7/31/2019 14 Conversion of FAT into Carbonhydrate in Plants the Glyoxylate Pathway

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GLUCOSE

GlutamateGlutamate 2-Oxoglutarate2-Oxoglutarate

MITOCHONDRIONGLYOXYSOME

CYTOSOL

Succinate

Fumarate

Malate

Oxaloacetate

Oxaloacetate

-OOCCH2CH2COO-

-OOCCH=CHCOO-

-OOCCH2CH(OH)COO-

-OOCCH2COCOO-

-OOCCH2COCOO-

CH(OH)COO-

CHCOO-

CH2COO-

CH2COO-

C(OH)COO-

CH2COO-

-OOCCH2CH(NH3)COO-+

-OOCCH2CH(NH3)COO-+

Succinate-OOCCH2CH2COO-

Aspartate Aspartate

Malate

Malate

Oxaloacetate

Acetyl-CoA

Acetyl-CoA

Phosphoenolpyruvate

CH3COSCoA

CH3COSCoA

CH2=(OP)COO-

Fatty Acid

Fatty Acid

Citrate

Glyoxylate

Isocitrate

-OOCCH2CH(OH)COO-

OHCCOO-

-OOCCH2COCOO-

CONVERSION OF FAT INTO CARBOHYDRATE IN PLANTS -

THE GLYOXYLATE PATHWAY

FATTY ACID

An important metabolic advantage possessed by plants (and some bacteria) and not by animals is their

ability to convert fat into carbohydrate. This is especially important to seeds, the germination of which

depends on carbohydrates which cannot (in the dark) be formed by photosynthesis. At such stages the

organism is equipped with special organelles, glyoxysomes, the function of which is to make possiblethis conversion. The unique enzymes present in the glyoxysomes are isocitrate lyase which cleaves

isocitrate into succinate and glyoxylate, and malate synthase which enables this glyoxylate to react

with a second molecule of acetyl-CoA to form malate. This malate can then pass through the

mitochondrial membrane into the cytosol where it is oxidised to oxaloacetate for conversion to glucose

by gluconeogenesis.

NAD

NAD

NADH+H+

NADH+H+

UQ

UQH2

2CH3COSCoA + 2NAD+ + UQ + 3H2O -OOCCH2COCOO- + 2HSCoA + 2NADH + 2UQH2 +4H+

Acetyl-CoA Oxaloacetate

4.1.3.1

4.1.3.1 Isocitrate lyase

4.1.3.7

4.1.3.7 Citrate (si)-synthase

4.1.3.2

4.1.3.2 Malate synthase

4.2.1.3

4.2.1.3 Aconitate hydratase

1.1.1.37

1.1.1.37 Malate dehydrogenase

1.1.1.37

4.1.1.32

4.1.1.32

1.3.5.1

1.3.5.1 Succinic dehydrogenase (ubiquinone)

4.2.1.2

4.2.1.2 Fumarate hydratase

2.6.1.1

2.6.1.1 Aspartate transaminase

2.6.1.1

Phosphoenolpyruvate carboxylase

ENZYMES

Designed by Donald Nicholson c 2002 IUBMB