Carbohydrate Biosynthesis in Plants
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Transcript of Carbohydrate Biosynthesis in Plants
Carbohydrate Biosynthesis in Plants
CH353 January 15, 2008
Overview of Plant Metabolism
Overview of Carbon Assimilation
Occurs in Chloroplasts
Stage 1: Fixation• 1 step – RUBISCO
unique to plantsStage 2: Reduction• 3 steps – analogous
to gluconeogenesis (uses NADPH)
Stage 3: Regeneration• 9 steps; 7 enzymes
analogous to pentose phosphate pathway
• Stage 1: FixationRubisco
ribulose 1,5-bisphosphate + CO2 → 2 3-phosphoglycerate
• Stage 2: Reduction3-phosphoglycerate kinase
3-phosphoglycerate + ATP → 1,3-bisphosphoglycerate + ADP
glyceraldehyde 3-phosphate dehydrogenase1,3-bisphosphoglycerate + NADPH →
glyceraldehyde 3-phosphate + NADP+ + Pi
triose phosphate isomeraseglyceraldehyde 3-phosphate ↔ dihydroxyacetone phosphate
Stages of Carbon Assimilation
Carbon Assimilation Stage 3:Regeneration of Acceptor
Transketolase Reactions
• Transketolases transfer “active aldehyde” from a ketose (donor) to an aldose (acceptor) with cofactor thiamine pyrophosphate (TPP)
• Transketolase reactions for carbon assimilation in chloroplast are identical to those for pentose phosphate pathway in cytosol
Donor1 Acceptor1 Acceptor2 Donor2
TPP
orSedoheptulose
7-phosphate
orRibose
5-phosphate
Transaldolase Reaction
• Transaldolases transfer dihydroxy-acetone phosphate (donor) to an aldose (acceptor) forming an aldol condensation adduct
• Involves Schiff base enzyme bound intermediate
• Transaldolase reaction (pictured) is identical to aldolase reaction in glycolysis/gluconeogenesis; other is unique to carbon assimilation
• Donor: dihydroxyacetone phosphate• Acceptors: erythrose 4-phosphate
and glyceraldehyde 3-phosphate
Sedoheptulose 1,7-bisphosphateor
orErythrose 4-phosphate
↓↑
+
Donor Acceptor
Stage 3: Regeneration of Acceptor
glyceraldehyde 3-phosphate
transaldolase ↑↓ + dihydroxyacetone phosphate
fructose 1,6-bisphosphatebisphosphatase ↓ - Pi
fructose 6-phosphate
transketolase ↑↓ + glyceraldehyde 3-phosphate
erythrose 4-phosphate + xylulose 5-phosphate
transaldolase ↑↓ + dihydroxyacetone phosphate
sedoheptulose 1,7-bisphosphatebisphosphatase ↓ - Pi
sedoheptulose 7-phosphate
transketolase ↑↓ + glyceraldehyde 3-phosphate
ribose 5-phosphate + xylulose 5-phosphate
transaldolase has same ketose as substrate
transketolase has same aldose as substrate
bisphosphatases make process irreversible
Stage 3: Regeneration of Acceptor
2 xylulose 5-phosphate 1 ribose 5-phosphate
ribulose 5-phosphate epimerase ↑↓ ↑↓ ribose 5-phosphate isomerase
3 ribulose 5-phosphateribulose 5-phosphate kinase ↓ + 3 ATP → 3 ADP
3 ribulose 1,5-bisphosphate
Stage 3 Net:
Input: 15 C Output: 15 C
2 dihydroxyacetone phosphate 3 ribulose 1,5-bisphosphate 3 glyceraldehyde 3-phosphate 3 ADP3 ATP 2 Pi
Stoichiometry of Carbon Assimilation
• Assimilation of 3 carbons and 1 phosphorous per cycle
• Inorganic phosphate must be replaced for sustained ATP synthesis in chloroplast
Overall Process:3 CO2 + 9 ATP + 6 NADPH → glyceraldehyde 3-phosphate + 9 ADP + 6 NADP+ + 8 Pi
Phosphate–Triose Phosphate Antiporter• Exchanges dihydroxyacetone phosphate or 3-phosphoglycerate
for phosphate• In light: triose phosphate transported to cytosol with antiport of
phosphate to chloroplast stroma• Phosphate is released in cytosol with sucrose biosynthesis
ATP and Reducing Equivalents Exchange
• Exchange of ATP and reducing equivalents mediated by antiporter
• only 3-phosphoglycerate or dihydroxyacetone phosphate transported
• ATP and NADPH used on stromal side and ATP and NADH generated on cytosolic side
• no net flux of phosphate or triose phosphate
Regulation of Enzymes
• Rubisco– Rubisco activase removes substrate from inactive enzyme
(ATP hydrolyzed)– Carbamoylation of active site lysine (CO2 + Mg+2)– Nocturnal inhibitor binds
