1

Shuttles

Balancing the Redox potential between the

cytosol and mitochondria

• Malate - Aspartate Shuttle

• Glycerol-Phosphate-Shuttle

2

Shuttles

• For gluconeogenesis: Metabolites must be transported between cytosol and mitochondria. There is no transporter for OAA.

• The redox potential requires balancing between cytosol and mitochondria.

Malate-Aspartate Shuttle for 'transport' of NADH and metabolites.

Glycerol-3-Phosphate Shuttle for 'transport' of NADH

Malate-Aspartate Shuttle

Gluconeogenesis

cytosol Mitochondrion

pyruvate

OAAMalate Malate

OAA

Aspartate

pyruvate

Aspartate

3

C–OH|C–OH|C–OH

glycerol

C–OH|C = O|C–OPO3

DHAP

H–C = O|C–OH|C–OPO3

GA3P

C–OH|C–OH|C–OPO3

G3P

ATP

ADP

NADH NAD+

TPI

G3P-DH

Glycerol kinase

Glycerol Metabolism

Can be used in glycerol-phosphate Shuttle

4

5

Pentose Phosphate Pathway (PPP)

orHexose Monophosphate

Shunt

6

R5P = ribose-5-phosphateX5P = xylulose-5-phosphate

Glucose

Pyruvate

GlycogenR5P G6P

NADPHglycolysis

G6Pase (li

ver)

nucleotidebiosynthesis

Generates reducing equivalents for reductive biosynthesis and reduction of glutathione.

Energy Production

EnergyStorage

G6P ---> ---> ---> Ribulose-5-Phosphate

2 NADP+ 2 NADPH

Reductive portion

R5P

X5P7

Important Facts about the PP-Pathway

1. Generates 2 NADPH for every G6P

oxidized. Needed for:

• Reductive biosynthesis

• Reduction of glutathione

2. It produces R5P for nucleotide

synthesis.

3. It produces X5P – an allosteric effector

of carbohydrate metabolism.

4. Pathway is not a dead-end. It generates

metabolites that can feed back into

glycolysis.

5. G6P-DH catalyzes the 1st step:

• Inhibited by high NADPH.

• Induced in liver by high carb diet.8

9

Importance of Hexose Monophosphate (HMP)

Shunt in Red Blood Cells

• Role in maintaining viability of red blood cell.

• Provides NADPH to protect against oxidative damage.– NADPH is needed for reduction

of glutathione.– Glutathione can reduce

reactive oxygen species (H2O2, organic peroxides).

– Reducing conditions are necessary for keeping Fe+2 in hemoglobin from being oxidized.

10

Glutathione• A tri-peptide

(-glutamyl-cysteinylglycine)• Important for destroying harmful

oxidants.

Glutathione reductase

GSSG2 GSH

NADPH + H+NADP+

Pentose Phosphate Pathway (G6P-DH)

2 GSH + R–O–O–R GSSG + R–OH + H2O

Glutathion peroxidase

11

Glutathione in Red Cell

• Helps to maintain sulfhydryl groups of proteins such as hemoglobin. These groups can undergo spontaneous oxidation to disulfides.

2 protein–SH + 1/2 O2 ---> protein–S–S–protein + H2O

GSSG 2 GSH

Glutathione reductase

NADPH NADP+

Important Facts about the PP-Pathway

1. Generates 2 NADPH for every G6P

oxidized. Needed for:

• Reductive biosynthesis

• Reduction of glutathione

2. It produces R5P for nucleotide

synthesis.

3. It produces X5P – an allosteric effector

of carbohydrate metabolism.

4. Pathway is not a dead-end. It generates

metabolites that can feed back into

glycolysis.

5. G6P-DH catalyzes the 1st step:

• Inhibited by high NADPH.

• Induced in liver by high carb diet.12

G6P

Ribose-5-P

Ribulose-5-P

2 NADPH

PP2A(phosphoprotein phosphatase 2A)

2 NADP+

Nucleotidesynthesis

F6P + GA3P

Bifunctional Enz ChREBP(transcription factor)

X5P

Role of Metabolites in PPP

+

+ CO2

Intermediates in glycolysis

Allosteric activation

13

Remember the BiFunctional Enzyme

14

F6P

F-2,6BP

Allosteric activatorPFK-1

Allosteric inhibitorF-1,6BPase

BiF-Enz

kinase Ptase

Carbohydrates to FatsChREBP = Carbohydrate response

element binding protein• A transcription factor• Highly expressed in liver, kidney, &

adipose tissues.• Inactive state: present in cytosol &

specific ser/th sites are phosphorylated.• Activated by dephosphorylation.

PP2A = Phosphoprotein phosphatase 2A• Function = it dephosphorylates:

– Bifunctional Enzyme– ChREBP

• Activated by Xylulose-5-Phosphate

(X5P)

15

ChREBP• When dephosphorylated by PP2A it

translocates to nucleus, gets dephosphorylated

again, binds DNA, and upregulates

transcription of:– pyruvate kinase– acetyl-CoA carboxylase– FA synthase proteins

ChREBP

P P

ChREBP

P

ChREBPChREBP

P

DNA Response Elements mRNA transcripts

PP2Acytosol

nucleus

PP2A

16

G6P

NADPH

PP2A

Bifunctional E ChREBP

X5P

Allosteric Effects of X5P in Liver

Activates PFK-2↑[F2,6-BP]

Activates Glycolysis

Effects Transcription:• Pyruvate kinase• Acetyl-CoA carboxylase• FA synthase

+Allosteric activation

Glucose

Promotes Carbs ---> Fats

Which activity?Effect?

Protein Induction

17

Glucose

G6P

F1,6-BP

F6P

PEP

pyruvate

acetyl-CoA

OAAcitrate

acetyl-CoA

malonyl-CoA

citrateOAA

malate

malate

X5P

pyruvate

NADH

NAD+

FAs

F2,6-BP+

PP2A

ChREBP

ACC

FAS

NADPH

PK +

+

+BiF-E

NADH

NAD+

18