1.2 Biologically Important Molecules (Proteins)

Proteins

Functions of Proteins

• Speed up chemical reactions (catalysts)

• Transport substances throughout the body

• Provide structural support in cells and tissues

• Facilitate movement in organisms

• Carry cellular messages

• Regulate cellular processes

• Fight infection by acting as antibodies

• Many more functions in biological systems!

Building Blocks of Proteins

• Amino Acids:

  • The monomers of proteins.

  • The body uses 20 different types of amino acids to construct proteins.

  • Structure of an amino acid:

    • Central carbon atom

    • Hydrogen atom

    • Amino group (–NH2)

    • Carboxylic group (–COOH)

    • R-group (side chain) that determines the function of the protein.

Types of Amino Acids

• Polar Amino Acids:

  • Prefer an aqueous (water) environment;

  • Usually found on the surface of proteins.

• Non-Polar Amino Acids:

  • Avoid aqueous environments;

  • Typically located in the core of proteins.

• Electrically Charged Amino Acids:

  • Positively or negatively charged;

  • Hydrophilic and often play roles in interactions and bonding.

Amino Acid Examples

• Non-essential amino acids:

  • Can be synthesized by the body: Glycine, Alanine, etc.

• Essential amino acids:

  • Must be obtained through diet: Leucine, Lysine, etc.

• Total Amino Acids: 20 (8 essential and 12 non-essential).

Peptide Bonds

• Formation of Proteins:

  • Proteins are formed when amino acids are linked via peptide bonds (condensation reaction).

  • Broken down by hydrolysis, adding water to break these peptide bonds.

• Peptides:

  • Dipeptide – 2 amino acids linked by a peptide bond.

  • Tripeptide – 3 amino acids linked.

  • Polypeptide – More than 3 amino acids linked.

Levels of Protein Organization

Primary Structure

• Structure:

  • Linear sequence of amino acids forming a polypeptide chain.

  • Critical for the final structure and function of the protein.

  • Example: In sickle cell anemia, a single error in amino acid sequence affects folding, resulting in disease pathology.

Secondary Structure

• Coils and Folds:

  • Formed by interactions between elements of the amino acid backbone, often through hydrogen bonds.

  • Common patterns include:

    • Alpha-helix

    • Beta-pleated sheets.

Tertiary Structure

• Further folding of the polypeptide into a functional shape.

• Involves R-group interactions:

  • Hydrogen bonds - with polar side chains;

  • Ionic bonds - with charged side chains;

  • Hydrophobic interactions - among non-polar R groups;

  • Disulfide bridges - covalent bonds between sulfur-containing R groups.

Quaternary Structure

• Final Shape:

  • Some proteins consist of a single polypeptide chain, while others involve multiple polypeptide chains, each with its own structures.

  • Example: Hemoglobin, a protein made of multiple chains.

Denaturation of Proteins

• Unfolding Process:

  • Occurs when normal bonding of R-groups is disturbed, breaking intermolecular bonds affecting secondary, tertiary, and quaternary structures.

  • Result: Loss of 3D shape, leading to loss of function.

• Causes of Denaturation:

  • Extreme temperatures (hot or cold).

  • Extreme pH (high or low).

  • Exposure to chemicals.

Consequences of Denaturation

• Useful Case:

  • Gastrin, a digestive enzyme, works in stomach (low pH) but is inactive in small intestine (high pH).

• Dangerous Case:

  • Prolonged fever can lead to denaturation of critical brain enzymes, possibly resulting in death.