Biochem Lipids and Carbohydrates Flashcards
Membranes
serve as a boundary for the cell separating the inside of the cell from the exterior environment
Sequester nutrients, fuels
Exclude toxins
Composed of lipids, proteins and a variety of small molecules
Primary cell membrane is the plasma membrane
Eukaryotes have membrane-enclosed organelles within cells, prokaryotes do not
Fatty Acids
Polar “head”: oxygen atoms
Non-polar “tail” : hydrocarbon
Note shape of chain: depends on double bonds
Saturated = no double bonds
Unsaturated = double bonds
More “kinks” are better. Chains don’t pack as closely so the chains are more “fluid-like”
Triacylglycerol (TG): Fatty Acid Storage
storage mechanism for fatty acids: tightly packed, low water content
Highly hydrophobic, excludes water
fatty acids cleaved at acyl group by lipases
results in free fatty acids and either diacyl- or monoacylglycerol, or glycerol
Clinical Relevance of Lipids and Membranes
Fatty acid / lipid metabolism is fundamental to cell health and development.
- Membranes allow cells to live.
Glycerophospholipids and sphingolipids are key components of membranes required for cell function.
Disorders in lipid metabolism fundamental to many metabolic disorders including obesity, energy status and diabetic ketoacidosis.
Improper membrane structure and/or function is the basis of many pathological states, including multiple sclerosis, respiratory distress syndrome in newborns and sphingolipidoses.
Properties of Fatty Acids
16- and 18-carbon chains most common in humans, but 2-3 carbons important metabolically
Saturated (no double bonds in chain)
Pack tightly, less fluid-like, high melting point, can be solid at body temperatures
Unsaturated (one or more double bonds)
Typically 1 to 4 double bonds: monounsaturated = 1 double bond, polyunsaturated = more than 1 double bond
Cis configuration predominates
Disrupts packing of chains: “kinks” prevent the regular packing of long linear chains.
Decreases melting point of chains: allow them to be fluid-like at body temperature
More fluid like is good overall, but saturated fats important too…help “stiffen” lipid bilayers to just the right amount of “fluidity”
Trans chains in unsaturated fatty acids are bad
More like saturated chains, less fluid, more packed
Common Fatty Acids and their Functions
arachadonic acid is a key fatty acid in cell signaling: used for prostaglandins, thromboxanes, leukotrienes
linoleic and linolenic acid are considered “essential fatty acids”
linoleic acid – essential FA for making arachidonic acid in humans (which we can’t synthesize de novo)
α-linolenic acid – essential FA for making EPA, which makes DHA.
Glycerophospholipids and Sphingolipids
Glycerophospholipids: Membrane Lipids
similar to triacyl glycerols, except carbon 3 of the glycerol backbone has a phosphate moiety
Compound shown here is “phosphatidic acid” – parent compound of all phospholipids
specific molecules can be attached to the phosphate group to give the lipid specific structure and functional properties
Glycerophospholipid Head Groups
Note: charges on head groups define functional role of the lipids.
Note particularly the head group inositol – key signaling molecule (next slide)
Phosphatidylinositol (PI) Phosphates
PI contains an inositol group bound to glycerol C-3, and fatty acids bound to C1 and C2.
The inositol group can be phosphorylated at positions 4 and 5 or positions 3, 4 and 5.
Serve two functions in signal transduction (more on this later):
1) PI-4,5-bisP can be cleaved by phospholipase C (PLC) to form IP3 and DAG
Signaling molecules
2) PI-4,5-bisP can be phosphorylated by PI 3-kinase to form PI-3,4,5-trisP
Docking site for signal transduction proteins
Sphingolipids: signaling and cell recognition
18 carbon amino alcohol backbone
Primary difference between sphingolipids and phospholipids
Note amide linkage of one fatty acid make a “ceramide”
Addition of a phosphate group and a head group conveys structure and function properties
Sphingomyelin commonly found in cell membranes, especially in the myelin sheath
Glycolipids: Cerebrosides and Gangliosides
Attach a single sugar and a cerebroside is formed
Note: no phosphate
Attach a polysaccharide and a ganglioside is formed
Very specific structures and functions: primarily cell surface recognition and binding of specific proteins and factors to cell surface.
Properties of Lipid Bilayers
Lipids are amphiphilic molecules which form lipid bilayers in aqueous solution
sequesters non-polar fatty acid chains away from water and exposes polar head groups to water.
Typical “hydrophobic interaction” weak force
Hydrophobic interior of bilayers excludes water and polar molecules.
forms an impermeable barrier to all but the smallest polar molecules.
Non-polar molecules will partition into hydrophobic core
Small, slightly polar molecules can pass easily
CO2, O2, NO, ethanol and others.
Membrane Proteins
Lipids provide the basic structure of membranes, but most specific functions are carried out by proteins associated with the bilayer
Different membranes have different functions, thus contain different proteins
Gives rise to concept of membrane function
Mitochondrial membranes and Endoplasmic Reticulum membranes do very different things
the amount of protein in a membrane can vary from about 20% to about 80%
typically, the plasma membrane (PM) is about 50% protein, 40% lipid and 10% carbohydrate
the carbohydrate is attached to proteins or lipids and is entirely extracellular
Membrane Protein Types
Integral proteins: proteins embedded in the bilayer that totally span the membrane
polar parts of protein exposed to aqueous phase, non-polar parts exposed to hydrophobic core
Peripheral proteins: proteins that associate with the bilayer but do not penetrate it.
association is non-covalent with either bilayer lipids or integral proteins
