Plant lecture 11 - Crassulacean acid metabolism

Question 1

Crassulacean acid metabolism (CAM) phases

Answer 1

• 2nd form of CO2-conc. mechanism
• Phases
• All occurs in 1 cell
  1. Acidification, net CO2 fixation, PEPC, malic acid ↑ (Dark period)
  2. PEPC → Rubisco
  3. Deacidification, Co2 refixation + RUBISCO
  4. Net Co2 fixation, RUBISCO/PEPC
  (Light period for 2-4, malic E ↓
• Take up most CO2 during night (accumulates malic acid + degrades glucan)
• Transient burst of CO2 fixation during start of day

Question 2

Carbon flow in CAM

Answer 2

Dark

• All C flow is in just mesophyll (C4 = mesophyll + BS)
• CO2 taken up from atmosphere by open stomata at night, hydrated to form carbonic anhydrase bicarbonate. Binds to PEP → OAA (PEPC) → Malate (MDH)
• Can’t use photosynthesis as night
• Some malate equilibrates w/ pool in mit + enters TCA (not lots)
• Want to accumulate malate as a C source for light period. If accumulated in cytosol, would feedback + inhibit PEPC, so needs to be removed to vacuole.
• Accumulates as malic acid + ↓ pH6 to pH3 in vacuole by end of dark period
• ATP needed to drive proton pump to bring H+ into vacuole to ↓ pH
• 1 ATP used to drive proton pump per malate accumulated in vacuole + 0.5 ATP per malate produced in glycolysis = 0.5 per malate

Light
- Efflux of malic E out of vacuole + charge-balancing H+ out
- In cytosol, decarb. by malic E, releases CO2 which is liberated in cell cytoplasm at ↑ conc.
- Then processed in Calvin cycle
- Stomata closed so CO2 builds up to 1% in phase 3 (10x ↑ than C4)
- ↑ CO2 conc. maximises carboxylation of Rubisco + almost completely suppresses oxygenase. TCA v efficient
2 ATP per malate for PPDK, 1 per malate for PGK + 0.5 per malate for AGPase = 3.5ATP

Overall = 4ATP needed + E requirements of Calvin cycle
- More complicated in reality, around 641 reactants

Question 3

C4 vs CAM

Answer 3

• Both ↑ carboxylation efficiency of Rubisco by repressing photorespiration
• Both have evolved many times (>60) independently, strong evidence for functional significance in improving efficiency of photosynthesis
  C4
• Characteristic of warm, high light but not necessarily H20 limited environment
• NADP Mc subtype = found in ↓ arid environment than PEPCK + NAD-Me
• Similar C flow to CAM but PEPC active during light so malate formed + immediately transported from mesophyll to BS where decarboxylated + CO2 conc. around Rubisco
  CAM
• Warm, high light. Almost always assoc. w/ H20-limited environments as closure of stomata in day + confining CO2 to dark period = associated w/ conserving H20
• CO2 fixed by PEPC→OAA→malate, stored in vacuole then releases CO2 in light period + is assimilated through Calvin cycle

Question 4

E.G. of Cam plant environment

Answer 4

• Economic importance e.g. Vanilla planifolia - cultivation in Madagascar

Question 5

Developmental + environmental induction of CAM

Answer 5

• CAM may be facultatively expressed in response to stress e.g. exposed to hot dry conditions, has epidermal storage cells that can take up NaCl
• Can be used to investigate regulation

Question 6

Regulation of PEPC

Could add circadian clocks if want!!!!

Answer 6

• Ideally, PEPC is active in dark + off in light
• Max catalytic capacity actually x change that much day-night
• Instead, malate sensitivity of PEPC (Ki malate) is ↓ at night than day. Dramatic change, 5% activity in 2mM malate in day vs 80% at night
• Substrate affinity of PEPC is higher at night than during day (0.2mm in night vs 0.8mm in day for 1/2 max activity)
• Night time E = PEPC, malic-acid accumulation
• Day time = malic-acid release, decarboxylation, Rubisco