Lecture 9

Glucose Disposal and Carbohydrate Structure

Glycogen Synthase

• Catalyses the addition of ‘activated’ glucose onto an existing glycogen molecule– UDP-glucose + glycogenn UDP + glycogenn+1

• Regulated by reversible phosphorylation (covalent modification)– Active when dephosphorylated, inactive when phosphorylated

• Phosphorylation happens on a serine residue– Dephosphorylation catalysed by phosphatases (specifically

protein phosphatase I)– Phosphorylation catalysed by kinases (specifically glycogen

synthase kinase)• Insulin stimulates PPI

– And so causes GS to be dephosphorylated and active– So insulin effectively stimulates GS

Phosphofructokinase

• Catalyses the second ‘energy investment’ stage of glycolysis– Fructose 6-phosphate + ATP fructose 1,6

bisphosphate + ADP

• Regulated allosterically– Simulated by concentration changes that reflect a low

energy charge• An increase in ADP/AMP and a decrease in ATP• These molecules bind at a site away from the active site –

the allosteric binding sites.

– Many other molecules affect PFK allosterically but all are effectively indicators of ‘energy charge’

Coupling (again!)

• The stimulation of glycogen synthesis by insulin creates an ‘energy demand’– Glycogenesis is anabolic– The activation of glucose prior to incorporation into glycogen

requires ATP – This drops the cellular [ATP] and increases the [ADP]

• This drop in ‘energy charge’ is reflected by a stimulation of PFK– A good example of how an anabolic pathway requires energy

from a catabolic pathway– Insulin has ‘indirectly’ stimulated PFK and glucose oxidation

even though it does not have any direct lines of communication to this enzyme

Carb structure - general• - CHOH- with -C=O ....

– makes it a good reducing agent (it, itself can be oxidised)• Aldoses and ketoses

– -C=O in aldehyde or ketone position– simplest 3C trioses glyceraldehyde and dihydroxyacetone– no chiral centres in the latter, but one in the former (L & D)

• Tetroses (4C) – another chiral carbon appears• Pentoses (5C) pentoses

– Can also form ring structure (happens very fast)- -C=0 reacts with one of the far-away CHOHs

– creates another stereo-centre • anomeric carbon - alpha or beta - (changing all the time in solution).

• Hexoses (6C) - now four chiral centres 24=16 in the aldose– most commonly occurring in nature is the form that has all the -OHs in a

plane – D-glucose– Can form a ring in solution – continually opening and closing– Chair and boat configuration

More ‘chemistry’• Stereoisomers

– molecules with the same formula but different spatial arrangements• What you DON’T need to know

– Enantiomers• Stereoisomers that are mirror images of each other

– Diastereomers • Stereoisomers that are not mirror images

• What you DO need to know…– Epimers - differ in orientation around just one carbon atom.

• Glucose and mannose, glucose and galactose. – Anomers – differ in the carbon formed by the ring– Numbering of glucose. May seem pedantic but important when dealing with

radioactivity!• Glycosidic Bonds between monosaccharides.

1-4 and 1-6 glycogen, starch. 1-4 cellulose 1-2 sucrose, 1-4 lactose

• No longer reducing sugars when in these bonds.– Ring opening/closing no longer possible

Nomenclature1CHO

2C OHH

3C HHO

4C OHH

5C OHH

6CH2OH

1CH2OH

2C O

3C HHO

4C OHH

5C OHH

6CH2OH

D-glucose aldose D-Fructose ketose

Numbering of sugars

Ring Formation

O

H

HO

H

HO

H

OH

OHHH

OH

O

H

HO

H

HO

H

H

OHHOH

OHCHO

OHH

HHO

OHH

OHH

CH2OH D-glucose

D-glucose

Attack of O on C5 to C1. O on C1 becomes new OH group

Haworth Projections

O

OH

OH

OH

OH

CH2OH

CH2OH

OHCH2OH

OH

OH

O

beta-D-fructosebeta-D-glucose

Pyranose ring Furanose ringOO

pyran furan

Maltose

O

OH

OH

OH

CH2OH

maltose

O H

OH

OH

OH

CH2OH

glucose glucose

reducing end

O

Glycosidic bonds between 1 (alpha anomer) and 4… 14

Still a reducing end (glucose ring on the right can still open)

Lactose

OOH

H

OH

OH

CH2OH

lactose

O H

OH

OH

OH

CH2OH

galactose glucose

O reducing end

Glycosidic bonds between 1 (beta anomer) and 4… 14

Sucrose

O

OH

H

OH

OH

CH2OH

CH2OH

HCH2OH

OH

OHO

O

sucrose

Glycosidic bonds between 1 (alpha anomer) and 2… 12

glucose fructose

Glycogen/Starch

O

OH

OH

CH2

O H

OH

OH

OH

CH2OH

glucose glucose

reducing end

OO

O

OH

OH

OH

CH2OH

O

glucose

branch point

Glycosidic bonds between 1 (alpha anomer) and 4… 14 but also 16

Cellulose

O

OH

OH

CH2OHO

OH

OH

CH2OH

glucose glucose

O

cellulose

Glycosidic bonds between 1 (beta anomer) and 4… 14

Enables lots of hydrogen bonds between chains and a lattice fibre structure

Sugar Tests

• Free anomeric carbon – a reducing sugar!– transient formation of the aldehyde in solution– Capable of reducing H2O2, ferricyanide, some metal ions (Cu2+,

Ag+)– Fehling’s test (Cu) – red ppt– Tollen’s test (Ag) – silver mirror

• Most usually done enzymatically and spectrophotometrically– Glucose oxidase – production of H2O2

– Colour changes or measured electrochemically

• The aldehyde group also makes glucose quite dangerous to have in your body at high concentrations for long periods of time