BS332: Biomembranes and Bioenergetics

BS332: Biomembranes and Bioenergetics

• The course consists of 17 lectures delivered by Professor Vass Bavro and Dr Dima Svistunenko
• A 2-hour revision workshop is scheduled
• Assessment is 100% based on a 3-hour summer exam scheduled in week 33-36
• Course materials are available on Moodle at https://moodle.essex.ac.uk/course/view.php?id=12353
• Recommended reading materials include "Bioenergetics 4" by David G. Nicholls and Stuart J. Ferguson, "Molecular Biology of the Cell" (various editions), "Brock Biology of Microorganisms", and "Atkins' Physical Chemistry"
• The course covers topics including membrane structure, lipid types, membrane protein function, thermodynamics, energy, and membrane transport.

Today's Lecture Topics

• Introductions to biomembranes and lipids
• Organization of prokaryotic and eukaryotic membranes
• Membrane components and their functions
• Membrane asymmetry
• Organization of bacterial and eukaryotic membranes
• Organelles and endosymbiosis theory

Learning Outcomes

• Students will be able to describe membrane components (proteins, polysaccharides, and lipids) and their functions
• Students will understand how proteins and lipids move within membranes
• Students will explain why membranes are fluid, membrane asymmetry, and its maintenance
• Students will describe the differences between eukaryotic and prokaryotic membranes

What are Membranes?

• Complex entities of lipids and proteins
• Some membranes contain water and sugar
• Fluid dynamic structures that comprise 50% of animal membrane mass

Why have Membranes?

• Permeability barriers maintaining cellular compartments
• Essential for biochemical reactions
• Allow the buildup of gradients (ions, protons, charge, and metabolites) enabling energy conversion
• Surround organelles providing specialized functions and compartmentalization
• Provide higher reagent concentration in enzymatic reactions for more efficient reactions

How Lipids Pack into Aqueous Environments

• Cone-shaped lipids form micelles
• Cylindrical phospholipids form bilayers
• Hydrophilic moieties interact with solution; hydrophobic tails bury within

The Bilayer

• Bilayer structure based on amphipathic phospholipids first proposed by Gorter and Grendel (1925)
• They observed that lipid extracted from erythrocytes has twice the surface area of the erythrocytes, suggesting a bilayer structure
• Danielli and Davson (1935) introduced the idea of proteins in membranes with their "trilamellar sandwich model"
• The fluid mosaic model of Singer and Nicolson (1972) provides the current understanding of membrane structure

Lipid plus Protein = Biomembrane

• Membrane proteins act as passive diffusion gates (controlled or not), ion channels, active transporters, and energy generators
• Proteins crucial for signaling, immune response, and defensive mechanisms

Basic Building Blocks - Lipids

• Lipids such as phospholipids and sphingolipids are crucial components for membrane structure and function

Diversity of Membrane Lipids

• Cells contain over 1000 different lipid species in eukaryotic cells including: glycerides, sphingolipids, phospholipids, cholesterol derivatives, and glycolipids

Main Membrane-Forming Lipids

• Phospholipids, including phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, and others
• Glycolipids, including glucosylcerebrosides
• These phospholipids and glycolipids are crucial components of cellular membranes

The Phosphoglyceride Molecule

• A phospholipid (or glycerophospholipid)
• Has a glycerol backbone and two fatty acid tails
• Has a hydrophilic head group and hydrophobic tails
• Has different types of head groups like choline
• Saturated or unsaturated fatty acid chains have different kinds of impact on membrane fluidity

Main Lipids of Mammalian Membrane

• Some lipids can be charged (e.g., phosphatidylserine, pH-dependent negative charge)

Cholesterol Affects Diffusion

• Up to 1 molecule of cholesterol per phospholipid in eukaryotes
• Cholesterol interacts with the polar head groups and hydrocarbon chains of phospholipids, affecting membrane fluidity and permeability

