Protein Structure and Function

Introduction to Proteins

Proteins are the most abundant and versatile macromolecules in living organisms. They are essential for a wide range of biological functions and are composed of 20 different amino acids, each with unique side chains.

• Proteins: Large, complex molecules made up of amino acid chains.

• Amino acids: Organic molecules serving as the building blocks of proteins.

• Macromolecules: Large molecules necessary for life, including proteins, nucleic acids, carbohydrates, and lipids.

Example: Hemoglobin is a protein responsible for oxygen transport in blood.

Overview of Protein Structure

Proteins have a hierarchical structure, which determines their function and diversity in living cells. The four levels of protein structure are:

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall three-dimensional shape formed by interactions among side chains (R-groups).

• Quaternary structure: Association of multiple polypeptide subunits into a functional protein complex.

Additional info: The sequence of amino acids (primary structure) ultimately determines the higher levels of structure and the protein's function.

Amino Acids and Their Polymerization

Structure of Amino Acids

Each amino acid consists of a central carbon atom (alpha carbon) bonded to four groups:

• Hydrogen atom (H)

• Amino group (NH2)

• Carboxyl group (COOH)

• R-group (side chain): Unique to each amino acid and determines its properties

Ionization in Water: In aqueous solutions, amino and carboxyl groups can ionize, affecting solubility and reactivity.

Nature and Properties of R-Groups

The R-group, or side chain, is the variable part of an amino acid and is responsible for the diversity of amino acids. R-groups can be:

• Charged (acidic or basic)

• Uncharged polar

• Nonpolar

These properties affect the solubility and chemical reactivity of amino acids.

Polarity and Charge of R-Groups

R-groups determine whether an amino acid is hydrophilic (water-loving) or hydrophobic (water-fearing):

• Polar and charged R-groups: Hydrophilic, interact with water

• Nonpolar R-groups: Hydrophobic, do not interact with water

To classify an amino acid, ask:

1. Does the side chain have a negative charge? (Acidic)

2. Does the side chain have a positive charge? (Basic)

3. If uncharged, does it have an oxygen atom? (Uncharged polar)

4. If none of the above, it is nonpolar.

Example: Methionine is a nonpolar amino acid.

Polymerization: Formation of Proteins

Amino acids link together via peptide bonds formed by condensation reactions between the carboxyl group of one amino acid and the amino group of another.

• Peptide bond: C-N covalent bond joining amino acids

• Polypeptide: Chain of more than 50 amino acids

• Protein: Complete, functional form of a polypeptide

Directionality: Polypeptides have an N-terminus (amino end) and a C-terminus (carboxyl end).

Levels of Protein Structure

Primary Structure

The primary structure is the unique sequence of amino acids in a protein. This sequence is fundamental to all higher levels of structure.

• Single amino acid changes can drastically affect protein function.

• Example: Sickle cell anemia is caused by a change from glutamic acid to valine in hemoglobin.

Secondary Structure

Secondary structure arises from hydrogen bonding between the backbone atoms of the polypeptide chain, forming:

• Alpha-helix (α-helix)

• Beta-pleated sheet (β-sheet)

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions among R-groups:

• Hydrogen bonding (between polar side chains)

• Hydrophobic interactions (among nonpolar side chains)

• Van der Waals interactions (weak electrical attractions)

• Covalent bonding (disulfide bonds between cysteine residues)

• Ionic bonding (between charged side chains)

Quaternary Structure

Quaternary structure results from the assembly of multiple polypeptide subunits into a single functional protein complex.

• Dimers: Proteins with two subunits

• Homodimers: Two identical subunits

• Macromolecular machines: Complexes of multiple proteins (e.g., ribosome)

Table: Levels of Protein Structure

Protein Folding and Function

Protein Folding

Protein folding is crucial for function and is often spontaneous, driven by chemical bonds and interactions. Folded proteins are more stable and have lower potential energy than unfolded (denatured) proteins.

• Molecular chaperones: Proteins that assist in proper folding and prevent aggregation (e.g., Hsp90).

• Misfolded proteins can cause diseases (e.g., prions in mad cow disease).

Protein Flexibility and Regulation

Proteins are dynamic and may exist in multiple shapes until they bind specific molecules, which can trigger their final functional form. Some proteins require binding partners to complete folding.

Infectious Protein Misfolding

Misfolded proteins, such as prions, can induce normal proteins to adopt the infectious, disease-causing shape.

• Prion protein (PrP): Responsible for transmissible spongiform encephalopathies (e.g., mad cow disease).

Diverse Functions of Proteins

Major Protein Functions

Proteins perform a wide variety of functions in cells:

• Catalysis: Enzymes speed up chemical reactions

• Structure: Shape cells and form body structures

• Movement: Motor proteins move cells or molecules

• Signaling: Convey signals between cells

• Transport: Move molecules across membranes or throughout the body

• Defense: Antibodies attack pathogens

Enzymes as Catalysts

Enzymes are proteins that function as biological catalysts, holding substrates in precise orientation to facilitate chemical reactions.

• Active site: Region on the enzyme where substrates bind and react

Equation:

Proteins and the Origin of Life

Proteins in Early Life

Researchers have shown that amino acids could polymerize under prebiotic conditions, but proteins alone do not possess all attributes of life (information, replication, evolution). Nucleic acids are required for information storage and replication.

• Stanley Miller's experiment: Demonstrated amino acid formation under simulated early Earth conditions.

• Limitation: Proteins cannot self-replicate or evolve without nucleic acids.