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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
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
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
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.
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.
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.
• 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