4. Lipids and Carbohydrates

What are the functions of lipids?

• energy storage (abundant in cells)

• structure (abundant in cells)

• signalling (lower conc in cells)

How do lipids play a role in animal cells?

• plasma membrane provides semipermeable barrier with outside wall

• membrane bound organelles creating compartments with different environments

lipid bilayer

What are some characteristics of lipids?

• generally non-polar (entirelly or in part)

• therefore low solubility in water

How is energy stored in lipids?

fatty acids

• Triacylglycerol/triglyceride

• waxes

• phospholipids

Examples of storage lipids?

• fatty acids

• triglycerides

• waxes

Examples of membrane lipids?

• phospholipids

• glycolipids

• cholesterol

Examples of signalling lipids?

• phospholipid derivatives

• steroid hormones (cholesterol derivatives)

• eicosanoids (paracrine hormones)

• lipid soluble vitamins (vitamin A)

What are fatty acids?

carboxylic acids (-1) with hydrocarbon chains containing 4-26 carbons

• almost all have an even number of carbon

• most natural fas are unbranched

• amphipathic (carboxyl = hydrophilic, hydrocarbon chain - hydrophobic)

Difference between saturated, monounsaturated and polyunsaturated

saturated: no double bonds b/w carbons in chain

monounsaturated: one double bond b/w carbons in alkyl chain
polyunsaturated: more than one double bond in alkyl chain

Nomenclature

1) C1 = carboxyl group carbon

2) 18:0 means 18 C and no C=C

3) 18:1 Δ^9,12 means 18 C and 2 C=C starting at carbon 9 and carbon 12

saturated fatty acid trends

• increasing melting point as increased chain size

• increasing melting point as decreasing double bonds

• decreasing water solubility as chain size increases

why do fatty acids pack into stable aggregates?

• fully saturated C backbone is usually usually in a fully extended conformation

• therefore pack into a nearly crystalline array, stabilized by

  extensive hydrophobic interactions of the hydrocarbon chain

Why increased size of fatty acid = increased boiling point?

Longer carbon chains require more energy to disrupt the packing (more entropy required to expel water) => higher melting temperatures

cis or trans unsaturated?

double bond usually cis (not trans)

unsaturated fatty acid trends and why?

lower melting point

• Unsaturated cis fatty acids pack less orderly due to the kink → less-extensive favourable interactions

• It takes less thermal energy to disrupt disordered packing of unsaturated fatty acids

How do trans fatty acids form?

partial dehydrogenation of unsaturated fatty acids

• done to increase shelf life or stability at high temperature of oils used in cooking

trans double bonds allows…

adopt an extended conformation

trans vs. cis fatty acids

pack more regularly and show

• higher melting points than cis forms

trans or cis fatty acids good for you?

trans increases risk of cardiovascular disease

Why is olive oil a liquid while butter is a soft solid?

composition of saturated and unsaturated fatty acids

olive oil = more C18 (unsaturated) therefore disrupted packing and more space between molecules - liquid

butter = lower C16/C18 unsaturated and more C16/C18 saturated fatty acids, pack more easily therefore a solid

What are omega-3 fatty acids?

• essential nutrients, humans can’t synthesise

• ALA, DHA, EPA

can fatty acid double bonds be conjugated?

no! must be separated by C=C-C-C=C

Where does the Omega-3 name come from?

• omega = last letter of greek alphabet

• therefore C1 is the last C (opposite end to COOH)

• therefore C3 is the first double bond

What are triacylglycerols?

• fat molecule, glycerol backbone and three acyl trains

• glycerol OH’s provide 3 sites for ester linkages (carboxylic acid reacts with hydroxyl)

• HYDROPHOBIC

• provide stored energy and insulation

What is glycerol?

three carbon alcohol

simple vs. complex triacylglycerols

simple: all three fatty acids identical

complex: fatty acids differ

What are advantages of triacylglycerols?

1. higher energy yield than oxidation of other fuel sources such as glycogen or starch

2. not hydrated (less weight)

What are adipocytes?

fat cells

lipid bilayer

• two sheets

• hydrophobic tails in middle

• hydrophilic heads outside

• proteins and sugars

  • channels and pores

  • glycoproteins

  • sterol

  • sphingolipids

  • phospholipids

phospholipids

• phosphate group in their polar head

• two non-polar tails

• two types:

  • glycerophospholipids (glycerol)

  • sphingolipids (sphingosine)

difference b/w glycerophospholipids and glycerophospholipids?

