BCH 451 - Exam 3
Terms in this set (34)
What is the basic composition of a lipid?
A hydrocarbon chain attached to a polar group.
What are the 5 main functions of a Lipid?
1) Structural components or biological molecules
2) Barriers (compartmentalize metabolites, pH, metabolic pathways)
3) Energy storage molecules
5) Chemical signals (regulation) (hormones & signal transduction)
What are the three ways the aliphatic tail of a fatty acid can differ from another one?
1) Length of the hydrocarbon tail - 12-20 carbons is typical and they are always multiples of two because fatty acids are synthesized with 2 carbon units.
2) Number of Double bonds - cis/trans conformation
3) Number of branches
What is the difference between saturated and unsaturated fatty acids?
Saturated = max number of hydrogens
--> no CC double bonds
--> typically solid at room temp
Unsaturated = Fatty acids with CC double bonds
--> Monounsaturated, polyunsaturated
--> Typically liquid at room temp
--> Naturally occurring double bonds = always in cis
Describe the factors that increase/decrease melting temperature.
Increased Melting Temperature:
--> Longer chain (the Van der Waals forces are increased)
--> No double bonds (also a straighter chain)
Decreased Melting Temperature:
--> Shorter chain
--> Adding double bonds (natural double bonds are always in cis)
Describe the number nomenclature for fatty acids.
The first number is the number of carbons, followed by the number of double bonds, and the location of the double bonds.
Draw the fatty acid denoted by 14:2^6,10
What are the building blocks of a triacylglycerol? Draw its basic structure. Are they hydrophobic or hydrophilic?
Building Blocks: 3 fatty acid chains and a glycerol molecule joined by ester bonds.
What are the 4 components that make up a glycerophospholipid? Draw its basic structure.
1) Glycerol backbone
2) 2 Fatty Acids (C1 & C2)
3) Phosphate group (C3)
4) Variable X group attached to phosphate group (this and the R groups give a lot of options for variation)
Most abundant lipids in most membranes
What is the backbone of a sphingolipid? Draw the backbone and label the relevant pieces.
Backbone = sphingosine
second most abundant lipid in animal and plant membranes
--> Numerous diseases are associated with sphingolipids and are usually caused by the absence of enzymes needed to degrade sphingolipids.
--> Gaucher's, Krabbe's, Niemann-Pick, Tay-Sach's
Can you identify cholesterol, testosterone, and estradiol? Only need to be able to identify cholesterol for the exam.
Describe cholesterol's structure and behavior.
Cholesterol is made of 4 ring structures: 3 six carbon ring structures and 1 five carbon ring structure. These rings structures make the molecule rigid which plays a crucial role when part of a lipid structure. Cholesterol is very hydrophobic and posses only one small polar group. Its synthesis also acts as the precursor for the synthesis of other steroids including testosterone and estradiol which act as hormones.
Define lipoprotein. What is their function? Where are they synthesized?
Any protein covalently linked to lipid groups. Typically divided into three subgroups: chylomicrons, LDL = low density lipoproteins, and HDL = high density lipoproteins
--> refers to a phospholipid that carries various lipids within its interior
--> Responsible for transporting cholesterol and lipids in the blood stream
--> synthesized in the liver and then exported to the bloodstream to do their job
What is the function of HDL?
High density lipoproteins collect cholesterol in the blood and transport it back to the liver where it is converted to bile acids and eliminated from the liver. The cholesterol collected primarily comes from dying cells and membranes undergoing turnover.
Liver cells are the only cells to possess HDL receptors therefore HDL cholesterol specifically exists to deliver cholesterol to the liver for turnover.
What is the function of chylomicrons?
Transport triacylglycerols from intestines to tissues.
What is the function of LDL?
Low density lipoproteins transport cholesterol to peripheral tissues. The cells have membrane surface receptors that bind Apoprotein B-100 and take up the LDL particles AKA LDL particles can bind to tissue
What is the healthy ratio of HDL to LDL?
Approximately how much cholesterol is synthesized each day? How is the majority of cholesterol transported?
800 mg or 80% of the total cholesterol. Majority of cholesterol is transported as cholesterol ester.
What enzyme is the key regulatory step in cholesterol biosynthesis?
3-hydroxy-3-methylglutaryl CoA reductase (inhibited by lipitor)
What is athlerosclerosis? How does it happen? What are foam cells? Which lipoprotein is associated with heart disease?
Athlerosclerosis is a disease where atheromas (soft masses) accumulated on the inside of arteries. Atheromas = plaques and most of the cholesterol found in these plaques is delivered by cells that become foam cells. Foam cells are macrophage cells filled with cholesterol and cholesterol esters. These eventually calcify and protrude into the lumen of the artery to form blockages. High LDL is associated with heart disease.
What is a biological membrane?
A bilayer structure composed primarily of lipids and proteins, that surrounds and contains the contents of cells and organelles (sometimes viruses)
What are the 5 functions of a biological membrane?
1) Serves as a barrier separating molecules and processes.
2) Selective pumps that regulate transport of ions and biological molecules
3) Generates and maintains proton gradients
4) Site of biological rxns and pathways
5) Recognizes extracellular signals and communicates with the cell interior.
What are the six membrane features?
1) Bilayer structure - two lipid layers with hydrophobic tails on inside and polar head groups on outside of cell and inside of cell.
