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Primary components of biological membranes
Amphipathic lipids and proteins
Describe amphipathic lipids
- 1. principal feature on lipid membrane
- 2. has a polar hydrophilic head and an uncharged hydrophobic tail
3 main categories of amphipathic lipids
- 1. phosphoglycerides
- 2. sphingolipids
- 3. sterols
Describe the hydrophobic tail of a phosphoglyceride
- a. 2 fatty acids are attached via ester linkage to C1 and C2 carbons of glycerol-3-phosphate
- b. long allopathic hydrophobic tails composed of 16-20 carbons
- c. shorter fatty acid chains tend to be saturated while longer tend to be saturated
Describe the hydrophilic head of a phosphoglyceride
Phosphate attached to the C3 of the glycerol molecule can form a second ester bond. Phosphoglycerides are named after the constituent that is being attached.
- 1.phosphoglyceride with no substituent attached to the phosphate group.
- 2. intermediate in the synthesis of other phosphoglycerides.
- 3.generated when other phosphoglycerides are degraded with phospholipase D.
- 1.choline attached to the phosphate group
- 2. Choline's + charge balances phosphate's - charge. Net charge is zero/
ethanolamine attached. Has no net charge
serine attached. since serine is a zwitterion, the overall charge of phosphatydilserine is (-)
- 1. Glycerol attachment
- 2. carries a negative charge
- 3. Important constituent of pulmonary surfactant which is needed for lung function.
- An important phosphoglyceride in the mitochondrial inner membrane.
- Attached to phosphatydilglycerol
- has inositol attached.
- Can be phosphorylated and serve as signaling proteins and also serves as substrates for the production of secondary messengers.
- Constructed around sphingosine.
- Usually also have 2 hydrophobic alipathic tails and a hydrophilic head.
- Sphingosine is formed by joining the carboxyl end of palmitate to the alpha carbon of serine
Describe the hydrophobic head of sphingolipids
- analogous to glycerol backbone of phosphoglycerides.
- one tail is allopathic chain of sphingosine.
- the other tail is a ling fatty acid chain via an amide linkage to the amino group of sphingosine.
- this is also called ceramide
Describe the hydrophilic head of sphingolipids
- Ceramide has a free hydroxyl group in which polar head groups can be attached.
- also named in the basis of their head group
ceramide with attached choline and phosphate group
Glycosphingolipids or sphingoglycolipids
- carbohydrates attached to free hydroxyl of ceramide
- carbohydrate can be highly variable ranging from small to multi branched
- Cerebrosides contain a single carb residue
- Gangliosides are more complex and contain sialic acid
- Glycosphingolipids commonly found in external leaflet of the plasma membrane
- distinct structure
- hydrophilic head is consist of a single OH group which can snuggle up with the phosphate groups of other membrane lipids
- less flexible steroid ring
- increases rigidity of the lipid bilayer
- decreases temp at which bio membrane will solidify
Structure of lipid bilayer
- 2 layers ( leaflets): phobic in, philic out
- usually form closed structures
- a barrier to transverse diffusion of hydrophilic substances
- membrane lipids need help to flip sides
- can laterally diffuse
describe membrane flipping
- Flipping is enzymatically controlled:a diverse family of flippases which have specific actions
- some membranes do not flip: too large to be flipped or does not contain flippases
- 2 leaflets usually have different composition: outer vs inner leaflet. example: outer leaflet composition has signaling molecules
Membrane Protein classification in relation to interaction with lipid bilayer
- Integral membrane proteins
- peripheral membrane proteins
- transmembrane proteins
- lipid anchors
- peripheral membrane proteins
Integral VS peripheral membrane protein
- Integral: contains a hydrophobic region allowing it to embed into the lipid bilayer.
- Peripheral: does not extend into the lipid bilayer
- Most integral proteins are transmembrane
- has an extracellular domain, transmembrane domain and cytoplasmic domain
- Transmembrane domain: non polar hydrophobic region, alpha helix is suited for this. can be single pass or multiples
- Cytoplasmic or extracellular domain: resemble soluble proteins and is tethered to the transmembrane domain
- Glycosylation or glycocalyx: has glycoprotein attachment, and the carbohydrate is attached to the extracellular domain of the protein
membrane proteins are covalently linked to lipids instead of attachment to hydrophobic portion of a protein.
peripheral membrane proteins
- soluble proteins that are attached through interaction with the cytosolic or extracellular domains of integral proteins or with the hydrophilic head groups of membrane lipids.
- act in concert with integral membrane proteins for signaling to structural functions.
Functions of membrane proteins
- Transport proteins: subdivided into channels, carrier proteins, and pumps
- Signaling proteins: receptors. serve to detect signals in the cell's environment by binding to specific molecules in the extracellular. cause conformational changes allow cells to adapt.
- Structural/and anchoring proteins: integral and peripheral proteins carry this function.
- Can have multiple functions
Describe structural/anchoring proteins
- Integral membrane proteins serve to attach cells to their surroundings: extracellular domain bind to macromolecules in the EC space while the cytoplasmic domain bind to cytoskeleton.
- Peripheral membrane protein or membrane cytoskeleton: cytoplasmic surface has a meshwork of peripheral membrane proteins which provide structural support for membrane. Key protein is spectrin, an elongated molecule that binds to a complex of other proteins and attaches this complex to a network of other peripheral membrane proteins.
- Dystrophin: a spectrum like molecule in which a mutation can cause muscular dystrophy
- Ankyrin: Anchors spectrum network to plasma membrane
- Hydrophilic tails of membrane lipids can have longer chains.
- longer tails have tendency to stick together.
- these tails are enriched in sphingolipids and cholesterol
- membrane proteins also have different lengths and it is energetically favorable to match the length of transmembrane region of the protein to the length of the hydrophobic core of the lipids, some membrane proteins show preference to be in these rafts. Other proteins are excluded for these rafts.
- Can serve as platforms for specific signaling pathways