11. Carbohydrates

aldose

polyhydroxy aldehyde

ketose

polyhydroxy ketone

3 -7 carbon atoms

triose → heptoses

D vs L

D is common form in nature vs L. aa;s are common

glyceraldehyde and dihydroxyacetone

are the simplest sugars (trioses)

D or L

designation determined by stereochemistry at chiral carbon that is furthest away from carbonyl

enantiomers

mirror images

diastereomers

stereoisomers that differ at 1 or more chiral centers and are not enantiomers (so diff mpt etc)

epimers

differ at only one stereocentre (diff mpt, special diastereomer)

D-glucose and L-glucose

are enantiomers

D-erthryose and D-threose are

epimers at C2 (also diasteromers)

D-glucose and D-mannose are

epimers at C2

D-glucose & D-galactose are

epimers at C4

D-mannose and D-galactose

are NOT epimers but are diastereomers at C2 and C4

linear sugar structures sponataneously cyclize

intramolecular nucleophilic attack by hydroxyl group at the carbonyl group to make cyclic form of sugar

alcohol + aldehyde

hemiacetal

alcohol + ketone

hemiketal

B-d-Glucopyranose

hawarth projection of D-glucose

B-D-Fructofuranose

hawarth projection of D-fructose

furanose

5-membered ring with 4 carbons

pyranose

6-membered ring (5 c)

anomeric carbon

carbon attached to ring O and a OH (note: is the original carbonyl carbon that underwent nucleophilic attack by a -OH group)

cyclization causes the

former carbonyl carbon to become a chiral centre

alpha

6CH2OH is on opposite side of sugar ring to anomeric OH (up, down)

beta

6CH2OH is on the same side of sugar ring to anomeric OH (up,up)

anomers

diastereomers that differ at the anomeric carbon

alpha anomer:

OH substituent of anomeric C is opposite to CH2OH at C that designates D/L configuration (C5 in hexoses)

Beta-anomer

OH substituent of anomeric C is on same side of CH2OH at C that designates D/L configuration

sugars prefer chair

(mosre stable) as opposed to boat (less stable) conformation

boat configuration only seen in

lysozyme

it is generally sterically favourable (more stable) to

to put bulky substituents in equitorial rather than axial bositions

for glucopyranose, B is more stable than alpha, why?

in alpha, the anomeric C:OH is axial (less sterically stable) vs B-equitorial

rings can reopen and close readily in solution unless

three is no hydrogen on the anomeric hydroxyl (e.g another R group (sugar is attached to the anomeric C)

equilibrium of solution of glucose is

63.6% beta and 36.4% alpha delta G=-1.36 kj/mol

mutarotation

reopening and reclosing of a cyclic sugar which interconverts alpha or beta anomers

oxidation of the aldehyde to a carboxylic acid yields

an aldonic acid (oxidized at C1; occurs relatively readily)

oxidation of primary alcohol group to a carboxylic acid

yields a uronic acid

reaction of OH and COOH will always

produce an ester and h20

sugar acids internally esterify e.g

glucuronic acid, L-asorbic acid (vitamin c)

reducing sugars have a

terminal aldehyde group (in its linear form) which can act as a reducing agent

amino sugars: hydroxyl group(s) replaced by amino group(s)

which can react further e.g with acetic acid to form N-acetyl substituent

reduced sugars: carbonyl group reduced to alcohol e.g

reduction of glyceraldehydes gives glycerol

reduction of glucose gives

sorbitol (nutritive sweetener)

Glycosidic bonds

formation of bonds to the anomeric carbon of a cyclic sugar note: product can no longer open to aldehyde or undergo mutarotation (switching between alpha & beta) at neutral pH

glycosidic bonds cleaved by

glycosidase enzymes

linkage can be head-to-tail

(an anomeric carbon linked to a non-anomeric carbon) or head-to-head (2 anomeric carbons linked together)

linear polymer of alpha(1→ 4) linkage has

only one reducing sugar end or “reducing end”

sucrose glycosidic linkage & monomers

alpha,Beta(1→ 2), alpha d-glucose + beta d-fructose

lactose (alpha form) glycosidic linkage and monomers

B(1-4), B-d-galactose + alpha-d-glucose

Cellobiose (beta form) glycosidic linkage and monomers

B(1-4), B d-glucose + a d-glucose)

maltose (alpha form) glycosidic linkage and monomers

a(1→4) a d-glucose + a d-glucose

b(1-4) glycosidic linkage cleaved by

B-galactosidase (lactase)

polymers of sugars

homopolymeric or heteropolymeric

polyglucose(3)

