Protein Structure and Function: Biochemistry Study Notes

Protein Functions in the Human Body

Overview of Protein Functions

Proteins are essential macromolecules responsible for a wide variety of functions in biological systems. Their diverse roles are determined by their unique structures, which are encoded by genetic information.

• Enzymatic Activity: Proteins act as enzymes, catalyzing biochemical reactions. Example: Proteases, catalyzing the breakdown of proteins.

• Structural Support: Proteins provide structural integrity to cells and tissues. Example: Collagen in connective tissues.

• Transport: Proteins transport molecules across cell membranes or within the bloodstream. Example: Transmembrane receptors.

• Signaling: Proteins are involved in cell signaling and communication. Example: Insulin regulates glucose levels.

• Defense: Proteins protect the body from pathogens. Example: Antibodies in the immune system.

• Storage: Proteins store amino acids or other substances for later use. Example: Ferritin stores iron.

• Movement: Proteins facilitate movement of cells and muscles. Example: Actin and myosin in muscle contraction.

Protein Structure: Levels and Types

Overview of Protein Structure

Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. Each level contributes to the protein's final shape and function.

• Primary Structure: The linear sequence of amino acids determined by DNA. Key Point: The sequence is encoded by the genetic code and dictates the protein's properties.

• Secondary Structure: Localized folding patterns such as alpha helices and beta sheets, stabilized by hydrogen bonds.

• Tertiary Structure: The overall 3D structure of a single polypeptide chain, stabilized by hydrophobic interactions, ionic bonds, disulfide bridges, and hydrogen bonds.

• Quaternary Structure: Assembly of multiple polypeptide chains into a functional protein complex.

Primary Structure of Proteins

Genetic Coding and Amino Acid Sequence

The primary structure of a protein is the specific sequence of amino acids, determined by the genetic code in DNA. This sequence is transcribed into mRNA and translated by ribosomes, which link amino acids together via peptide bonds.

• Codons: Triplets of nucleotides in mRNA specify particular amino acids.

• Peptide Bonds: Covalent bonds formed between the carboxyl group of one amino acid and the amino group of another.

Peptide Bond Formation and Structure

Chemical Structure and Amide Definition

Peptide bonds are formed by a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing water and forming an amide linkage.

• Amide: A nitrogen linked to a carbonyl resulting from a carboxylic acid and amine.

Peptide Bond Formation Equation:

$\text{Carboxylic Acid} + \text{Amine} \rightarrow \text{Amide} + H_2O$

Resonance and Geometry of Peptide Bonds

Peptide bonds exhibit resonance, resulting in partial double-bond character and restricted rotation. This leads to a planar geometry, typically in the trans configuration.

• Resonance Structure: The electrons are delocalized between the C=O and C-N bonds.

• Geometry: Peptide bonds are usually trans due to steric hindrance.

Resonance Structure Equation:

$\ce{O=C-NH}$ \leftrightarrow $\ce{O^{-}-C=N^{+}H}$

Secondary Structure of Proteins

Alpha Helix and Beta Sheet Structures

Secondary structures are stabilized by hydrogen bonding between backbone atoms. The two main types are alpha helices and beta sheets.

• Alpha Helix: Right-handed coil stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen four residues ahead.

• Beta Sheet: Extended strands connected by hydrogen bonds between backbone atoms of adjacent strands. Can be parallel or antiparallel.

Stabilizing Forces:

• Hydrogen bonds (backbone-backbone)

• Side chain interactions (in non-repetitive structures)

Alpha Helix in Membrane-Spanning Proteins

Alpha helices are ideal for membrane-spanning regions due to their hydrophobic side chains and internal hydrogen bonding. Proline often disrupts helices due to its rigid structure, acting as a helix breaker.

• Hydrophobic Interactions: Favorable for spanning lipid bilayers.

• Proline: Introduces kinks and breaks in the helix.

Tertiary and Quaternary Structure

Tertiary Structure

The tertiary structure is the overall 3D shape of a single polypeptide, stabilized by various interactions:

• Hydrophobic interactions

• Hydrogen bonds

• Ionic bonds

• Disulfide bridges

• Van der Waals interactions

Functional Domains:

• Extracellular Domain: Binds ligands outside the cell.

• Transmembrane Domain: Spans the lipid bilayer.

• Intracellular Domain: Interacts with signaling molecules inside the cell.

Quaternary Structure

Quaternary structure involves the assembly of multiple polypeptide chains into a functional protein complex. Examples include hemoglobin and antibody molecules.

Comparison of Protein Structure Levels

Summary Table: Protein Structure Levels

Level	Description	Stabilizing Forces	Example
Primary	Linear sequence of amino acids	Peptide bonds	Insulin
Secondary	Local folding (alpha helix, beta sheet)	Hydrogen bonds	Keratin (alpha helix)
Tertiary	3D structure of single polypeptide	Hydrophobic, ionic, hydrogen bonds, disulfide bridges	Myoglobin
Quaternary	Assembly of multiple polypeptides	Same as tertiary, plus subunit interactions	Hemoglobin

Key Terms and Definitions

• Amino Acid: Organic molecule with amino and carboxyl groups; building block of proteins.

• Peptide Bond: Covalent bond linking amino acids in a protein.

• Alpha Helix: Spiral secondary structure stabilized by hydrogen bonds.

• Beta Sheet: Sheet-like secondary structure stabilized by hydrogen bonds.

• Quaternary Structure: Protein structure formed by multiple polypeptide chains.

Examples and Applications

• Enzyme Catalysis: Proteases, kinases, and polymerases are proteins that catalyze essential biochemical reactions.

• Structural Proteins: Collagen provides tensile strength to connective tissues.

• Transport Proteins: Hemoglobin transports oxygen in the blood.

• Signaling Proteins: Insulin regulates blood glucose levels.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and some examples have been inferred from standard biochemistry knowledge.