Liposomes are spherical structures where phospholipid head groups point towards water both outside and inside, creating a stable formation.
They are not typically found in biology but are used in experiments.
RNA vaccines use liposomes to deliver RNA to cells, which then produce viral proteins and mount an immune response.
Micelles vs. Liposomes
Micelles consist of a single layer of phospholipids with tails pointed inward.
They are unstable because water molecules can disrupt the head groups causing the structure to fall apart.
Liposomes have a bilayer structure, with head groups facing water on both sides.
Liposome Size and Curvature
Liposomes range from 50 to 500 nanometers.
Typical cells are around 10 micrometers (10,000 nanometers) in diameter.
Small structures have high curvature.
As liposomes grow, they form planar bilayers with minimal curvature from one phospholipid to the next.
Analogy to Earth's Curvature
Each phospholipid head group is approximately one nanometer.
A cell with a 10-micrometer diameter has a circumference of about 30,000 nanometers.
The Earth's equator is about 24,500 miles; locally it appears flat, similar to how a large planar bilayer appears flat despite its curvature.
Lipid Mosaic Model of the Membrane
Biological membranes consist of a planar bilayer of phospholipids punctuated by transmembrane proteins.
This arrangement is known as the lipid mosaic model.
Proteins are polymers of amino acids linked by peptide bonds.
Basic Amino Acid Structure
All amino acids have a common structure:
An amine group (nitrogen and two hydrogens).
A central carbon atom.
An R group that varies among the 20 different amino acids.
A carboxylic acid group (carbon with a double-bonded oxygen and an OH).
Peptide Bond Formation
Peptide bonds link amino acids together through a dehydration synthesis.
The hydroxyl group from the carboxylic acid of one amino acid combines with a hydrogen from the amine group of another amino acid to form water (H2O).
This generates a covalent bond between the nitrogen of one amino acid and the carbonyl carbon of the adjacent amino acid.
Primary Structure of Proteins
The primary structure is the sequence of amino acids in a polypeptide chain.
Proteins can have several hundred amino acids.
The amino terminus is one end of the molecule, and the carboxy terminus is the other end.
Orientation of Amino Acid Sequence
Proteins have an orientation due to the way amino acids are linked.
One end is the amino terminus, and the other is the carboxy terminus.
This nomenclature is critical for understanding protein structure.
Secondary Structure of Proteins
It results from hydrogen bonding within the protein.
Two main forms: alpha helix and beta pleated sheet.
Alpha Helix
Alpha-helical regions are often short.
Essential for proteins crossing the phospholipid bilayer.
The helix is stabilized by hydrogen bonds between amino hydrogens and carbonyl oxygens.
The pattern for hydrogen bonding is I+4, where the carbonyl oxygen of amino acid i interacts with the hydrogen on amino acid i+4.
Analogy: spiral staircase where each step (amino acid) twists by approximately 100 degrees (3.6 amino acids per full turn) and rises by 0.15 nanometers.
Beta Pleated Sheet
The other form of secondary structure, involving hydrogen bonds between adjacent strands.
Can be arranged in parallel or antiparallel orientations.
Antiparallel arrangements have a repeating ROH pattern between strands.
Parallel arrangements have offset hydrogen bonds that are somewhat weaker due to the twist.
Common Amino Acids in Alpha Helices
m a l e k (Methionine, Alanine, Leucine, Glutamic acid, Lysine) are frequently found.
m a r k l (Methionine, Alanine, Arginine, Lysine, and Leucine) and are pretty similar.
Glycine and proline are typically avoided due to their tendency to introduce kinks.
Role of R Groups in Secondary Structures
R groups aren't required, but some are favored over others.
Hydrogen bonding between the amine hydrogen and carbonyl oxygen drives the structure.
Protein Structure Representation
Flat ribbon-like structures indicate beta sheets, with arrows showing direction (antiparallel or parallel).
Helices are often shown as twisted coils.
Proteins in aqueous environments tend to hide alpha helices between beta sheets to protect them from water's dipole moments.
Tertiary structure
Involves the three-dimensional shape of the protein.
Requires R groups, which are divided into those that are hydrophilic and hydrophobic.
Amino Acid Properties
Amino acids are classified as nonpolar (hydrophobic), polar, or charged (hydrophilic).
Amino Acid Structures
Glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, and methionine are nonpolar.
Serine, threonine, cysteine, tyrosine, asparagine, and glutamine are polar.
Aspartate, glutamate, lysine, arginine, and histidine are charged.
Three Important Amino Acids
Serine, threonine, and tyrosine have hydroxyl groups and can be phosphorylated.
They're important in G protein-coupled receptors and kinases.
Glutamate is a common neurotransmitter in the nervous system.
Lysine and arginine have long side chains and are charged.
Histidine binds to metals.
Protein Folding
In aqueous environments, hydrophilic R groups point toward water, and hydrophobic R groups point inward, away from water.
Importance of Three-dimentional Shape
The three-dimensional shape of a protein determines its function.
It influences how antigens bind to antibodies and how neurotransmitters bind to receptors.
Representation of Protein Structure
Hydrophilic R groups (blue gumdrops) point outward, and hydrophobic R groups (white gumdrops) are folded inward.
This folding minimizes the interaction between hydrophobic residues and water.
Binding Pockets
The three-dimensional shape of an enzyme creates binding pockets for substrates.
The shape and charge of the binding pocket determine how substrates fit.
Myosin Example
Myosin is an enzyme that converts ATP to ADP plus inorganic phosphate.
It has a long tail and a head with an ATP-binding pocket.
ATP Binding
ATP has a ribose sugar, adenine, and a triphosphate group.
The triphosphate group is heavily charged.
For ATP to bind, it must match the charge and shape of the binding pocket.
Myosin cleaves ATP into ADP and inorganic phosphate in the binding pocket.
When the myosin head binds to actin, the ADP and inorganic phosphate leave, causing a structural change and movement.
Energy Conversion
Myosin converts chemical energy in ATP into mechanical energy in movement.
Charge changes in the binding pocket drive the movement.
The three-dimensional shape of the myosin head and its interaction with actin are crucial for this process.