• one polar head group + 2 non-polar tails

• g: uses glycerol

• s: uses sphingosine

most common glycerophospholipid?

phosphatidylcholine

glycerophospholipids

• further defined by additional components in head group

  ie. -H is phosphatic acid, -NH3 is …

• charges on head range from -4 - 0

sphingolipids

• backbone derives from amino alcohol sphingosine

  • carbons 1,2,3 of backbone considered equivalent to the 3 carbon glycerol but also contributes 1 of two tails

• simplest is ceramide, X of alcohol is H

• DONT ALWAYS CONTAIN A PHOSPHATE GROUP

  • glycolipids

• structural and signalling

glycolipids

• mono or oligosaccharaide unites in head groups

• lipid with sugar

  • sphingolipids

  • galactolipids (sulfolipids)

• common on OUTER membrane

sterols

• four fused rings decorated with alkyl side chains

• cholesterol

• polar head group (OH) on carbon ring

• structural, signalling precursor

biological membranes are…

lipid bilayers

thickness of membranes?

3nm

what is the fluid mosaic model of membranes

• incorporates proteins within the lipid bilayer

• proteins embedded within the bilayer are held by hydrophobic interactions

• interactions among components are non-covalent (allowing fluid, dynamic properties)

• charges of the lipid head groups contribute significantly to surface properties of membranes

what shape do fatty acids form?

micelles (conical individuals)

what shape do phospholipids form?

bilayer (cylindrical individuals)

• exposed hydrophobic region on edges are exposed to water → unstable

• bilayers fold to dorm hollow vesicle (liposome)

what is the aqueous cavity?

what impacts the membrane fluidity?

• fatty acid composition

  • length of fatty acyl chains

  • degree of unsaturation

    • more saturated = better packing = more rigid

• cholesterol content

  • moderates membrane fluidity

  • high conc = stiffens

  • low conc = breaks up packing = more fluid

membrane and temp

37ºC = all biological membranes are fluid

phase transition temperature - temp at which membrane goes from paracrystaline state to fluid state

• increased temp = more fluid

lipid movement

• lateral diffusion very fast at 37ºC

• transverse diffusion (flip-flop) is very slow

  • therefore monolayers can have different lipid compositions

lateral diffusion

• measured experimentally by fluorescence recovery after photobleaching (FRAP)

membrane rafts

sterols and sphingolipids cluster together in membrane rafts/lipid rafts/micro domains

• slightly thicker, more ordered (less fluid) and harder to dissolve in non-ionic detergents than surrounding region)

• behave like a liquid-ordered RAFT in the SEA of liquid-

  disordered phospholipids

function of membrane proteins

1. transporters (selective entry)

2. receptors for recognition signals

3. provide structural support

can proteins move around the membrane like lipids?

yes! they are free to diffuse laterally

how do proteins interact with membrane?

1. integral membrane protein

2. peripheral protein

integral membrane proteins

• deeply embedded in the membrane - firmly attached

  • strong hydrophobic interactions b/w amino acids on surface of protein and acyl chain of lipids

peripheral membrane proteins

• associate with outside surfaces, not attached/embedded

  • ionic interactions and H-bonding with:

    • polar head group of lipids

    • integral membrane proteins

how can peripheral membrane proteins be released?

interact non-covalently therefore released by reagents that disrupt ionic interactions

1. high salt (salt bridges)

2. change pH

3. chelating agent

how can integral membrane proteins be released?

reagents that disrupt hydrophobic interactions eg. detergents like SDS (sodium dodecyl sulphate)

• remember extracting DNA from strawberries prac

true or false: all membrane proteins have a unique orientation in the membrane?

true!!

how can you determine orientation/arrangement of membrane proteins? ie which bits are inside/outside

protease sensitivity of proteins from intact cells

• enzyme which chops proteins up, to determine which bits are outside

• trans membrane and inside protein are intact

what can we assume about transmembrane proteins?

• sequence will consist of hydrophobic amino acids

• conformation will be an alpha-helix

• span (segment of protein) equal to width of membrane

how many amino acids fit in a membrane?

~ 20 hydrophobic residues

membrane = 3nm thick

alpha-helix = 3.6 residues/turn

each turn 0.54nm length

0.15nm/residue

3/0.15 = 20

what is hydropathy index?

a measure of polarity of each amino acid

• +ve value = hydrophobic

• -ve value = basic/hydrophilic

hydropothy plot

y axis: hydropathy index

x axis: residue number

+ve values will hydrophobic and be the inside of the membrane

true or false: hydropathy plots can have multiple hydrophobic peaks?

true!

where are the N- and C- termini found?

