Protein: Structure and Function – Study Notes (General Biology)

Protein: Structure and Function

Introduction to Proteins

Proteins are the most abundant and versatile macromolecules in living organisms. They are essential for nearly every biological process, and their structure determines their diverse functions in cells.

• Proteins are polymers made from 20 different amino acids, each with a unique side chain (R group).

• Protein structure is organized hierarchically: Primary, Secondary, Tertiary, and Quaternary levels.

• The specific structure of a protein determines its function and its role in living cells.

Macromolecules in Biology

Types of Biological Macromolecules

Macromolecules are large molecules essential for life, composed of thousands of atoms. Living organisms are primarily made up of four major classes of macromolecules:

• Proteins

• Carbohydrates

• Lipids

• Nucleic acids

Each class plays a distinct role in cellular structure and function.

Amino Acids: Building Blocks of Proteins

Structure of Amino Acids

Amino acids are the monomers that make up proteins. Each amino acid has a central carbon atom (the alpha carbon) bonded to:

• An amino group (–NH2)

• A carboxyl group (–COOH)

• A hydrogen atom

• A unique side chain (R group) that determines the properties of the amino acid

Amino acids can exist in nonionized and ionized forms depending on the pH of the solution.

• Nonionized form: Both amino and carboxyl groups are uncharged.

• Ionized form: The amino group gains a proton (H+), and the carboxyl group loses a proton, resulting in a zwitterion.

Classification of Amino Acids

The 20 amino acids are classified based on the properties of their side chains:

• Charged side chains: Hydrophilic; can act as acids (donate H+) or bases (accept H+); participate in ionic bonds.

• Polar side chains: Hydrophilic; contain partial charges; form hydrogen bonds; dissolve easily in water.

• Nonpolar side chains: Hydrophobic; do not interact well with water; often found in the interior of proteins.

• Special side chains: Cysteine contains a sulfhydryl group (–SH) that can form disulfide bridges, stabilizing protein structure.

Protein Structure

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, linked by peptide bonds. This sequence determines all higher levels of structure and ultimately the protein's function.

• Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next.

• Proteins have directionality: the N-terminus (amino end) and the C-terminus (carboxyl end).

Secondary Structure

Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms. The two main types are:

• Alpha helix (α-helix): A coiled structure stabilized by hydrogen bonds.

• Beta sheet (β-sheet): Segments of the polypeptide chain align side by side, forming a sheet-like structure.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, resulting from interactions between R groups and the peptide backbone.

• Hydrogen bonds between R groups

• Hydrophobic interactions

• Disulfide bridges (covalent bonds between cysteine residues)

• Ionic bonds

Quaternary Structure

Quaternary structure arises when two or more polypeptide chains (subunits) join to form a functional protein complex (e.g., hemoglobin).

• Stabilized by the same types of interactions as tertiary structure.

• Allows for complex functions and regulation.

Protein Folding and Denaturation

Importance of Folding

Proper folding is essential for protein function. If a protein loses its shape (denatures), it loses its ability to function.

• Factors such as temperature, pH, and chemicals can cause denaturation.

• Some proteins can refold with the help of chaperone proteins (e.g., heat shock proteins).

• Misfolded proteins can cause diseases (e.g., prion diseases).

Functions of Proteins

Major Types of Protein Functions

Proteins perform a wide variety of functions in cells. The following table summarizes major types and examples:

Type	Function	Example
Enzymatic	Catalyze chemical reactions	Digestive enzymes (e.g., pepsin)
Defensive	Protection against disease	Antibodies
Storage	Storage of amino acids	Casein (milk protein)
Transport	Transport substances	Hemoglobin (oxygen transport)
Hormonal	Coordination of activities	Insulin
Receptor	Response to chemical signals	Cell membrane receptors
Contractile/Motor	Movement	Actin, myosin
Structural	Support	Keratin, collagen

Enzymes: Protein Catalysts

Enzyme Structure and Function

Enzymes are proteins (and some RNA molecules) that act as biological catalysts, speeding up chemical reactions without being consumed.

• Active site: The region of the enzyme where the substrate binds and the reaction occurs.

• Enzyme specificity is often described by the "lock and key" model, but both the enzyme and substrate can change shape (induced fit model).

• Enzymes lower the activation energy ($E_a$) required for a reaction.

Equation for activation energy:

$E_a$

• Enzymes may require cofactors (metal ions or organic molecules) to be active. An enzyme without its cofactor is called an apoenzyme; with its cofactor, it is a holoenzyme.

• Enzymes are organized into metabolic pathways, where the product of one reaction becomes the substrate for the next.

Examples of Enzymes

• Pepsin: Breaks down proteins in the stomach by hydrolyzing peptide bonds.

• Chymotrypsin: Another digestive enzyme that targets specific peptide bonds.

Summary Table: Levels of Protein Structure

Level	Description	Bonds/Interactions
Primary	Sequence of amino acids	Peptide bonds
Secondary	Local folding (α-helix, β-sheet)	Hydrogen bonds (backbone)
Tertiary	3D shape of polypeptide	Hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges
Quaternary	Assembly of multiple polypeptides	Same as tertiary

Additional info:

• Protein misfolding can lead to diseases such as Alzheimer's, Parkinson's, and prion diseases.

• Chaperone proteins assist in proper folding and refolding of denatured proteins.

• Enzyme-catalyzed reactions are essential for metabolism and cellular regulation.