Plant Carbohydrate Biosynthesis

1. Glyoxylate cycle

2. Biosynthesis of starch and sucrose

3. Synthesis of cell wall polysaccharides

4. Integration of carbohydrate metabolism in the plant cell

Glyoxylate cycle shares some enzymes with citric acid cycle

p.624

Citrate synthase

Aconitase

Isocitrate lyase

Malate synthase

Malate dehydrogenase

oxaloacetate

acetyl-CoA

citrate

isocitrate

glyoxylateSuccinate

malateNAD+

NADHcitrate

CoA-SH

isocitrate

glyoxylateacetyl-CoA

malateCoA-SH

oxaloacetate

Plants use glyoxylate cycle to convert lipids to carbohydrates

Citric acid cycle and glyoxylate cycle are regulated reciprocally

Glyoxylate cycle Citric acid cycle

Isocitrate lyase

isocitrate

Intermediates of citric acid cycle, glycolysis, AMP, ADP

PK

PPase

P-KGIsocitrate DHase

GlcoGlco

Glco

Glco

Glco

Starch synthase

Xa

Xb

PP

PPGlco

Glco

Glco

GlcoGlco

PPGlco

Glco

Glco

PPGlco

GlcoGlco

Glco

Glco

Glco

PPGlco

PPGlco

Glco

Glco

Glco

Glco

Glco

Starch biosynthesis is growing from reducing end

Sucrose biosynthesis

• Sucrose is synthesized in cytosol by sucrose 6-phosphate synthase and sucrose 6-phosphate phosphatase.

p. 773UDP-glucose F 6-P

Sucrose 6-phosphateUDP

SucrosePi

Sucrose 6-phosphate synthase

Sucrose 6-phosphate phosphatase

UDP-Glc

G 3-PATPNADPH

Calvincycle

DHAP

Pi DHAP G 3-P

F 1,6-BPF 6-PPi

G 6-PF 6-P

G 1-P

UTPPPi

2Pi

Sucrose 6-PUDP SucrosePi

Regulation of sucrose biosynthesis

FBPase-1/PP-PFK-1

Sucrose 6-phosphate synthase

FBPase-1/PP-PFK-1 • FBPase-1 and PP-PFK-1

are regulated indirectly by the products of photosynthesis and oxidative phosphorylation.

p. 773

CalvincycleG 3-PATP

NADPH

DHAP

F 2,6-BP

F 6-P

PF

K-2

FB

Pase-2

Sucrose

F 1,6-BP

F 6-P

PF

K-1

FB

Pase-1

p. 773

ADP

Calvincycle

F 2,6-BP

F 6-P

PF

K-2

FB

Pase-2

DHAP+G 3-P

F 1,6-BP

F 6-P

PF

K-1

FB

Pase-1

ATP Pi

p. 773

Sucrose 6-phosphate synthase

is also regulated

• Sucrose 6-phosphate synthase is regulate by phosphorylation/dephosphorylation.

Sucrose 6-phosphate synthase

P

SPS kinase

SPS PPase

G 6-P Pi

Starch biosynthesis is regulated by ADP-glucose

pyrophosphorylase

Plant cell wall biosynthesis

• Plant cell wall is made of cellulose microfibrils, which is consisted of about 36 chains of cellulose, a polymer of (14)glucose.

Cellulose biosynthesis

• Cellulose is synthesized by terminal complexes or rosettes, consisting of cellulose synthase and associated enzymes.

Terminal complex (rosette)

p.777

Cellulose synthase

• Cellulose synthase has not been isolated in its active form, but from the hydropathy plots deduced from its amino acid sequence it was predicted to have eight transmembrane segments, connected by short loops on the outside, and several longer loops exposed to the cytosol.

Initiation of new cellulose chain

synthesis• Glucose is

transferred from UDP-glucose to a membrane lipid (probably sitosterol) on the inner face of the plasma membrane.

p.776

New cellulose chain synthesis (1)

• Intracellular cellulose synthase adds several more glucose residues to the first one, in (14) linkage, forming a short oligosacchairde chain attached to the sitosterol (sitosterol dextrin).

New cellulose chain synthesis (2)

• Next, the whole sitosterol dextrin flips across to the outer face of the plasma membrane, where most of the polysaccharide chain is removed by endo-1,4--glucanase.

New cellulose chain synthesis (3)

• The dextrin primer (removed from sitosterol by endo-1,4--glucanase) is now (covalently) attached to another form of cellulose synthase.

New cellulose chain synthesis (4)

• The UDP-glucose used for cellulose synthesis is generated from sucrose produced from photosynthesis, by the reaction catalyzed by sucrose synthase (this enzyme is wrongly named).

New cellulose chain synthesis (5)

• The glucose associated with UDP is -linked.

• Its configuration will be converted by glycosyltransferases so the product (cellulose) is -linked.

Biosynthesis of peptidoglycan

• Peptidoglycan is the major component of bacterial cell wall.

Peptidoglycan synthesis (1)1. N-acetylglucosamine

(GlcNAc) condenses with UTP to form UDP-GlcNAc.

2. UDP-GlcNAc reacts with PEP to form UDP-Mur2Ac.

3. Five amino acids are then added.

Peptidoglycan synthesis (2)4. The Mur2Ac-

pentapaptide moiety is then transferred from UDP to dolichol.

5. Another GlcNAc is added to this molecule.

6. Five glycines are added to the lys residue of the pentapeptide.

7. The whole disaccharide decapeptide is added to the nonreducing end of an existing peptidoglycan molecule.

Peptidoglycan synthesis (3)

Peptidoglycan synthesis (4)

8. Transpeptidase catalyze a transpeptidation reaction to crosslink adjacent polysaccharide chains.

Penicillin inhibit transpeptidase

• Penicillins and related antibiotics contain the -lactam ring.

• Different substitution at position 6 determines their differential pharmacological properties.

Acid stable

Acid labile

Penicillin actions

• Penicillin acts as suicide inhibitors for transpeptidase.

• A -lactamase froms a temporary covalent adduct with the carboxyl group of the opened -lactam ring, which is immediately hydrolyzed, regenerating active enzyme.

-lactamase inactivates penicillin

photosynthesis

PPP

glycolysis