Metabolism of Hexoses & the pentose pathway

Question 1

What are some other sugars that can be used in glycolysis?

Answer 1

Fructose
Galactose
Mannose

Question 2

Describe fructose?

Answer 2

A major sugar that is rich in fruit and sucrose (fructose + glucose)

It has two pathways of metabolism one in muscles and one in the liver

Question 3

Describe the pathway of fructose in the muscles?

Answer 3

Hexokinase (due to a broad specificity) converts fructose into fructose-6-phosphate, using ATP
Which can then go striaght into glycolysis

Very simple and one step

Question 4

Describe the pathway of fructose in the liver?

Answer 4

Fructokinase (using ATP) converts fructose into fructose-1-phosphate
Fructose-1-phosphate is cleaved into Dihydroxy-acetone phosphate (DHAP) and glyceraldehyde using aldolase (type b)
Glyceraldehyde can enter as glyceraldehyde-3-phosphate (GAP) using glyceraldehyde kinase to be converted
DHAP can be converted to GAP using TIM

Question 5

Is excess fructose harmful?

Answer 5

Due to fructose bypasing PFK, therefore avoiding a major metabolic control point - leading glycolysis flux towards lipid synthesis
There could be some links to obesity as consumption has increased nearly 10-fold in the last 25 years

Question 6

Is a lack of fructose harmfull?

Answer 6

Deficiency in Type B aldolase = fructose intolerance
F1P accumulates, depleting the liver store of Pi and a drop in ATP = liver damage
F1P also inhibits glycogen phosphorylase (glycogen degradation) and fructose-1,6-bisphosphatase (gluconeogenesis)
This leads to hypoglycaemia which can be life threatening

Question 7

Describe galactose?

Answer 7

It is obtained from hydrolysis of lactose (glucose + galactose)
Glucose and galactose are epimers at the C4 position
It needs to be epimerised before entering glycolysis

Question 8

Describe the pathway of galactose?

Answer 8

Galactose is converted by galactokinase and ATP into galactose-1-phosphate
It uses a coupled reaction using galactose-1-phosphate uridylyl transferase to epimerise into glucose-1-phosphate
Glucose-1-phosphate is converted by phosphoglucomutase into glucose-6-phosphate - which can now enter glycolysis

Question 9

What is the coupled reaction in the galactose pathway?

Answer 9

UDP-galactose is converted via UDP-galactose-4-epimerase and NAD+ into UDP-glucose
galactose-1-phosphate uridylyl transferase is then used to transform Ga1P to Glu1P and also transform UDP-glucose back to UDP-galactose

Question 10

What is a disease of galactose?

Answer 10

Galactosemia is the inability to convert galactose to glucose
This can lead to mental retardation and liver damage
Due to a lack of galactose-1-phosphate uridylyl transferase
Build up of toxic by products, can form galactitol in the lens of the eye = cataracts
Treated by a galactose free diet

Question 11

Describe mannose?

Answer 11

Mannose is derived from dietary sugars and glycoproteins
The C2 epimer of glucose
Converted to fructose by a 2-step reaction

Question 12

Describe the pathway of mannose?

Answer 12

Mannose is converted to mannose-6-phosphate using hexokinase and ATP
Mannose-6-phosphate is then converted to fructose-6-phosphate using phophomannose isomerase

Question 13

What is the pentose phosphate pathway?

Answer 13

This pathway produces ribose and NADPH

Using free energy of metabolite oxidation for reductive biosynthesis

Question 14

What is NADPH used for?

Answer 14

Reductive biosynthesis of fatty acids and cholestrol (in addition to ATP)

NADPH is not metabolically interchangable with NADH as the dehydrogenases in each pathway are highly specific for their respective co-enzyme

Question 15

How many reactions are involved in the pentose phosphate pathway?

Answer 15

8 reactions that are split into three sections:

1. Oxidative reactions which yield NADPH and ribulose-5-phosphate
2. Isomerisation and epimerisation reactions which transform Ru5P into ribose-5-phosphate or xylulose-5-phosphate
3. A series of C-C bond cleavage and formation reactions that convert two Xu5P and one R5P to two F6P and one GAP

Question 16

What reactions take place in section 1 of the pentose phosphate pathway?

