CARBOHYDRATES - LECTURE 1

glycolysis

glucose → energy

gluconeogenesis

non carbs → glucose

glycogenesis

glucose → glycogen

glycogenolysis

glycogen → glucose 

old definition of carbs

compounds with molecular formula Cn(H2O)n

limitations with the old definition of carbs

• There were molecules which were very clearly carbs that did not follow the Cn(H2O)n formula as there was an oxygen missing

  • There were molecules which were not carbs which followed this definition such as acetic acid

how are carbs classified

sugars (saccharides)

non-sugars (polysaccharides)

how are sugars classified 

mono, di, oligosaccharides (3-10 units)

how are monosaccharides classified

trioses

tetroses

pentoses

hexoses

what are the 2 types of hexoses - give examples

aldohexose - glucose

ketohexose - fructose

how are disaccharides classified

reducing - lactose, maltose, cellobiose

non reducing - sucrose

how are oligosaccharides classified

trisaccharides

tetrasaccharides

pentasaccharides

how are non sugars classified

homopolysaccharides 

heteropolysaccharides 

fischer projections

• One of the simplest ways to represent carbs is in the straight chain form

• Developed by emil fischer

how do fischer projections indicate stereochemistry

• Horizontal = wedge i.e. coming out of the page

  • Vertical = dash i.e. pointing away from the page

how to identify R/S in fischer

• Redraw the chiral centre in 3D if needed

• Assign CIP priorities

• Determine configuration

• If lowest priority is on a vertical line - its ok

  • If lowest priority is on a horizontal line - disregard it, get the R/S configuration - then SWITCH

describe the process of ring closure of glucose in aqueous solution

• in water glucose does not remain in the straight chain form

• predominantly forms a ring structure due to  intramolecular attack

  • It is a simple nucleophilic attack where hydroxyl on carbon 5 attack the carbonyl carbon of the aldehyde

  • A hemiacetal is formed and produces a six membered ring

haworth projections - what do they represent

a ring in a flat planar structure

difference between alpha and beta anomers

• The anomeric carbon is newly formed and can have OH either

  • Down -> alpha anomer - this is when the nucleophilic attack comes from the bottom

  • Up -> beta anomer - this is when the nucleophilic attack comes from the top

what are the two ways substituents can be presented in chair conformations

axial - pointing straight up or down

equatorial - projecting outward roughly in the plane of the ring

which carbon is C6

CHOH group above the ring

what are D and L isomers in relation to one another

enantiomers

D isomer

when hydroxy furthest away from the carbonyl group is on the right

L isomer 

when hydroxy furthest away from the carbonyl group is on the left

epimer

an  epimer is a type of stereoisomer that differs from another molecule at only one specific chiral centre while all other chiral centres are the same

AKA diasteromer

mutarotation

The change in optical rotation due to the interconversion between alpha and beta anomers

it is catalysed by both acids and bases

when does mutarotation occur 

when the cyclic form briefly opens to the linear aldehyde/ ketone form then recloses 

approx equilibrium for glucose in water

• α-D-glucose: 36%

• β-D-glucose: 64%

• Open-chain form: 0.02%

describe the formation of hemiacetals

• A hemiacetal forms when a hydroxyl group attacks a carbonyl group intramolecularly

• Hemiacetal carbon has both - OH and -OR groups

• Reaction is reversible which allows the interconversion between alpha and beta

why is the beta anomer of glucose more stable

• Beta anomer of glucose is more stable due to equatorial OH groups in chair form

how do you form acetals from hemiacetals

• Adding extra alcohol in an acid catalysed reaction converts hemiacetals to acetals

steps:

• Protonate OH - this then leaves as water

• Alcohol attacks - we get an acetal with 2 -OR groups

  • Acetals are stable and do not form equilibria

  • Breaking acetals requires a strong acid and heat (boiling)

reducing sugars

can open to form free aldehyde or ketone

aldehydes reduce other compounds and become carbox acids 

hemiacetals allow ring opening, enabling this to happen

why is sucrose non-reducing

• Both anomeric carbons are full acetals - no free OH

• Cannot open to form aldehydes/ ketones

• Does not undergo mutarotation

• Only hydrolysed under strong acidic boiling conditions

anomeric carbon

carbonyl carbon

anomers

isomers which differ in placement of hydroxyl on C1

slightly different chemical and physical properties

which monosaccharide is a structural isomer but NOT an anomer, epimer, diastereomer or enantiomer of D-glucose

fructose

how does fructose form a ring

fructose is a ketose

its ring forms when a hydroxyl group attacks the ketone at C2

what are the 2 possible ring forms of fructose

furanose and pyranose 

why is beta-D-fructopyranose the most stable form

• Most hydroxy groups are equatorial, minimizing steric strain.

• Beta anomer places the anomeric OH on C2 in a favorable orientation.

explain how fructose is a reducing sugar

• Under basic conditions, fructose undergoes keto-enol tautomerization to form glucose and mannose intermediates.

• These intermediates have an aldehyde group that can be oxidized.

• Therefore, fructose gives a positive reducing sugar test

what is the optical rotation of fructose and what effects it

Fructose has a negative specific rotation, which changes slightly depending on which ring form predominates.

structure of D glucose

structure of D mannose

structure of D galactose

structure of D-fructose