent bonds). * Protein subunits assemble in a specific orientation. * Example: Hemoglobin (with four subunits). ## Polypeptide Flexibility * The polypeptide backbone is flexible due to rotation around the bonds connecting amino acids. * Torsion/dihedral angles occur between the bonds. ## Polypeptide Synthesis * Polypeptides are synthesized by ribosomes, polymerizing amino acids in a sequence dictated by mRNA. * Peptide bonds form between amino acids. * Proteases digest/release individual amino acids. * The precise amino acid sequence is crucial for protein function and 3D structure. ## Protein Denaturation * Unfolding/loss of secondary/tertiary structure (caused by physical or chemical agents). * Loss of function due to disruption of the protein's active domains. ## Additional Notes * Images of diagrams and 3D models of collagen, elastin,myoglobin and hemoglobin are included in the document. * Specific examples of proteins' structures were mentioned, for instance, collagen and elastin secondary structures.