Protein Structure and Function: Cell Biology Study Notes

Protein Structure and Function

Introduction to Proteins

Proteins are essential macromolecules in all living cells, performing a vast array of functions due to their diverse structures. Understanding protein structure and function is fundamental for cell biology, as proteins are involved in catalysis, transport, regulation, and structural support.

• Proteins are polymers made of amino acids.

• They exhibit a wide variety of shapes and sizes, which are closely linked to their functions.

Classification of Proteins by Function

Proteins can be classified into several functional groups based on their roles in the cell:

• Enzymes: Catalyze biochemical reactions.

• Structural proteins: Provide support and shape to cells and tissues (e.g., collagen).

• Transport proteins: Move molecules across membranes (e.g., hemoglobin).

• Motor proteins: Enable movement (e.g., myosin).

• Receptors: Receive and transmit signals.

• Regulatory proteins: Control gene expression and cellular processes.

Overview Table: Protein Functions and Examples

Function	Example	Description
Enzyme	Hexokinase	Catalyzes glucose phosphorylation
Structural	Collagen	Provides strength to connective tissues
Transport	Hemoglobin	Transports oxygen in blood
Motor	Myosin	Facilitates muscle contraction
Regulatory	p53	Regulates cell cycle and apoptosis

Amino Acids: Building Blocks of Proteins

Structure of Amino Acids

Amino acids are organic molecules that serve as the monomers of proteins. Each amino acid has a central carbon (α-carbon) bonded to:

• An amino group (NH2)

• A carboxyl group (COOH)

• A hydrogen atom

• A unique side chain (R group) that determines its properties

General structure:

• $\mathrm{NH_2{-}CHR{-}COOH}$

At physiological pH (~7.4), amino acids exist as zwitterions, with the amino group protonated and the carboxyl group deprotonated:

• $\mathrm{H_3N^+-CHR-COO^-}$

Functional Groups in Amino Acids

The side chains (R groups) of amino acids contain seven major functional groups important in biochemistry:

• Hydroxyl (-OH)

• Sulfhydryl (-SH)

• Carboxyl (-COOH)

• Amino (-NH2)

• Phosphate (-PO4)

• Carbonyl (C=O)

• Methyl (-CH3)

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains:

• Nonpolar (hydrophobic): e.g., alanine, valine, leucine

• Polar (hydrophilic): e.g., serine, threonine, asparagine

• Electrically charged:

  • Positively charged (basic): lysine, arginine, histidine

  • Negatively charged (acidic): aspartic acid, glutamic acid

Special Amino Acids

Certain amino acids have unique properties:

• Cysteine: Contains a sulfhydryl group (-SH) that can form disulfide bonds ($\mathrm{S{-}S}$), stabilizing protein structure.

• Methionine: Contains a thioether group; often the first amino acid in protein synthesis.

• Serine, threonine, tyrosine: Can be phosphorylated, affecting protein activity and signaling.

Amino Acid Codes

Amino acids are represented by three-letter and one-letter codes for convenience in protein sequence notation.

Amino Acid	Three-letter Code	One-letter Code	Side Chain Property
Alanine	Ala	A	Nonpolar
Serine	Ser	S	Polar
Glutamic acid	Glu	E	Negatively charged
Lysine	Lys	K	Positively charged
Cysteine	Cys	C	Special (disulfide bonds)

Protein Structure: Levels of Organization

Primary Structure

The primary structure of a protein is its linear sequence of amino acids, linked by covalent peptide bonds. This sequence determines the protein's final shape and function.

• Peptide bond formation: $\mathrm{COOH}$ of one amino acid reacts with $\mathrm{NH_2}$ of another, releasing water ($\mathrm{H_2O}$).

• $\mathrm{Peptide~bond:~CONH}$

• Proteins typically contain 30–2000 amino acids.

Secondary Structure

Secondary structure refers to regular local folding patterns of the polypeptide chain, stabilized by hydrogen bonds.

• α-helix: Right-handed coil stabilized by H-bonds between every fourth amino acid.

• β-sheet: Sheet-like structure formed by H-bonds between segments of polypeptide chains running parallel or antiparallel.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, resulting from interactions among side chains.

• Stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Example: The Ras protein, a GTP-binding protein involved in cell proliferation.

Quaternary Structure

Quaternary structure arises when two or more polypeptide chains (subunits) assemble into a functional protein complex.

• Example: Hemoglobin, composed of four polypeptide subunits.

Chemical Bonds Stabilizing Protein Structure

• Hydrogen bonds: Between backbone atoms and side chains.

• Ionic bonds: Between charged side chains.

• Hydrophobic interactions: Nonpolar side chains cluster away from water.

• Disulfide bonds: Covalent bonds between cysteine residues.

Protein Sequence vs. Composition

The function of a protein depends not only on the types of amino acids present but also on their specific sequence. Different arrangements of the same amino acids can result in proteins with distinct properties and functions.

• Sequence: Order of amino acids (e.g., Met-Asp-Tyr-Leu)

• Composition: Types and numbers of amino acids present

Summary Table: Levels of Protein Structure

Level	Description	Stabilizing Bonds	Example
Primary	Linear sequence of amino acids	Peptide bonds	Insulin
Secondary	Local folding (α-helix, β-sheet)	Hydrogen bonds	Keratin (α-helix)
Tertiary	Three-dimensional shape	Hydrogen, ionic, hydrophobic, disulfide bonds	Myoglobin
Quaternary	Assembly of multiple polypeptides	Same as tertiary	Hemoglobin

Key Equations and Concepts

• Peptide bond formation: $\mathrm{Amino~acid_1~+~Amino~acid_2~\rightarrow~Dipeptide~+~H_2O}$

• Zwitterion at neutral pH: $\mathrm{H_3N^+-CHR-COO^-}$

Examples and Applications

• Enzyme catalysis: Hexokinase initiates glycolysis by phosphorylating glucose.

• Structural support: Collagen provides tensile strength to skin and bones.

• Transport: Hemoglobin carries oxygen from lungs to tissues.

• Regulation: p53 protein controls cell division and prevents tumor formation.

Additional info: The notes expand on the slide content with definitions, examples, and tables for clarity and completeness, suitable for college-level cell biology study.