Plant Carbohydrate Biosynthesis

1
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

2
Glyoxylate cycle shares some enzymes with citric
acid cycle
p.624
3
Plants use glyoxylate cycle to convert lipids to
carbohydrates
4
Citric acid cycle and glyoxylate cycle are
regulated reciprocally
5
Starch biosynthesis is growing from reducing end
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
Sucrose biosynthesis

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

11
Sucrose biosynthesis

• Sucrose biosynthesis is beginning with
  dihydroxyacetone phosphate exported from the
  chloroplast.
• Dihydroxyacetone phosphate is then converted to
  glyceraldehyde 3-phosphate by triose phosphate
  isomerase.

12
Sucrose biosynthesis

• After condensation of glyceraldehyde 3-phosphate
  and dihydroxyacetone phosphate by aldolase,
  fructose 1,6-bisphosphate is dephosphorylated by
  FBPase-1 to produce fructose 6-phosphate.

13
Sucrose 6-phosphate synthase catalyze the
formation of sucrose 6-phosphate
14
Sucrose 6-phosphate phosphatase catalyze the
formation of sucrose by dephosphorylation
15
Regulation of sucrose biosynthesis

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

16
FBPase-1/PP-PFK-1

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

17
(No Transcript)
18
(No Transcript)
19
Sucrose 6-phosphate synthase is also regulated

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

20
Starch biosynthesis is regulated by ADP-glucose
pyrophosphorylase
21
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.

22
Cellulose biosynthesis

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

23
Terminal complex (rosette)
p.777
24
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.

25
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
26
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).

27
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.

28
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.

29
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).

30
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.

31
Biosynthesis of peptidoglycan

• Peptidoglycan is the major component of bacterial
  cell wall.

32
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.

33
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.

34
Peptidoglycan synthesis (3)

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

35
Peptidoglycan synthesis (4)

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

36
Penicillin inhibit transpeptidase

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

37
Acid stable
Acid labile
38
Penicillin actions

• Penicillin acts as suicide inhibitors for
  transpeptidase.

39
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.

40
glycolysis
PPP
photosynthesis