Ch 8 Lipids and proteins are associated in biological membranes

lipid

marginally soluble in water readily soluble in solvents

• open chain compounds with polar heads and nonpolar tails

• fused ring compounds

fatty acids

• amphipathic: polar end (water soluble) and another with hydrocarbon nonpolar (insoluble in water)

  • unbranched hydrocarbon tail

• even number of C atoms

saturated fatty acid

C double bond in the nonpolar chain usually cis, lower melting point

• cis points kink in tail trans = tail remains fully extended

unsaturated fatty acid

only single carbon bonds

Triacylglycerol/ triglyceride

• a lipid formed by esterification of three fatty acids to glycerol

• can store fatty acids

• doesn’t exist in membranes

• concentrated store of metabolic energy

  • Complete oxidation of fats yields 9kcal/g vs 4kcal/g carbohydrates

glycerol

a three-carbon compound that contains three hydroxyl groups, one bound to each carbon

lipases

enzymes that hydrolyze lipids

• breaks ester linkages of triacylglycerols into 3 fatty acids

saponification

uses a salt base like KOH or NaOH to produce glycerol, and the salts of the fatty acids. Salts=soap

Phosphoglycerides

roader class of glycerol‑based phospholipids containing two fatty acids and a phosphate head group.

• make up membranes

phosphatidic acid

glycerol with two of the OH groups esterified with fatty acids and 3rd OH is esterified with phosphoric acid

phosphatidyl ester (Phosphoacylglycerols)

• phosphatidic acid with another alcohol esterified to the phosphoric acid moiety

• the polar head group is charged, because the phosphate group is ionized at neutral pH.

wax

• mixtures of esters of long-chain acids and long-chain alcohols

• frequently used as protective coating

sphingolipids

• lipids whose structure is based on sphingosines, not glycerol

• simplest compounds=ceramides: 1 fatty acid group linked to sphingosine

sphingomyelins

compounds which the primary alcohol of sphingosine is esterified to phosphoric acid which is also esterified to another amino alcohol

glycolipids

• a lipid to which a sugar moiety is bonded

• ceramides often act as parent compound

cerebroside

a glycolipid that contains sphingosine and a fatty acid in addition to the sugar moiety

steroids

lipids with a characteristic fused ring (A,B, and C) and 1 five-membered ring

cholesterol

• steroid that occurs in cell membranes the precursor of other steroids

• only contains a single OH group-mainly hydrophobic

• can modify role of membrane proteins

membranes

separate inside cell from environment, transport specific substances into or out of cell, house enzymes and proteins

lipid bilayer

• aggregate of lipid molecules in which the polar head groups are in contact with water and the hydrophobic tails are not

• held together by noncovalent interactions like van-der-waals and hydrophobic interactions

membrane fluidity with saturated fatty acids

no cis double bonds= close packing of molecules=rigidity

membrane fluidity with unsaturated fatty acids

cis kinks= disorder in the packing of chains= more open structure=more fluidity

stabilizes the extended straight chain arrangement of saturated fatty acids by van der waals

order of increasing fluidity: animal, plant, prokaryote

how does cholesterol restrict fluidity to the membrane

lipid bilayer asymmetry

• various mixture of lipids on inner and outer surface

• concentration of bulky molecules is higher in the outer layer, which has more room facing the outer environment

disorders, more

heat___ bilayers making them ___ fluid

flip-flop, laterally

lipids don’t often ____, but can move____ within each layer of the bilayer

peripheral proteins

proteins loosely bound to the outside of the membrane, usually bound to charged heads by polar/electrostatic interactions

integral proteins

proteins embedded in a membrane

• completely span membrane (often in a-helix or beta-sheets)

• anchored to lipids via covalent bonds from cysteines or free amino groups on protein to lipid anchors

how are proteins attached to the membrane?

