BIO 120 Chapter Five F2024

Chapter 5: Protein Structure, Function, and Synthesis

5.1 Overview of Proteins

  • Diversity: Proteins are diverse, ubiquitous, and versatile classes of macromolecules.

  • Functionality: Play critical roles in mediating reactions and cellular processes essential for cellular function.

5.2 Amino Acids: The Building Blocks of Proteins

  • Components: Each amino acid comprises four main components, bound to the alpha carbon:

    • Carboxyl group

    • Amino group

    • Hydrogen atom

    • R group (side chain)

  • Each amino acid's unique R group determines its properties.

5.3 R Group and Protein Folding

  • Impact on Folding: The chemical properties of the R group affect how each amino acid influences the protein's three-dimensional shape, which in turn, governs its function.

5.4 Structure of Amino Acids

  • Classification of Amino Acids:

    • Interaction with water (Hydrophilic or Hydrophobic)

    • Basic or Acidic: affects polarity

    • Polar or Nonpolar: affects solubility in biological systems.

5.5 Hydrophobic vs. Hydrophilic Amino Acids

  • Hydrophobic Amino Acids:

    • Tend to aggregate in the center of proteins, away from aqueous environments.

  • Hydrophilic Amino Acids:

    • Basic and acidic types are strongly polar, typically found on the protein surface, interacting with water via hydrogen bonding.

5.6 Special Amino Acids

  • Glycine: Small, nonpolar, increases protein flexibility.

  • Proline: Cyclic structure restricts folding, affects bends in protein chains.

  • Cysteine: Can form disulfide bridges, crucial for maintaining protein structure.

5.7 Evolution of Histone Proteins

  • Histones evolve slowly due to their crucial role in binding with DNA's phosphate groups, requiring specific basic amino acids (lysine and arginine).

5.8 Peptide Bonds and Polypeptides

  • Peptide Bonds: Form covalent links between adjacent amino acids via dehydration synthesis, limiting rotation around these bonds.

5.9 Levels of Protein Structure

  • Primary Structure: Linear sequence of amino acids, determines ultimate protein structure.

  • Secondary Structure: Refers to local folding patterns, such as alpha helices and beta sheets.

    • Alpha Helix: Stabilized by hydrogen bonds; allows for non-adjacent molecular interactions.

    • Beta Sheet: Can be parallel or antiparallel, stabilized by hydrogen bonds between different sections of the polypeptide chain.

  • Tertiary Structure: The overall three-dimensional shape, determined by R group interactions, hydrophobic interactions, and secondary structures.

    • All forces (van der Waals, covalent, hydrogen, ionic) contribute to the tertiary structure.

  • Quaternary Structure: Characterizes proteins consisting of multiple polypeptide subunits, important for functionality (e.g., hemoglobin).

5.10 Translation of mRNA into Protein

  • Ribosomes: Composed of proteins and ribosomal RNA; read mRNA codons to synthesize proteins.

  • Codon-Anticodon Interactions: Anticodon on tRNA pairs with codon on mRNA in an antiparallel manner.

  • Functional Sites in Ribosomes:

    • A Site: Accepts tRNA.

    • P Site: Peptide bond formation occurs.

    • E Site: Exit site for uncharged tRNA.

  • Translation Stages: Initiation, elongation, and termination involving various factors and ribosomal subunits.

5.11 Protein Evolution and Regulation

  • Proteins exhibit evolutionary conservation and can be grouped into families based on structural and functional similarities. Mutations can lead to beneficial changes allowing for adaptation through natural selection.

  • Regulation of Protein Synthesis: Involves controlling gene expression at multiple levels, ensuring proteins are synthesized only when needed.

  • Protein Sorting: Essential for directing proteins to their correct cellular locations post-translation.

    • Specific signal sequences guide proteins to the endoplasmic reticulum (ER) and the endomembrane system.

5.12 Conclusion

  • The structure of proteins is intricately linked to their function, governed by the sequence of amino acids and various interactions at multiple structural levels.