Protein Structure and Function

Levels of Protein Structure

Proteins are complex macromolecules essential for cellular structure and function. Their structure is organized into four hierarchical levels:

• Primary Structure: The unique sequence of amino acids in a polypeptide chain, held together by peptide bonds.

• Secondary Structure: Local folding of the polypeptide chain into structures such as α-helices and β-pleated sheets, stabilized by hydrogen bonds.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, formed by interactions among R-groups (side chains), including hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.

• Quaternary Structure: The association of two or more polypeptide chains into a functional protein complex.

Example: Hemoglobin is a protein with quaternary structure, consisting of four polypeptide subunits.

Cell Membranes and Transport

Fluid Mosaic Model of the Membrane

The fluid mosaic model describes the structure of cell membranes as a mosaic of various proteins floating in or on a fluid lipid bilayer.

• Phospholipid Bilayer: Forms the fundamental structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded within or attached to the bilayer, serving functions such as transport, signaling, and structural support.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, important for cell recognition.

Additional info: The fluidity of the membrane allows for lateral movement of components, essential for membrane function.

Membrane Fluidity

Membrane fluidity refers to the viscosity of the lipid bilayer, affecting the movement of proteins and lipids within the membrane.

• Factors Affecting Fluidity:

  • Fatty acid composition (unsaturated vs. saturated)

  • Cholesterol content (in animal cells)

  • Temperature

• Biological Importance: Proper fluidity is crucial for membrane function, including transport and cell signaling.

Membrane Proteins

Membrane proteins are integral or peripheral proteins associated with the cell membrane, each with specific roles:

• Integral Proteins: Span the membrane and are involved in transport and signal transduction.

• Peripheral Proteins: Loosely attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton.

Selective Permeability and Membrane Proteins

Cell membranes are selectively permeable, allowing some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) diffuse freely.

• Polar or charged molecules require transport proteins (channels or carriers).

• Membrane proteins facilitate the movement of specific substances across the membrane.

Passive and Active Transport

Transport across membranes can be passive or active:

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple diffusion: Direct movement through the lipid bilayer.

  • Facilitated diffusion: Movement via transport proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

  • Example: Sodium-potassium pump ( out, in per ATP hydrolyzed).

Bulk Transport

Bulk transport involves the movement of large particles or volumes of fluid across the membrane via vesicles.

• Endocytosis: Uptake of materials into the cell by vesicle formation.

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of extracellular fluid.

  • Receptor-mediated endocytosis: Specific uptake via receptor proteins.

• Exocytosis: Release of materials from the cell by vesicle fusion with the membrane.

Cell Types and Organelles

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on structural differences.

Surface Area:Volume Ratios

The ratio of surface area to volume is a key factor in cell size and function.

• Smaller cells have a higher surface area:volume ratio, facilitating efficient exchange of materials with the environment.

• Larger cells may have difficulty transporting materials quickly enough to support cellular processes.

• Formula: For a sphere, ,

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Synthesis and transport of proteins and lipids, detoxification, and waste processing.

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles involved in energy conversion.

• Mitochondria: Site of cellular respiration, converting glucose and oxygen into ATP.

• Chloroplasts: Site of photosynthesis in plants and algae, converting light energy into chemical energy.

• Both contain their own DNA and ribosomes, supporting the endosymbiotic theory.

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, cell shape, and facilitates movement.

• Microfilaments: Composed of actin, involved in cell movement and shape.

• Intermediate filaments: Provide mechanical support.

• Microtubules: Hollow tubes made of tubulin, important for cell division and intracellular transport.

Extracellular Components and Cell Junctions

Cells interact with their environment and each other through extracellular components and specialized junctions.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells, providing support and signaling.

• Cell Junctions:

  • Tight junctions: Prevent leakage between cells.

  • Desmosomes: Anchor cells together.

  • Gap junctions: Allow communication between animal cells.

  • Plasmodesmata: Channels between plant cells for transport and communication.