Lec7/8 - Carbohydrates, Nucleic Acids, Lipids and Membranes

Question 1

Describe how the Fisher projection works

Answer 1

The most oxidised carbon (C=O) is viewed at the top; the first C-C bond points slightly out of the plane; horizontal bonds (C-H and C-OH) point out of the plane

Question 2

How can the Fisher projection be used to determine if a carbohydrate is L- or D-form

Answer 2

The Chiral Carbon that is FURTHEST from the most oxidised carbon is used: with the horizontal bonds pointing OUT of the plane, if the OH group is to the right, it is D form; if the OH group is to the left, it is L form

Question 3

What are the two families of monosaccharides based on the position of the most oxidised carbon?

Answer 3

Aldoses (most oxidised is at the end of the chain) and Ketoses (most oxidised is C2, or occasionally C3)

Question 4

What are Oligosaccharides?

Answer 4

Carbohydrates with 2-20 carbon atoms in the chain

Question 5

What are Glycoconjugates?

Answer 5

Glycoproteins or Glycolipids

Question 6

What are diastereomers?

Answer 6

Any stereoisomer that is NOT an enantiomer (i.e. not a mirror image)

Question 7

Are L- or D-isomers of carbohydrates more common in nature?

Answer 7

D are more common

Question 8

How are hemiacetal/hemiketals formed (and what four bonds is the C left with)?

Answer 8

When an aldehyde or ketone group reacts with an alcohol group (the C then has one ether link (C-O-X), one OH group, one X group and one H OR X group (H for hemiacetal, X for hemiketal)

Question 9

What is the difference between a hemiacetal/hemiketal and an acetal/ketal?

Answer 9

If the carbon has TWO C-O-X bonds instead of one, then it is a full acetal/ketal (in a hemiacetal or hemiketal, one is an OH group)

Question 10

What are pyranoses and furanoses?

Answer 10

Pyranose = 6-membered ring (5C + O)
Furanose = 5-membered ring (4C + O)

Question 11

How can alpha- and beta-glucose be distinguished?

Answer 11

If the OH on the C1 (anomeric) carbon is the SAME orientation as the OH of C4, it is ALPHA;
If it is the OPPOSITE orientation, it is BETA

Question 12

Name the 4 main families of lipids

Answer 12

Triglycerols, Sphingolipids, Glycerophospholipids, Isoprenoids

Question 13

Summarise the structure of the 4 lipid families

Answer 13

Triacylglycerols: 3 FA chains, Glycerol Backbone

Glycerophospholipids: Glycerol Backbone, 2 FAs, polar head group

Sphingolipids: Sphingosine backbone unit with an AMIDE linked fatty acid, polar head group

Isoprenoids: NO FAs, variable polar group content

Question 14

What is the difference between a fatty acyl group and a fatty acid?

Answer 14

Fatty acyl group = 12 carbons or less

Question 15

Are double bonds in fatty acids trans or cis?

Answer 15

ALWAYS CIS

Question 16

Describe the structure of glycerophospholipids

Answer 16

Glycerol backbone with 2 esterified FAs; ONE PHOSPHATE is ESTERIFIED to the 3rd carbon of glycerol; this phosphate can be further esterified to a polar group X (which makes the molecule amphipathic)

Question 17

Name 4 possible “X” groups for glycerophospholipids

Answer 17

Ethanolamine, Choline, Serine, Inositol

Question 18

Name 4 types of sphingolipids (and the esterified polar groups that define each one)

Answer 18

Ceramide (just H attached to O)

Sphingomyelin (Phosphocholine or ethanolamine)

Cerebroside (Glucose/Galactose)

Ganglioside (complex oligosaccharide)

Question 19

Summarise the structure of steroids

Answer 19

They are a type of isoprenoid (with around 30 carbons) that originate from 5-C isoprene units; they form a 4-ring system

Question 20

Summarise the structure and importance of cholestrol

Answer 20

Structural component of membranes (fluidity buffer), precursor to other steroids (e.g., hormones, bile), fused ring system, amphipathic

Question 21

State the approximate ratio of proteins to lipids in a plasma membrane

Answer 21

25-50% lipid; 50-75% protein

Question 22

Which lipids are found in plasma membranes?

Answer 22

Sphingolipids, Phospholipids, Cholestrol in certain eukaryotes

Question 23

How do the outer and inner leaflets of the lipid bilayer tend to differ in composition?

Answer 23

More SPHINGOLIPIDS in OUTER
More GLYCEROPHOSPHOLIPIDS in CYTOSOLIC

Question 24

What are the two main forms of movement for lipids in the bilayer, and which is more rapid?

Answer 24

Lateral Diffusion is very rapid
Transverse Diffusion (Flip Flop) is very slow

Question 25

What are the three classes of membrane proteins?

Answer 25

Integral, Peripheral (charge-charge, H bonding w/ integrals or lipids) and Lipid-Anchored (covalent bond)

Question 26

What are the four types of Lipid-Anchored membrane proteins?

Answer 26

1. ESTER/THIOESTER between Ser/Cys and a Fatty Acyl Group
2. AMIDE BOND between N-terminal Gly and a Fatty Acyl Group
3. THIOESTER between Cys and an isoprenoid chain
4. Anchored by C-terminus to GPI in OUTER leaflet

Question 27

What two things do the branches in amylopectin increase?

Answer 27

The compactness, and the number of chain ends (faster formation + degradation)

Question 28

Describe where the various glucose storage molecules might be found

Answer 28

Animals and Fungi: GLYCOGEN (similar to amylopectin but larger and more branches)

Plants: STARCH (amylose + amylopectin)

Question 29

Describe the structure of cellulose

Answer 29

Straight and unbranched chains due to ß1-4 glycosidic bonds; adjacent chains form H bonds; fibrils associate into a high strength insoluble polymer

Question 30

How can carbohydrates be linked to proteins?

Answer 30

O-glycosidic linkages (Ser, Thr)
N-glycosidic linkage (Asn)

Question 31

How are DNA dinucleotides formed?

Answer 31

A phosphodiester linkage forms, with the loss of inorganic Pyrophosphate (PPi)