Plant lecture 9 - Photorespiration

Question 1

Problems with Rubisco

Answer 1

• Slow turnover 3s-1
• ↑ Kc so typically -50% saturated w/ CO2 in C3 plants
• Rather poor selectivity for CO2 via O2 under physiological conditions

Question 2

Experimental evidence for photorespiration

Answer 2

1. O2 inhibition of photosynthesis
   - Soybean plants maintained at 2 different CO2 conc. (275ppm + 73ppm)
   - As [O2] ↑, rate of CO2 uptake ↓
   - For 73ppm, CO2 uptake became -ve
2. Post-illumination ‘burs’ of CO2
   - Measured gas exchange characteristics of a leaf
   - Leaf initially exposed to v ↑ light intensity + exposed to successfully lower intensities
   - W/ each transition, photosynthesis rapidly ↓ + ‘overshoots’. Balanced back to new steady state- post illumination burst
   - Thought light-dependent transport processes immediately change but takes longer for other metabolic processes to adjust

Question 3

Properties of Rubisco + Structure

Answer 3

• Carboxylase: RuBP + CO2 → glycerate 3P + O2
• Oxygenase: RuBP + O2 → phosphoglycolate
• Balance btw photoresp: photosynthesis ↑ w/ ↑ temperature
• At ↑ temp, V(O2) ↑ more than V(CO2) + solubility of CO2 ↓ more than O2

Structure

• 550kDa hexadecameric structure
• 8 large subunits (LSU), 55kDa
• 8 small subunits (SSU), 13-15kDa
• Tetramer of LSU capped by pair of SSU tetramers

Question 4

Photorespiration

Answer 4

• In chloroplast:
• RuBP + O2 0 → phosphoglycolate (toxic)
• Immediately dephosph. → glycolate (phosphoglycolate phosphatase)
• Moves to peroxisome
• In peroxisome:
• Glycolate ox to glyoxylate which is converted by transaminase to glycine
• Glycine moves into mit.
• In mit.
• Glycine decarboxylation releases CO2
• Serine formed passes back into peroxisome.
• Transaminated to hydroxypyruvate which is reduced to glycerate (hydroxypyruvate reductase)
• Glycerate moves back into chloroplast in exchange for glycolate
• Glycolate → glycerate 3P + returns to cytoplasm

Question 5

2 key E of photoresp.

Answer 5

1. Glutamine synthase
   - Chloroplast
   - Assimilates NH3 into organic form
   Glutamate + NH3 + ATP → Glutamine + ADP + Pi
2. Glycine decarboxylase
   - Mitochondria
   - 4 protein system, ↓ Glycolic = ↑ GDC activity, ↓ levels of CO2 acceptor RubP so important feedback signal
   - Releases CO2 + NH3 (toxic if accumulate)
   2 Glycine + H2O + NAD+ → 1 Serine + CO2 + NH3 + NADH

Question 6

Purpose + cost of photorespiration

Answer 6

• Detoxifies phosphoglycolate
• In glycine decarboxylase reaction, 1 CO2 released, 3CO2 recycled
• Can salvage 75% of C that would be lost w/o complicated mechanisms (25% CO2 lost directly tho)
• Some benefits e.g. produces Ser + dissipates excess excitation E from chloroplast to mit.

But metabolic cost

• ATP:glycerate kinase in final step + glutamine synthetase reaction = essential for re-assimilation of NH3
• Reduced ferredoxin needed for GOGAT reaction
• H202 in peroxisome, NH3 need to be detoxified

Question 7

Why does photorespiration exist

Answer 7

• If so disadvantageous, would be lost w/ evolution
• Kozaki + Takeba experiment 1996:
• Hypothesised E cost of photoresp. in certain times could be adv. if plant has excess photon E or reducing equiv.
• Transgenic tobacco plants either ↑ or ↓ of GS + control
• Antisense GS show ↓ post illumination bursts of CO2 when lights switched off
• Control = normal
• GS overexpression = enhanced post illumination bursts of CO2 output
• Examined different responses to photoinhib. damage in CO2-free air→ GS over expressing = less reductions in e- transport rate/ETR, control = progressive decline, GS antisense = greater reduction in ETR in high light
• Consistent w/ photoresp. playing role in ↓ photoinhibitroy damage to e- transport system under high light

Question 8

Is Rubisco lazy

+

Evidence Rubisco specificity = evolutionary selection

Answer 8

• Early relic as early atmosphere had ↓ O2 so not influenced by ability to distinguish btw
• 1000x ↑ selectivity for CO2 than o2. But O2 conc. = 500x that of CO2 in air
• Specificity factor = Kcat/Km
• Anaerobic Bacteria = ↑ CO2, ↓ O2. Sc/o = 6-41
• C4 higher plants w/ CCM, Sc/o = 70-82
• Given evolutionary span, rather limited range of values, thought Rubisco operates under restraint. But range shows can respond to selection pressures
• Variation = compromise: a TS for CO2 aids discrimination btw CO2 + O2 (↑ for CO2) but resemblance to 6C carboxyketone intermediate causes the int. to bind so tightly, catalytic max = restricted
• Inverse relationship btw Sc/o + Kcat (as make TS ↑ specific to CO2, ↓ opportunity for tighter binding/ ↓ ROR)

Question 9

Rate limiting step (RLS) Rubisco photorespiration

Answer 9

• Step where E-substrate complex is resolved to release 2 3PGA molecules = kinetic RLS
• Trade-off to max. selectivity of CO2 - make resolving step difficult

Question 10

Nitrate assimilation + photorespiration

Answer 10

• Inhibition of photosynthesis inhibits nitrate assimilation + absorption (Bloom et al 2010)
• Most plants get N from environment in form of inorganic nitrate or ammonium + legumes can assimilate N2
• Used 3 conditions: normal O2+ CO2 (1), ↑ CO2 + normal O2 (2), normal CO2, ↓ O2 (3)
• 2+3 suppress photoresp + N assimilation
• Possible as in cytosol photorespiration produces reducing equiv. needed for N assimilation
• Change in assimilatory quotient which reflects shoot NO3- assimilation is ↓ at ↑ CO2
• Photoresp. = embedded in N assimilation