Lecture 8 - Biological membranes

Question 1

Lipids: what are they used for and what are some key characteristics?

Answer 1

• Energy reserves (triacylglycerols)
• Lipids and their derivates serve as vitamins and hormones

Insoluble in aqueous solution (hydrophobic)
Lipophilic bile acid readily degrades lipids

Question 2

Four main lipid families

Answer 2

Triacylglycerols - glycerol backbone, 3 fatty acids (acyl) groups ~ hydrophobic

Glycerophospholipids - glycerol backbone, phosphate moieties, polar head group (glycoconjugates can form) ~ amphipathic

Sphingolipid - sphingosine backbone, often glycoconjugates ~ amphipathic

Isoprenoids - include steroids, lipid vitamins, and hormones - mainly hydrophobic, may be variable depending on the head group

Question 3

C₂₂ - what is the compound?

Answer 3

Docosahexaenoic acid

Question 4

Fatty acid nomenclature

Answer 4

The carboxyl group is C1
The carbons after c1 are α,β,γ,δ,ε etc.
The last carbon in the chain is ω

Question 5

Unsaturated acids: what types are there and what conformation do they normally have?

Answer 5

Monounsaturated - one double bond
Polyunsaturated - more than one double bond

Cis conformations are practically always present in fatty acids

Question 6

Triglycerols

Answer 6

The carbons at the top of the chain each bound to their own fatty acid by an ester bond

Question 7

Glycerophospholipids: what is the relative abundance, the structure, and the further polar group possible forms?

Answer 7

The most abundant lipids in membranes

Possess a glycerol backbone with two fatty acids (R1 and R2) esterified to glycerol and one phosphate esterified to 3rd carbon of glycerol

The phosphate can be further esterified to a polar group (X)

Question 8

Sphingolipids: what is the abundance and what is the structure?

Answer 8

A large family of membrane lipids

Instead of a glycerol backbone like TAGs and phospholipids, they have a sphingosine unit (with 3 carbons that can be considered similar to those in glycerol)

Like phospholipids, have two nonpolar (hydrophobic) tails and a polar (hydrophilic) headgroup (= amphipathic)

Question 9

Membrane lipids: sphingolipids - what are the 4 main types?

Answer 9

Ceramide
Sphingomyelin - phosphocholine, ethanolamine
Cerebroside - glucose/galactose
Ganglioside - complex oligosaccharide

Question 10

Membrane lipids: steroids - what are the names or functions of the carbon compounds

Answer 10

C10 - essential oil components
C15 - farnesol (membrane anchor for proteins, artemisinin)
C20 - e.g. retinal; the vitamin E precursor phytol
C30 - steroids
C40 - carotenoids

polyterpenes: rubber

Question 11

Membrane lipids: cholesterol - what does it do?

Answer 11

Cholesterol allows fluidity to be regulated: low temps are more fluid with cholesterol and high temps are less fluid with cholesterol (in comparison to how it would be without cholesterol)

Question 12

The difference between the two leaflets (monolayers) of the lipid bilayer

Answer 12

The two leaflets (or monolayers) of a bilayer membrane tend to differ in their lipid composition:

More sphingolipids in the outer leaflet,
More glycerophospholipids in the cytosolic leaflet.

Question 13

Movement of phospholipids in the membrane

Answer 13

Lateral diffusion (moving left or right within one leaflet) occurs often and is rapid

Transverse diffusion (between each leaflet of the bilayer) is rare and happens slowly

Question 14

Three classes of membrane proteins

Answer 14

Integral proteins
Peripheral proteins
Lipid-anchored membrane proteins

Question 15

Integral proteins

Answer 15

Contain hydrophobic regions within the hydrophobic layer of the membrane

Usually span the bilayer

Can provide channels formed from β-strands (when forming channels, interior is hydrophilic and exterior is hydrophobic)

Question 16

Peripheral proteins

Answer 16

Associated with membrane through charge-charge or hydrogen bonding to membrane polar head groups or integral proteins

More readily dissociated from the membrane, pH/ionic changes make them release easily

Question 17

Lipid-anchored membrane proteins

Answer 17

Tethered to the membrane through a lipid-protein covalent bond:

Cystolic leaflet bonds:
* Ester/thioester bond between amino acid (Ser/Cys) to a fatty acyl group
* amide bond linking N-terminal (Gly) to a fatty acyl group
* Thioether bond linking amino acid (Cys) to an isoprenoid chain (prenylated protein)

Outer leaflet bond:
* Protein anchored by C-terminus to glycosylphosphatidylinositol (GPI)

Question 18

Two types of active transport

Answer 18

Direct energy source (ATP/light/electron transport)
Secondary energy source (ion concentration gradient)