Amino Acid Polymers: Peptide Bond Structure

Amino Acid Polymers

• Amino acid polymer is a peptide bond formed between amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus)
• The backbone of a peptide chain consists of repeating patterns of carbon and nitrogen atoms

Biological Importance of Proteins

• Proteins are the workers of a cell, performing various functions to enable a cell to move, use energy, respond to stimuli, and stay alive
• Proteins have various functions, including:
  • Catalysis (enzymes)
  • Structure (muscle protein)
  • Movement (myosin and actin)
  • Defense (antibodies)
  • Regulation (enzymes and hormones)
  • Transport (globin)
  • Storage (Mb and Ferritin)

Classification of Amino Acids

• Amino acids can be classified into:
  • Essential amino acids (9): cannot be synthesized by the body and must be obtained through diet
  • Non-essential amino acids (11): can be synthesized by the body using essential amino acids
  • Semi-essential amino acids (e.g., arginine, cysteine, and tyrosine): cannot be synthesized in sufficient quantities during certain conditions (e.g., pregnancy, adolescent growth, or recovery from trauma)

Classification of Amino Acids According to Polarity

• Amino acids can be classified into:
  • Polar amino acids: contain polar hydrophilic side chains (e.g., OH, SH, NH2, COOH)
  • Non-polar amino acids: do not contain polar hydrophilic side chains

Classification of Amino Acids According to Chemical Structure

• Amino acids can be classified into:
  • Acidic amino acids (2): aspartic acid and glutamic acid
  • Basic amino acids (3): lysine, histidine, and arginine
  • Neutral amino acids (15): do not have acidic or basic properties

Metabolic Classification of Amino Acids

• Amino acids can be classified into:
  • Ketogenic amino acids: produce ketone bodies on metabolism
  • Mixed ketogenic and glucogenic amino acids: produce both ketone bodies and glucose on metabolism
  • Glucogenic amino acids: produce glucose on metabolism

Structures of Proteins

• Proteins are formed by a large number of amino acids linked together by peptide bonds to form a polypeptide chain
• Disulfide bonds: strong covalent bonds between sulfur amino acids (cysteine)

Quaternary Structure

• Quaternary structure is the overall protein structure that results from the aggregation of two or more polypeptide subunits

Protein Folding

• Protein folding is the physical process by which a linear polypeptide folds into its characteristic and functional three-dimensional structure
• Protein mis-folding leads to loss of function and causes a wide range of diseases (e.g., Alzheimer's disease and Parkinson's disease)

Classification of Proteins

• Classification based on protein function:
  • Catalytic proteins (e.g., enzymes)
  • Structural proteins (e.g., collagen, elastin, keratin)
  • Contractile proteins (e.g., myosin, actin)
  • Transport proteins (e.g., hemoglobin, myoglobin, albumin, transferrin)
  • Regulatory proteins or hormones (e.g., ACTH, insulin, growth hormone)
  • Genetic proteins (e.g., histones)
  • Protective proteins (e.g., immunoglobulins, clotting factors)

Classification Based on Composition

• Simple proteins: contain amino acids only
• Conjugated proteins: contain some non-protein moiety in its structure along with protein part (e.g., lipoprotein, glycoprotein)

Classification of Protein Based on Structure

• Fibrous proteins: long, narrow fibers, insoluble, more stable (e.g., actin, myosin, collagen)
• Globular proteins: rounded (spherical), soluble, less stable (e.g., albumin, hemoglobin, insulin)

Protein Denaturation

• Protein denaturation is the unfolding and disorganization of the protein's secondary and tertiary structures, without hydrolysis of peptide bonds
• Denaturing agents:
  • Physical agents (e.g., heat, UV light, violent shaking, high pressure)
  • Chemical agents (e.g., strong acids, strong alkalis, heavy metal salts, organic solvents)

Protein and Oral Health

• Protein helps promote steady growth and plays a role in the protection and repair of the periodontal tissue
• Consuming protein at the recommended or higher than recommended intake level is associated with better periodontal health
• Plant-based protein and periodontal health:
  • Fruits and vegetables, whole grains, nuts, and seeds contain polyphenols and fibers, which can reduce oxidative stress and inflammation
  • Protein helps increase absorption of calcium, supporting the mineralization of jaw bone

Amelogenins

• Amelogenins are tissue-specific proteins, rich in proline, leucine, histidine, and glutamyl residues
• Synthesized by the ameloblast cells of the inner enamel epithelium
• Amelogenins play a major role in the mineralization and morphological changes in enamel