Plant Carbohydrate Biosynthesis Glyoxylate cycle

1
Plant Carbohydrate Biosynthesis

• Glyoxylate cycle
• Biosynthesis of starch and sucrose
• Synthesis of cell wall polysaccharides
• Integration of carbohydrate metabolism in the
  plant cell

2
Glyoxylate cycle shares some enzymes with citric
acid cycle
p.624
acetyl-CoA
oxaloacetate
Citrate synthase
citrate
CoA-SH
citrate
Malate dehydrogenase
NADH
oxaloacetate
malate
NAD
Aconitase
isocitrate
Malate synthase
malate
CoA-SH
isocitrate
glyoxylate
Isocitrate lyase
Succinate
glyoxylate
acetyl-CoA
3
Plants use glyoxylate cycle to convert lipids to
carbohydrates
4
Citric acid cycle and glyoxylate cycle are
regulated reciprocally
isocitrate
Glyoxylate cycle
Citric acid cycle
P
Isocitrate lyase
Isocitrate DHase
a-KG
PPase
PK
Intermediates of citric acid cycle, glycolysis,
AMP, ADP
5
Starch biosynthesis is growing from reducing end
6
Sucrose biosynthesis
p. 773
UDP-glucose
F 6-P
Sucrose 6-phosphate synthase
Sucrose 6-phosphate
UDP

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

Sucrose 6-phosphate phosphatase
Sucrose
Pi
7
Calvin cycle
G 3-P
ATP NADPH
DHAP
Pi
DHAP
G 3-P
F 1,6-BP
F 6-P
Pi
G 1-P
2Pi
UDP-Glc
F 6-P
UTP
PPi
G 6-P
Sucrose 6-P
UDP
Sucrose
Pi
8
Regulation of sucrose biosynthesis

• FBPase-1/PP-PFK-1
• Sucrose 6-phosphate synthase

9
FBPase-1/PP-PFK-1

• FBPase-1 and PP-PFK-1 are regulated indirectly by
  the products of photosynthesis and oxidative
  phosphorylation.

p. 773
10
Calvin cycle
G 3-P
ATP NADPH
DHAP
F 1,6-BP
F 2,6-BP
PFK-2
FBPase-2
PFK-1
FBPase-1
Sucrose
F 6-P
F 6-P
p. 773
11
Calvin cycle
ADP
ATP
Pi
DHAPG 3-P
F 1,6-BP
F 2,6-BP
PFK-2
FBPase-2
PFK-1
FBPase-1
F 6-P
F 6-P
p. 773
12
Sucrose 6-phosphate synthase is also regulated
G 6-P
Pi

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

P
Sucrose 6-phosphate synthase
SPS kinase
SPS PPase
13
Starch biosynthesis is regulated by ADP-glucose
pyrophosphorylase
14
Plant cell wall biosynthesis

• Plant cell wall is made of cellulose
  microfibrils, which is consisted of about 36
  chains of cellulose, a polymer of b(1?4)glucose.

15
Cellulose biosynthesis

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

16
Terminal complex (rosette)
p.777
17
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.

18
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
19
New cellulose chain synthesis (1)

• Intracellular cellulose synthase adds several
  more glucose residues to the first one, in (b1?4)
  linkage, forming a short oligosacchairde chain
  attached to the sitosterol (sitosterol dextrin).

20
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-b-glucanase.

21
New cellulose chain synthesis (3)

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

22
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).

23
New cellulose chain synthesis (5)

• The glucose associated with UDP is a-linked.
• Its configuration will be converted by
  glycosyltransferases so the product (cellulose)
  is b-linked.

24
Biosynthesis of peptidoglycan

• Peptidoglycan is the major component of bacterial
  cell wall.

25
Peptidoglycan synthesis (1)

• N-acetylglucosamine (GlcNAc) condenses with UTP
  to form UDP-GlcNAc.
• UDP-GlcNAc reacts with PEP to form UDP-Mur2Ac.
• Five amino acids are then added.

26
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.

27
Peptidoglycan synthesis (3)

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

28
Peptidoglycan synthesis (4)

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

29
Penicillin inhibit transpeptidase

• Penicillins and related antibiotics contain the
  b-lactam ring.
• Different substitution at position 6 determines
  their differential pharmacological properties.

30
Acid stable
Acid labile
31
Penicillin actions

• Penicillin acts as suicide inhibitors for
  transpeptidase.

32
b-lactamase inactivates penicillin

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

33
glycolysis
PPP
photosynthesis