• Photosynthetic environment in chloroplast stroma↑ NADPH ↑ pH ↑ Mg2+
– Conditions stimulate enzyme activity– Rubisco activation (carbamoyllysine formation) is faster– Fructose 1,6-bisphosphatase activity ↑ 100x with illumination
• Reduction of enzymesRS–SR’ → RSH + HSR’
Regulation of Enzymes
• Photosynthetic environment in chloroplast stroma↑ NADPH ↑ pH ↑ Mg2+
Effect of pH and [Mg2+] on activity of fructose 1,6-bisphosphatase
Regulation of Enzymes
Activated by Reduction of Disulfides• glyceraldehyde 3-phosphate dehydrogenase• fructose 1,6-bisphosphatase• sedoheptulose 1,7-bisphosphatase• ribulose 5-phosphate kinaseInactivated by Reduction:• glucose 6-phosphate dehydrogenase
sulfhydryls(reduced)
disulfides(oxidized)
Rubisco Oxygenase Activity
• Rubisco accepts both CO2 and O2 as substrates
• Incorporation of O2 into ribulose 1,5-bisphosphate produces:– 3-phosphoglycerate– 2-phosphoglycolate
• No fixation of CO2 • Requires 2-phosphoglycolate
salvage
Glycolate Pathway
• Salvage of 2-phosphoglycolate• Involves metabolite transport and
enzymes in chloroplast, peroxisome and mitochondrion
• Glycine decarboxylase is key enzyme
• Process consumes O2 and evolves CO2 “Photorespiration”
• Wastes energy and fixed carbon and nitrogen
C4 Pathway
• Rubisco oxygenase activity favored by high temperature/low moisture environments
• C4 plants separate fixation of HCO3
- and CO2 in different but metabolically-linked cells
• Requires more energy (2 ATP’s) but avoids wasteful oxygenase reaction
• CAM plants temporally separate 2 fixations (store malate at night)
Starch and Sucrose Biosynthesis
Starch Biosynthesis• Carbohydrate storage• Occurs in plastids• ADP-glucose substrate• Adds to reducing end (unlike
glycogen synthesis)• α(1→4) glucose (amylose)
with α(1→6) branches (amylopectin)
Sucrose Biosynthesis• Carbohydrate transport• Occurs in cytoplasm• Fructose 6-phosphate &
UDP-glucose • Joins reducing (anomeric)
hydroxyls• Glucose(α1↔β2)Fructose
• Excessive amounts of triose and monosaccharide phosphates are converted to alternative forms in the light
• Liberates phosphate for ATP synthesis
Cellulose Biosynthesis
• Cell wall structure• Occurs in cytoplasm and at plasma membrane• Lipid-linked carrier and membrane protein complex• UDP-glucose is generated from sucrose and UDP by
sucrose synthase • UDP-glucose is substrate for cellulose synthase;
adds glucose monomers to non-reducing end • Cellulose is β(1→4) linked glucose
Regulation of Sucrose Biosynthesis
• Need phosphate for ATP synthesis and triose phosphate for carbon fixation
• Fructose 2,6-bisphosphate (F2,6BP) activates pyrophosphate-dependent phosphofructokinase-1 (PP-PFK-1) and inhibits fructose bisphosphatase-1 (FBPase-1)
• Its synthesis by phosphofructokinase-2 is inhibited by triose phosphates (light) and activated by phosphate (dark)
• In dark: ↑ Pi, ↑ F2,6BP, ↑ F1,6BP → glycolysis
• In light: ↑ triose phosphates, ↓ F2,6BP, ↑ F6P → sucrose biosynthesis
Regulation of Sucrose Biosynthesis
• Sucrose 6-phosphate synthase (SPS) is partially inactivated by phosphorylation by SPS kinase
• In light: glucose 6-phosphate (high gluconeogenesis) directly stimulates SPS and inhibits SPS kinase activating SPS (sucrose biosynthesis)
• In dark: phosphate directly inhibits SPS and inhibits SPS phosphatase inactivating SPS (no sucrose biosynthesis)
Regulation of Starch Biosynthesis
ADP-glucose pyrophosphorylase synthesizes starch precursor• inhibited by high [Pi] accumulating in the dark (ATP hydrolysis)• activated by high [3-phosphoglycerate] accumulating in the light
(carbon assimilation; diminished sucrose biosynthesis)
Gluconeogenesis from Fats
• Germinating seeds convert stored fats into sucrose
• β-oxidation (glyoxysome) fatty acid → acetyl-CoA
• glyoxylate cycle converts 2 acetyl-CoA → succinate
• mitochondrial citric acid cycle & cytoplasmic gluconeogenesis converts succinate → hexoses