Cholesterol in a Lipid Bilayer

• Cholesterol does not form a bilayer by itself
• Decreases the mobility of hydrocarbon chains in the phospholipids, enhancing membrane rigidity and reducing permeability
• Important in eukaryotic membranes at high concentrations

Membrane Asymmetry

• Phosphatidylserine (PS) is primarily found on the inner membrane leaflet
• Inositol phospholipids (PI) are concentrated in the inner membrane leaflet
• Glycolipids are primarily found on the outer membrane leaflet
• Inositol phospholipids involved in signaling

Asymmetry in Lipids is Functionally Significant

• Phosphatidylserine (PS) is a marker for cell status, important for coagulation cascade, and cell signaling.
• PS distribution used to distinguish living from dead/dying cells.

Phospholipid Translocators

• ER membranes are symmetric and randomized, with scramblase catalyzing flipping of phospholipids
• Plasma membranes (asymmetric), with flippase catalyzing specific phospholipids to the cytosolic monolayer.

Asymmetry in Lipids is Functionally Significant-Phosphatidyl Inositol Signaling

• A signal results in phosphorylation of phosphatidylinositol (PI)
• This recruits signaling molecules, resulting in downstream activation.

Membrane Asymmetry - Why is it Important?

• Unequal distribution of lipid components is a result of active transport
• Death program activation triggers the release of factors from the mitochondria

Asymmetry is not only across leaflets - lateral asymmetry creates mosaic of microdomains!

• Demonstration of lateral heterogeneity - Fluorescence Recovery After Photobleaching (FRAP)

Basic Organization of Eukaryotic vs Prokaryotic Membranes

• Eukaryotic membranes are more complex and diverse in lipid composition than prokaryotic membranes.

Membranes in the Eukaryotic Cell

• Eukaryotic cells contain internal membranes delimiting organelles (e.g., mitochondria, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes)

Eukaryotic Membranes

• Lipid bilayers are the basic structure, with embedded proteins
• Contain 50% lipids and 50% proteins by mass
• Phospholipids, sphingolipids, glycerophospholipids (contain glycerol) are primary types

Critical Differences Between Bacterial and Eukaryotic Membranes

• Eukaryotic membranes contain a wide variety of phospholipids, while bacteria usually have fewer types.
• Bacteria can contain other types of lipids including ornithine lipids, sulfolipids, and hopanoids.
• Eukaryotic membranes do not contain LPS (lipopolysaccharides) or peptidoglycan.
• Bacterial cells lack sterols, instead containing hopanoids which support membrane structure

The Bacterial Cell Envelope

• Gram-positive and Gram-negative bacteria have distinct cell envelope structures
• Gram-positive cell walls have a thick peptidoglycan layer; Gram-negative walls have a thin peptidoglycan layer and an outer membrane containing LPS

The Gram-negative Cell Envelope

• OM (outer membrane) lipoproteins and proteins (e.g. porins, B-barrels)
• OM outer leaflet is lipopolysaccharides (LPS)
• This is important for bacterial defense mechanisms, and virulence factors

Summary of Membrane Organizations

• Different types of membranes of bacteria (Gram+, Gram-), mycobacteria, and fungi

Micobacteria Membranes

• Microbial membranes have complex organization, which is still being researched

Origin of Eukaryotic Organelles

• Endosymbiotic theory explains the origin of mitochondria and chloroplasts
• Mitochondria and plastids likely originated from bacteria that were engulfed and adopted into eukaryotic cells

Endosymbiosis - Mitochondria and Chloroplasts

• Diagrams showing the stages of endosymbiosis

Summary

• Biological membranes are lipid bilayers with embedded proteins, dynamic and semi-fluid structures.
• Membrane lipid composition varies among eukaryotes (phospholipids, cholesterol, glycolipids) and prokaryotes (variations on phospholipids and hopanoids).
• Lipid asymmetry is important for membrane function and regulation
• Prokaryotic cells generally lack membrane-bound organelles.