Chapter 19 notes

Introduction to Proteins and Amino Acids

• Proteins are large, complex molecules essential for biological functions, formed from 20 different amino acids.

• Functions of proteins include:

  • Structural components (cartilage, muscles, nails, etc.)

  • Enzymatic activity (catalyzing biochemical reactions)

  • Transport functions (hemoglobin and myoglobin for oxygen transport)

  • Regulation (insulin, growth hormones)

  • Protection (immunoglobulins for immune response)

Classification of Proteins

• Proteins can be classified based on their function:

  • Structural: Collagen in tendons, keratin in hair.

  • Contractile: Myosin and actin make muscles move.

  • Transport: Hemoglobin carries oxygen, lipoproteins transport lipids.

  • Storage: Casein in milk, ferritin stores iron in the body.

  • Hormonal: Insulin regulates blood glucose, growth hormone for body growth.

  • Enzymes: Sucrase, trypsin facilitate biochemical reactions.

  • Protection: Immunoglobulins help recognize and eliminate foreign substances.

Structure of Amino Acids

• Amino Acids:

  • Contain a central carbon bonded to an ammonium group, carboxylate group, hydrogen atom, and an R group (side chain).

  • At physiological pH, they exist as zwitterions (both positive and negative charges).

• Classification of Amino Acids by R Groups:

  • Nonpolar (Hydrophobic): R groups include hydrogen, alkyl, or aromatic groups.

  • Polar Amino Acids:

    • Neutral: Hydroxyl, thiol, or amide groups.

    • Acidic: Carboxylate groups.

    • Basic: Ammonium groups.

• Amino Acid Abbreviations:

  • Three-Letter and One-Letter Abbreviations for 20 amino acids (e.g., Serine: Ser, S; Aspartate: Asp, D).

Peptide Bond Formation

• Peptide Bond: Covalent amide bond formed between the carboxyl group of one amino acid and the amino group of another.

• Forms peptides:

  • Dipeptides (2 amino acids)

  • Tripeptides (3)

  • Longer chains referred to as polypeptides.

Protein Structures

• Primary Structure: Linear sequence of amino acids, determines biological activity; e.g., insulin consists of two polypeptide chains linked by disulfide bonds.

• Secondary Structure: Local folding (alpha helices, beta-pleated sheets) stabilized by hydrogen bonds between backbone atoms.

• Tertiary Structure: Overall 3D shape of the protein formed by interactions among R groups (e.g., hydrophobic interactions, salt bridges, hydrogen bonds, disulfide bonds).

• Quaternary Structure: Assembly of multiple polypeptide subunits (e.g., hemoglobin with four subunits).

Denaturation of Proteins

• Denaturation: Disruption of secondary, tertiary, or quaternary structures without breaking peptide bonds, leading to loss of function.

• Causes of Denaturation:

  • Heat: Disrupts hydrogen bonds and interactions

  • pH changes: Alters ionic and hydrogen bonds

  • Organic compounds: Coagulates proteins

  • Heavy metals: Form ionic bonds with protein residues

  • Mechanical agitation: Stretches polypeptide chains.

  • Examples:

    • Tannic acid helps create a protective cover on burns by coagulating proteins.

    • Eggs hard boil as heat denatures proteins.

Health Links: Essential Amino Acids

• Essential Amino Acids (9 total) must be obtained from diet; examples include lysine, valine, phenylalanine, and leucine.

• Complete Proteins: Sources that contain all essential amino acids (e.g., eggs, meat).

• Incomplete Proteins: Plant sources lacking one or more essential amino acids (e.g., beans, nuts).

Health Issues Related to Protein

• Cystinuria: Genetic condition causing excess cystine in urine leading to kidney stones.

• Sickle-Cell Anemia: Caused by mutation in hemoglobin structure, leading to sickle-shaped red blood cells which affect oxygen transport.

• Alzheimer's Disease: Changes in protein folding result in formation of insoluble beta-pleated sheets causing plaques in the brain.

Conclusion

• Understanding proteins and amino acids is critical for recognizing their diverse biological functions and health implications, as well as the consequences of denaturation and diseases related to abnormal protein structures.