N/amino = outside

C/carboxyl = inside

how is water and other small molecules transported across membrane?

transport proteins like channel proteins

true or false: transport proteins allow many types of molecules to pass through?

false-ish

each transport protein transfers a particular type of molecule - specific

How are membrane transport proteins classed?

• carriers (transporters)

  • passive/active

• channels

  • passive

    • high conc to low conc

key feature of carriers

conformational change

• open on one side closed on the other → then switch

glucose transporter (GLUT1)

• nearly all mammalian cells

• passive transport ie. facilitated diffusion down conc gradient

• no other substance transported (no fructose/other sugar)

mechanism of glucose transporter

• two conformational states (T₁ and T₂)

• T₁ open on outside → binding of glucose → conformational change → T₂ allows glucose to relase insie cell

• also happens other way ie. glucose inside → outside

active transporters

• require energy input

• transport against conc gradient

• eg. ATP powered pumps

  • P-type ATPases

P-type ATPases examples

• sodium ion pump

• hydrogen potassium

• calcium ATPase

function of carbs/sugars?

1. source of energy/stored fuels

2. structure to cells and organisms (cellulose in plants, chitin in arthopods)

3. cell biology

   • major component of cell surface

   • important in influencing function of proteins

   • important in specific recognition interactions

   • cell-cell adhesion

how are glycoproteins synthesised?

protein = genes

sugars = ?

monosaccharides

• basic unit of carbs

• aldehyde or ketones that have two or more hydroxyl groups

• [C-H₂O]_n ‘carbon hyrdate’

• three or more carbons

structural formula of monosaccharide

[C-H₂O]_n

ketone vs. aldehyde?

ketone C=O

aldehyde C(=O)(-H)

ketose vs aldose?

ketose: ketone sugar

aldose: sugar with aldehyde group

most naturally occurring sugars are what chirality?

D (not L)

chirality as each substituent on C is dfferent

fisher projections

• provide a clear and simple view of stereochemistry at each carbon centre

• horizontal lines project out of the plan on paper

• vertical lines project behind the plane of the paper

monosaccharide nomenclature

three - triose

four - tetrose

five - pentose

six - hexose

seven - heptose

most common monosaccharide in nature

hexose

aldohexoses

four asymmetric chiral centres

D-Aldose monosaccharides

• “D” designates the configuration of the asymmetric carbon furthest from the aldehyde group

• D-glucose, D-mannose and D-galactose are abundant six carbon sugars

• D-glucose and D-mannose differ in configuration only at C-2

epimers

sugars that differ only in the config around one carbon atom

like D-Mannose and D-Glucose at C2

D-Glucose shape

ta too ta ta

ta too ta ta

right left right right (-OH)

pyranose rings

• Hexoses are not open chains

• Open chains cyclize into rings

• For an aldohexose like glucose the C-1 aldehyde reacts with the C-5 hydroxyl group to from an intramolecular hemiacetal

• Results in a 6-membered ring called a pyranose ring

fisher vs haworth

LURD

chair vs boat

chair more stable

Acetyle group

Formation of glycosidic bonds

• reactive group is anameric group

• C1, condemnation reaction, water created

• Ether linkage

What are multiple monosaccharide joined together called?

Oligosaccharide or polysaccharide

sucrose

lactose

maltose linkage

sucrose - alpha-1,2

lactose - beta-1,4

maltose - alpha-1,4

naming glycosidic links

• always involve C1 (hemiacetal) of first sugar

  • alpha or beta anomer of C1

  • alpha -OH down

  • beta -OH up

• linked to what carbon on second sugar?

how many ways can two D-glucose units be linked?

11

trisaccharide - 176

sucrose

lactose

maltose

sucrose: glucose + fructose

lactose: galactose + glucose

maltose: glucose + glucose

homopolysaccharide

polysaccharide where repeating units are all the same

true or false: carbs are unbranched

false! they can be branched (fuel storage) or unbranched (structure)

branched vs unbranched carbs

branched = fuel storage

unbranched = structure

glycogen branching

alpha-1,4 chains of glucose

alpha-1.6 glycosidic link to create branch

oligosaccharide ends

non-reducing vs reducing end

• monosaccharide can be cyclic or linear straight chains

  • straight chain aldehyde can reduce other groups

  • oxidised to carboxylic acid

what affects structure of polysaccharides?

monosaccharides and nature or linkage