2) Spontaneous, non covalent assemblies: Lipid and protein components can have covalently attached groups, the actual membrane is assembled via non-covalent forces.
3) Asymmetric: Each layer have different lipid concentrations and different proteins
4) Selectively Permeable: Because interior of membrane is hydrophobic, most polar or charged molecules cannot get through. Therefore specific proteins are used to help those molecules move in and out of the cell.
5) Fluid: Lipids and proteins in membranes can move within the membrane and often protein function is dependent on this movement. Fluidity is controlled by the amount of saturated vs. unsaturated fatty acid tails and cholesterol in the membranes.
6) Self-Annealing Capability: If disrupted, membranes can spontaneously reseal. Crucial for cell vitality and in membrane endo- and exocytosis.
What forces drive lipid layer formation? What are the thermodynamic considerations?
--> Hydrophobic forces drive lipid layer formation and Van der Waals stabilize the lipid molecules in the packing of the hydrophobic tails together.
--> Entropy is increased as the H2O molecules are no longer required to be ordered around each individual fatty acid tail and thus the free energy decreases. Meaning lipid layer formation is spontaneous and stable.
Describe a micelle.
Micelles are spherical monolayers that create a hydrophobic interior (thus good for transporting hydrophobic molecules).
Micelles form at lipid concentrations above the critical micelle concentration BUT single fatty acid chains and short fatty acid chains can promote micelle formation at lower lipid concentrations because they are better at fitting in the sharp curved structure.
Describe a lipid bilayer
Lipid bilayers form at high concentrations of lipids and are often composed of glycerophospholipids and sphingolipids as these are larger multichain fatty acids and thus do not bend as easily for micelles. Lipid bilayers are impermeable to most substances.
Liposomes are a spherical bilayer with a hydrophilic interior (thus able to transport hydrophilic molecules). Liposome formation completely separates the hydrophobic tails from the polar solvent and thus minimizes delta G.
Liposomes can be formed via sonication and often transport drugs to cells. Fatty acids are suspended in a solution containing the drug and sonication forms the liposome now containing some of the drug in solution. It is then transported to the cell via cell-liposome fusion.
These are easily created in a lab setting.
Name the three types of membrane proteins and describe where they're found.
1) Peripheral proteins --> Proteins that are associated with/bound to the membrane surface.
--> Often covalently modified with carbohydrates to form glycoproteins.
2) Lipid Anchored Peripheral Proteins --> Peripheral proteins bound to the membrane surface via covalently attached lipids.
Often proteins found on/exposed to surface function as membrane receptors or enzymes
3) Integral Proteins --> Proteins positioned within the membrane. Often these span the entire membrane (but not required).
--> Typically possess hydrophobic AAs in the region buried in the membrane.
What is the fluid mosaic model?
Describes the membrane as a fluid structure where lipids and proteins can move within the bilayers.
Describe the two methods of protein/lipid movement in a lipid bilayer as well as their energy demands.
Lateral Diffusion: Diffusion sideways/laterally within one layer of the lipid bilayer. Energetically favorable.
Transverse Diffusion (Flip Flop): Diffusion of a lipid/protein from one lipid monolayer to the other monolayer. Energetically unfavorable as the polar head group of a lipid would have to pass through two layers of hydrophobic lipid tails.
Takes 10^9 times longer for transverse diffusion than to move 50 angstroms in the same monolayer for a lipid. Proteins take even longer!
What regulates membrane fluidity for bacteria? Animals?
Membrane fluidity for bacteria is regulated by chain length and degree of saturation. Short and unsaturated chains are less ordered and fluid whereas long and saturated chains are rigid and more ordered.
Animals regulate membrane fluidity using cholesterol. Cholesterol's 4 rings form a hydrophobic, rigid, flat plane lipid that inserts itself among the hydrophobic tails (intercalates) with the polar OH group with the polar head groups of the lipids. At high temperatures cholesterol decreases fluidity to help prevent melting and at low temperatures cholesterol increases fluidity to allow movement within the membrane. In this way cholesterol acts much like a buffer.
Describe the types of passive diffusion. What is the difference between passive diffusion in bacteria and animals?
Two types of passive diffusion:
1) Passive-Simple Diffusion: Transport through membrane via simple diffusion through a "hole"
Pores in bacteria and channels in animals
Rate is determined by the concentration gradients and is a non saturable transport system (linear graph).
2) Facilitated Diffusion: Transport facilitated by binding a transporter protein in the membrane - much faster due to this binding. Rate of facilitated diffusion is saturable as eventually all transporter proteins will be in use so rate will reach its limit.
Describe the passive-simple diffusion transport example outlined in the notes.
Aquaporins are widely conserved (found in plants AND animals) water transporters with six alpha helices and 2 loops that function as tetramers. Each monomer (of which there are four) forms a channel that allows H2O through.
Describe the facilitated diffusion example outlined in the notes.
Lactose permease in e coli bacteria is an integral protein with a hydrophilic channel that has a lactose binding site. The enzyme has an inward and outward facing conformation where the outward facing conformation is open to the cell exterior, allowing for lactose to bind. Once lactose has bound, lactase permease undergoes a conformational change to the inward facing conformation where it is open to the interior of the cell and able to release the lactose.