Protein

1. Protein Characteristics

Proteins are organic macromolecules composed of amino acids linked by peptide bonds, serving as critical structural and functional components across all tissues and organs.

• Examples: Keratin (protective tissues, hair) and collagen (skin, bones, meninges).

• Functional Roles: Enzymes, antibodies, neurotransmitters, and hormones (e.g., growth hormone).

• Elemental Composition: Carbon, hydrogen, oxygen, nitrogen ($N$), and often sulfur ($S$).

1a. Structure of Proteins

• Monomers: Amino acids. The human body utilizes 20 distinct amino acids:

  • Non-essential (11): Synthesized internally by the body.

  • Essential (9): Must be obtained through diet.

• Amino Acid Composition:

  • Central carbon atom.

  • Hydrogen atom.

  • Basic amino group ($NH_2$).

  • Acidic carboxyl group ($COOH$).

  • Variable side chain ($R$ group), which determines polarity, size, and specific chemical properties.

  • Act as natural buffers to regulate acid-base balance.

• Peptide Formation:

  • Linked via dehydration synthesis through covalent peptide bonds.

  • Peptide: Very short amino acid chain.

  • Polypeptide: Chain containing fewer than 100 amino acids.

  • Protein: Chain containing 100 or more amino acids.

1b. Shape of Proteins

Protein function depends on its specific three-dimensional spatial configuration.

• Structural Levels:

  1. Primary Structure: Linear sequence of amino acids in a polypeptide chain.

  2. Secondary Structure: Local folding maintained by hydrogen bonds.

  • Alpha helix: Spiral structure.

  • Beta-pleated sheet: Accordion-like sheet.

  1. Tertiary Structure: Complex three-dimensional folding from interactions among secondary structures.

  • Maintained by hydrogen bonds and covalent disulfide bonds (sulfur-sulfur bonds between cysteine residues).

  1. Quaternary Structure: Association of two or more tertiary polypeptide subunits (e.g., hemoglobin consisting of two alpha and two beta chains).

• Protein Structural Types:

  • Fibrous Proteins: Elongated, strong, durable, hydrophobic structural molecules (e.g., collagen, keratin).

  • Globular Proteins: Spherical, hydrophilic, reactive molecules (e.g., hemoglobin, enzymes).

• Denaturation:

  • Alteration of functional structure due to heat, acidic or basic pH, or chemical exposure.

  • Causes loss of biological activity and function.

1c. Proteins Function as Enzymes

• Enzymes: Biological catalysts that accelerate chemical reactions without being consumed.

• Active Site: Specific region on an enzyme where a substrate (reactant) binds.

• Mechanism:

  1. Substrate binds to the enzyme active site, forming an enzyme-substrate complex.

  2. Temporary structural adjustments optimize reactant orientation.

  3. Reaction occurs, products are released, and the unchanged enzyme is free to catalyze further reactions.

1d. Other Functions of Proteins

• Hormonal Regulation: Act as chemical messengers (e.g., growth hormone).

• Acid-Base Balance: Serve as buffers to maintain systemic pH.

• Fluid and Electrolyte Balance: Attract water to balance fluids across vascular and cellular compartments.

• Cellular Transport: Membrane proteins regulate movement of substances in and out of cells.

• Conjugated Molecules: Combine with carbohydrates to form glycoproteins and proteoglycans.

• Energy Supply: Broken down for energy during severe carbohydrate/fat depletion, resulting in muscle tissue wasting due to lack of non-structural protein storage.