Proteins: Structure and Function

Proteins: Workhorses of the Cell

• Proteins account for about 15% of a cell's composition.
• Proteins have diverse functions, including enzymes, storage, transport, structural components, signaling, receptors, and gene expression regulators.

Amino Acid Structure

• Each amino acid contains an alpha carbon with four attached groups:

  • Amino group (-NH₂)
  • Carboxyl group (-COOH)
  • Hydrogen atom (-H)
  • R-group (variable side chain)

• Amino acids are linked by peptide bonds formed through dehydration reactions.

Peptide Bonds

• Peptide bonds are strong covalent bonds.
• The bond formation involves the carboxyl group of one amino acid reacting with the amino group of another, releasing water.

Protein Structure

Primary Structure

• The primary structure is the linear sequence of amino acids.
• This sequence dictates how the protein folds.

Secondary Structure

• Secondary structure refers to local folding patterns in the polypeptide chain.
• Common secondary structures include alpha helices and beta sheets.
• Alpha helices result from hydrogen bonds between every fourth amino acid.
• Beta sheets form when polypeptide chains are aligned beside each other, linked by hydrogen bonds.

Tertiary Structure

• Tertiary structure is the overall three-dimensional shape of the protein.
• It is determined by interactions between R-groups of amino acids.
• These interactions can include hydrogen bonds, disulfide bonds, hydrophobic interactions, and ionic bonds.

Quaternary Structure

• Some proteins consist of multiple polypeptide chains (subunits).
• The arrangement of these subunits forms the quaternary structure.

Families of Organic Molecules in Cells

• Organic molecules in cells are categorized into:
  • Sugars (carbohydrates)
  • Amino acids (proteins)
  • Fatty acids (lipids)
  • Nucleotides (DNA/RNA)
• Proteins are crucial for cellular functions, accounting for a significant portion (approximately 15%) of cellular composition.
• Proteins perform various roles, such as catalyzing reactions (enzymes), providing structural support, transporting molecules, generating movement (motor proteins), storing molecules, and carrying signals.

Special Amino Acids

• Some amino acids have specific properties due to their side chains.
• Histidine: its pKa is close to 7, allowing it to act as a base or an acid.
• Proline: its unique structure creates kinks in polypeptide chains, affecting secondary structure.
• Glycine: its small size allows for flexibility and is also crucial for certain structures.
• Cysteine: it can form disulfide bonds.

Hydrogen Bonds in Proteins

• Hydrogen bonds are crucial in protein folding and structure.
• They form between electronegative atoms like oxygen and nitrogen in the backbone or between amino acid side chains.
• Hydrogen bonds help stabilize secondary structures like alpha-helices and beta-sheets and play a critical role in protein tertiary structure.

Explore the essential roles of proteins in cellular composition and function. This quiz covers amino acid structure, peptide bond formation, and the various levels of protein structure including primary and secondary. Test your understanding of these fundamental biological concepts.