1. starch (alpha-amylose, amylopectin)

2. glycogen

3. cellulose

poly N-acetylglucosamine

chitin

starch: micture of glucose polymers that plants synthesize as their food reserve

• deposited in the cytoplsam of plant cells as insoluble granules which are composed of 2 major sugar polymers

  • alpha-amylose (linear)

  • amylopectin (branched)

glycogen: storage polysaccharide of animals

• found in all cells but especially skeletal muscle and liver

• similar to amylopectin but more highly branched

alpha-amylose: alpha (1-4) linked polyglucose (will have a reduced end!!!)

linear (unbranched) chains, thousands of glucose units long, coil into a helical shape (more compact vs extended)

amylopectin: alpha (1→4) linked poly glucose with ____

alpha (1→6) branches every 24-30 glucose residues; contains 10^6 glucose residues

glycogen: alpha (1→4) linked polyglucose with ___

alpha (1→6) branches very ~ 8-14 glucose residues (i.e more highly branched than amylopectin)

due to alpha linkages, these storage sugars are

relatively compact

alpha- amylose, amylopectin and glycogen have

only one reducing end

alpha-amylose have just one-reducing end (bc linear); however, amylopectin and glycogen have

many non-reducing ends

homopolymers with beta linkages

cellulose and chitin

cellulose: mixture of glucose polymers joined by ____ linkages, linear polymer up to ~ 15,000 glucose residues long

B1-4 linkages

• main component of cell walls

cellulose accounts for

~ 50% of the carbon in the biosphere

celullose has a long extended structure, highly H bonded compared to

alpha 1-4 polysaccharides for storage which are compact

H bonds form within cellulose strands, between strands within a sheet and between sheets, large multilayer structure is very hard to metabolize due to: (2)

1. extensive H bonding

2. inaccessibility of linkages

chitin: similar structure as for cellulose EXCEPT

sugar is N-acetylglucosamine (=NAG or GlcNac)

chitin is a major component of exoskeleton invertebrates such as crustaceans and insects; and cell walls of fungi and algae ALMOST

as abundant as cellulose

glycosaminoglycans: unbranched polysaccharides consisting of alternating

uronic acid and hexosamine (heteropolymer)

heparin is an anticoagulant, type of glycosaminoglycans, others ___

form gel-like substance in connective tissue (cartilage and blood vessel walls)

Proteoglycans: very diverse group of molecules in which a

core protein has at least 1 glycosaminoglycan chain covalently attached to via O-(ser/Thr) and N-(Asn linkages)

peptidoglycan: specialized structural polysaccharide in bacterial cell walls

consists of NAG and NAM which is covalently linked to tetrapeptide and pentaGly bridges

tetrapeptide in peptidoglycan attached to

NAM, and 3rd peptide attached to pentaGLy bridge

peptidoglycan highly crosslinked

convers high stability to cell wall

gram positive vs gram negative peptidoglycan

gram positive has a thick peptidoglycan (cell wall)

Glycoproteins (3)

• proteins have sugars covalently bonded to aa side chains

• linkages generally at aa on protein surface, often in Beta-turns

• sugar residues can vary greatly

2 major types of glycoproteins

1. O-linked

2. N-linked

O-linked via

hydroxyl group of Ser or Thr

N-linked through

amide groups of Asn

various types of sugar residues and linkages are observed in glycoproteins

linkages alpha or beta: 1-4, 1-3, 1-6, 2-3, etc

secreted proteins and membrane proteins are

generally glycosylated

carbohydrates on the cell surface play central roles in

cell-cell recognition

protein glycosylation is often highly heterogeneous

i.e individual molecules of the same protein differ with respect to amount/type of sugar(s) attached

ABO blood group antigens compared to type O

type a can ahve extra GalNAc and type B can have extra GaI

glycolipid: various oligosaccharides attached to lipids

e.g outer membrane lipid polysaccharide of gram (-) bacteria