Answer 16

1. Glucose-6-phosphate undergoes hydride transfer using NADP and G6P dehydrogenase to form 6-phosphoglucono-δlactone (cyclic ester)
2. 6-phosphogluconolactonase is hydrolysed with water to 6-phosphogluconate using 6-phosphogluconolactonase
3. 6- phosphogluconase catalyses the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate (Ru5P) and CO2. With a b-Keto acid intermediate (acting as an electron sink)

Question 17

What is generated in section 1 of the pentose phosphate pathway?

Answer 17

Two molcules of NADPH is generated in the first 3 steps

Question 18

What reactions take place in section 2 of the pentose phosphate pathway?

Answer 18

Ru5P is split into either:

1. Ribose-5-phosphate (R5P), catalysed by ribulose-5-phosphate isomerase
2. Xylulose-5-phosphate (Xu5P), catalysed by ribulose-5-phosphate epimerase

Question 19

How is it determined which product is formed in section 2 of the pentose phosphate pathway?

Answer 19

Relative amounts of each product depend on the needs of the cell
R5P important in DNA synthesis - pathway is often unregulated in cells undergoing cell division
If nucleotides are not required but NADPH is, then Xu5P and R5P are produced in 2:1 ratio to feed back into glycolysis later on

Question 20

What takes place in section 3 of the pentose phosphate pathway?

Answer 20

There is conversion of 5-C sugars to 6-C and 3C sugars

The pathways are interconnected and are catalysed by transketolase and transaldolase

Question 21

Describe transketolase?

Answer 21

It uses a cofactor TPP - thiamine pyrophosphate:
Active group is the thiazolium ring
C2 proton is acidic due to the adjacent positive charge on the nitrogen
This positive charge stabilises the negative carbanion when the proton dissociates
This carbanion is the active form of the coenzyme

Question 22

In section 3 of the pentose phosphate pathway - what does the first transketolase do?

Answer 22

Catalysed the transfer of a C2 unit from Xu5P to R5P, which yeilds Glyceraldehyde3-phosphate (GAP) and sedoheptulose-7-phosphate (S7P)
The reaction intermediate is a covalent product between Xu5P and TPP

Question 23

How does the transketolase and TPP first reaction work in section 3 of the pentose phosphate pathway?

Answer 23

1. TPP ylid adds to the Xu5P carbonyl group
2. C2-C3 bond cleavage yeilds GAP and a resonance stablised carbanion of the C2 unit
3. the C2 carbanion adds to the aldehyde carbon of R5P
4. TPP is eliminated yeilding S7P and regenerating TPP.E

Question 24

In section 3 of the pentose phosphate pathway what does transaldolase do generally?

Answer 24

It catalysed the transer of C3 units from S7P to GAP yeilding erythrose-4-phosphate (E4P) and F6P
The reaction occurs by aldol cleavage, starting with a schiff base formation between an amino group of a lysine residue and the carbonyl group of S7P

Question 25

Decribe how transaldolase works in section 3 of the pentose phosphate pathway?

Answer 25

1. The ɛ-amino group of Lys forms a Schiff base with the carbonyl group of S7P
2. A Schiff base–stabilized C3 carbanion is formed in an aldol cleavage reaction between C3 and C4 that eliminates E4P
3. The enzyme-bound resonance-stabilized carbanion adds to the carbonyl C atom of GAP, forming F6P linked to the enzyme via a Schiff base
4. The Schiff base hydrolyzes, regenerating active enzyme and releasing F6P

Question 26

What is the last stage of section 3 of the pentose phosphate pathway?

Answer 26

A 2nd transketolase, transfers a C2 unit from a second molecule of Xu5P to E4P to form GAP and another molcule of F6P

Question 27

What is the overall equation of section 3 of the pentose phosphate pathway?

Answer 27

2 Xu5P + 1 R5P ➙ 2 F6P + 1 GAP

Question 28

How can we link the pentose phosphate pathway?

Answer 28

The most important aspect is to generate NADPH - to be used in reductive reactions

The products of F6P and GAP can be fed back into glycolysis
R5P can be used to produce nucleotides