transport proteins

components of a membrane that mediate the entry of specific substances into a cell

receptor proteins

proteins on a cell membrane with specific binding sites for extracellular substances

fluid mosaic model

model for membrane structure in which proteins and a lipid bilayer exist side by side without covalent bonds between proteins and lipids

rafts

assemblages of lipids

• preferential association can take place among sphingolipids, sterols, and membrane proteins

laterally

proteins can also float ____ like the lipids of the bilayer

• structure and protection from environment

• transport

• receptor property

• catalysis

what are the functions of the cell membrane

semipermeable

• membranes are ______, some things can flow through

passive transport

process by which a substance enters a cell without using cells energy

• enter using energy from going down the concentration gradient (high—> low)

active transport

energy requiring process of moving substances into a cell against a concentration gradient (low—> high)

simple diffusion

• type of passive transport that involves the process of passing through a pore or opening in a membrane without requirement for a carrier or for the expenditure of energy

• only small nonpolar/uncharged molecules like O2, N2, and CO2

• rate of movement determined by steepness of concentration gradient across the membrane

facilitated diffusion

• process by which substances enter a cell by binding to a carrier protein, doesn’t require energy uses the energy from flowing down their concentration gradient

• Ex) Glucose flows through Glucose permease carrier protein from high concentration in blood to low concentration inside RBC

  • hyperbolic curve like Michelis-menten

primary active transport

carrier protein + hydrolysis of a high energy molecule

• necessary for resetting action potential so that Na+out>Na+inside and K+inside>K+outside

• 3 Na+ ions from inside cell bind and 1 subunit of protein hydrolyzes the ATP and transfers the phosphate group to an aspartate chain on another subunit

• phosphorylation causes confirmational change in protein (opens pore) so 3 Na+ released outside cell

• outside cell 2 K+ ions bind to pump enzyme which is still phosphorylated

• 2nd confirmational change regenerates enzymes original form such that 2K+ ions released inside cell

explain Na+/Ka+ pump

secondary active transport

transporter uses he energy from one molecule moving down it’s concentration gradient to drive another molecule up it’s gradient

proton pumps

integral membrane proteins that create a hydrogen ion gradient across the membrane (uses ATP)

A receptor only works when the key functional groups in its binding site are arranged in the correct 3‑D geometry so the ligand can fit and interact properly.

requirement for membrane receptors to work

• can be inhibited using inhibitors

• tyrosine kinase

• G-proteins

controlling membrane receptor activity

tyrosine kinase

regulate proteins by adding or removing phosphate groups

G-proteins

membrane associated signaling proteins that require GP hydrolysis to function

Low-density- lipoprotein(LDL)

principle carrier of cholesterol in the blood stream

• consists of various lipids (cholesterol and phosphoglycerates and protein)

• LDL protein portion binds to LDL receptor on cell membrane

• complex forms between LDL receptor and protein is endocytosed into the cell

• receptor protein recycled back to the surface

• cholesterol protein used in cells

• excess cholesterol inhibits synthesis of LDL receptor

  • *too few LDL receptors= excess cholesterol in bloodstream= possible deposit in arteries

how is cholesterol regulated

Vitamin A

• serves as the site of the primary photochemical reaction in vision

• beta-carotene: percussor to A, unsaturated hydrocarbon

• 
  

retinol

alcohol form of Vit A

retinal

aldehyde form of Vit A

rhodopsin

retinal+opsin, molecule crucial to vision; main found on rods (rods are 60% and 40% lipids)

Vitamin D

• regulated Ca+ and phosphorous metabolism

• Cholesterol + UV—> Vit D3 (cholecalciferol)

• D3: synthesis of Ca2+ binding proteins which increase absorption of dietary calcium

  • deficiency in children=rickets

Vitamin E

• serves as an antioxidant

• most active form= a-tocopherol

• reducing agent—> reacts with oxidizing (such as free radicals) agents before they can attack other biomolecules

free radical

highly reactive molecule that have at least 1 unpaired electron

Vitamin K

• regulatory function in blood clotting required to modify prothrombin and other proteins

prostaglandins

derivatives of arachidonic acid that contain a 5-membered ring and are at pharmaceutical importance (biologically active: BP control, smooth muscle contraction, inflammation)

arachidonic acid

a fatty acid that contains 20 carbon atoms and 4 double bonds; the precursor of prostaglandins and leukotrienes

• prevention of blood clots

leukotrienes

• substances derived from leukocytes and have 3 double bonds have pharmaceutical importance

• constrict smooth muscle especially in the lungs

• synthesis of leukotriene C facilitated by allergic reaction causes constriction of the airway

thromboxanes

also derived from arachidonic acid known to induce platelet aggregation and smooth muscle contraction