Proteins

• Proteins are naturally occurring, unbranched polymers in which the monomer units are amino acids
• The word "protein" is derived from the Greek word "proteios", meaning holding the first place
• Elemental composition of proteins includes carbon, oxygen, sulfur, hydrogen, and nitrogen
• Proteins may also contain other elements such as P, Fe, Cu, I, Mg, Mn, Zn, etc
• Primary Structure:

  • The linear sequence of amino acids forming the backbone of proteins
  • Represents the number and types of amino acids in the specific amino acid sequence
  • Proteins must have the correct sequence of amino acids to function properly
• Secondary Structure:

  • The spatial arrangement of proteins by twisting of the polypeptide chain
  • Common types are the α helix and the β pleated sheet
  • Held in shape by hydrogen bonds between carbonyl O and amino H of amino acids
• Beta-Sheet:

  • Two protein chain segments held together by hydrogen bonds
  • Can be parallel or antiparallel with different strengths of H-bonds
  • Motifs are combinations of secondary structures forming supersecondary structures
• Tertiary Structure:

  • The overall three-dimensional shape of a protein from interactions between widely separated amino acid side chains
  • Interactions include covalent-disulfide bonds, electrostatic interactions, hydrogen bonds, and hydrophobic interactions
• Quaternary Structure:

  • The highest level of protein organization
  • Found in multimeric proteins with two or more independent peptide chains
  • Held together mainly by hydrophobic interactions between amino acid R groups
• Hydrolysis of Proteins:

  • Heating a protein in a solution of strong acid or base hydrolyzes peptide bonds to produce free amino acids
  • Regenerates amine and carboxylic acid functional groups
• Protein Denaturation:

  • Partial or complete disorganization of a protein's 3D shape due to disruption of structural interactions
  • Results in loss of biochemical activity and coagulation
• According to Shape:

  • Fibrous Proteins: elongated shape with simple, regular, linear structures
  • Globular Proteins: folded into spherical shapes with hydrophobic side chains inside and hydrophilic side chains outside
• According to Composition:

  • Simple: composed of only amino acid residues (e.g., albumin, globulin)
  • Conjugated: amino acids with additional components like prosthetic groups
  • Derived: simple or conjugated proteins that underwent change (e.g., processes and peptones in hydrolytic breakdown)
• Alpha-Helix:

  • Resembles a coiled spring with hydrogen bonds between H atom attached to peptide N and O atom attached to peptide C
  • Proline cannot be found in α-helix
  • Contains only L-amino acids
  • R groups of aminoacyl residues face outward
  • Formed by maximum number of hydrogen bonds and van der Waals interactions
• Covalent-Disulfide - two cysteine residues react with each other

  Electrostatic - interaction between an acidic and basic side chain

  Hydrogen Bonds - occurs between2 polar R groups

  Hydrophobic - 2 non polar R groups are close to each other