Assimilation of Carbon in Plants

Module 0210101:

Molecular Biology and Biochemistry of the Cell

Lecture 18

Dale Sanders

17 March 2009

Objectives

By the end of the lecture you should understand:

1. How ribulose 1,5-bisphosphate can be regenerated after C-fixation;

2. The energetic cost of fixation of inorganic C to fructose 6-phosphate;

3. The pathways for metabolism of fructose 6-phosphate into the end-products:

(a) starch,

(b) sucrose;

4. What photorespiration is, and how it arises;

Reading

Besides the standard big biochemistry textbooks:

Buchanan, BB et al (2000) Biochemistry &Molecular Biology of Plants. pp. 630-675

Assimilation of Carbon in PlantsThe DARK reactions of photosynthesis:

1. CO2 fixation….. Rubisco

CO2 + Ribulose 1,5–bisphosphate 2 (3-phosphoglycerate)

1 + 5 2 x 3

2. Reduction …….. Gluconeogenesis

2 (3- Phosphoglycerate) Fructose 6-Phosphate

2 ATP 2ADP

2 NADPH 2 NADP+

2 x 3 6

3. Regeneration of Ribulose 1,5–bisphosphate ….

occurs via the CALVIN CYCLE

• Analogy with Pentose phosphate pathway:

making a 5C sugar from 6C + 3C sugars.

• Enzymes involved

– Transketolase: transfers a 2C unit from a ketose to an aldose

– Aldolase: Condensation between DHAP + an aldehyde

• These reactions are + at equilibrium

• Cycle “driven” by reactions with large –ve ΔG:

– Rubisco;

– Fructose 1,6-bisphosphatase;

– Phosphoribulose kinase

Energetics of CO2 Assimilation to

Fructose 6-phosphate (Fru 6-P)

From 6 CO2 …

1. 1 ATP consumed in phosphorylating each mole of 3-PGA

Since 6 CO2 + 6 Ru1,5-BP 12 PGA.

12 ATP consumed here

2. 1 NADPH consumed in reducing each mole of

1,3-BPG

12 NADPH consumed here

3. 1 ATP consumed in regenerating each mole ofRu1,5–BP in the Calvin cycle.

Since 6 Ru 1,5-BP required for fixation of 6 CO2

6 ATP consumed here

i.e. 18 ATP, 12 NADPH consumed/6 CO2 fixed

or 3 ATP & 2 NADPH/CO2 fixed.

The Fate of Fixed CO21. Starch synthesis

Starch: a storage polysaccharide, very similar to glycogenThe 2 component polymers:

• α-amylose: unbranched α 1 4 linked glucose

• amylopectin: Glucose residue backbone with α 1 4 linksbranch points: α 1 6 links

Starch synthesis is mainly in the chloroplast stroma

Fru 6-P Glu 6-P Glu 1–P

Phosphogluco Phosphogluco

ISOMERASE MUTASE

The 2 key-reactions in starch synthesis:ADP-glucose

Glu 1–P + ATP ADP–Glu + PPi

pyrophosphorylase

starch

ADP-Glu + (glucose)n (glucose)n + 1 + ADP

synthetase

Synthesis favoured when CO2 fixation rate exceeds the utilization rate ofreduced C

2. Sucrose synthesis occurs in the cytosol.

Two stages:

(a) export of reduced CO2 from the stroma

exchanger at inner envelope membrane ensures that P exportedas triose- is replenished (ultimately for phosphorylation ofADP).

Pi (ie DHAP)Triose P

(b) sucrose synthesis from triose- in the cytosol:

Sucrose: The major mobilizable sugar in plants: moved fromleaves developing tissues

roots, as respiratory metabolite

(4)Triose P (2)Fru 6 - P

Glu 1 - P

UDP – Glucose

UDP

UTP

PPi

UDP-Glucosepyrophosphorylase

Sucrose Psynthase

Sucrose 6 – P

Sucrose

PiO O

O

HOCH2

HOCH2

CH OH2

OH

OHOH

OH

HO

aldolase

This is the oxygenase function of Rubisco

The reaction is “unwanted”: an evolutionaryconsequence of [O2 ] in the atmosphere.

Rubisco is not a perfect enzyme -Photorespiration

Besides fixing CO2, Rubisco fixes O2!

Rubisco is not a perfect enzyme -Photorespiration

Ru 1,5-BP

– C – O – P

C = O

– C – OH

– C – OH

– C – O – P

O2

Rubisco+

– C – O – P

C

–O O + H2O + 2H+

Phosphoglycolate

O O–

C

– C – OH

– C – O – P3-PGA

+ H2O

O2 competes with CO2 at active site.

Recovery of C Skeletons Associated withPhosphoglycolate Formation

N.B 1. Processwasteful because C isoxidized and lost asCO2 .

2. However, ¾ of Crecovered as PGAsince glycerate inperoxisome can berecycled tochloroplast andphosphorylated.

Buchanan et al. (2000)

Summary

1. The Calvin cycle regenerates ribulose 1,5-bisphosphate as substrate for Rubisco.

2. The energetic cost of synthesizing hexose-phosphate from CO2 is 18 ATP hydrolysed and12 NADPH oxidized.

3. Fru 6P is the starting point for starch synthesis(chloroplast) and sucrose synthesis (cytosol).

4. Photorespiration involves the recovery of Cskeletons otherwise “lost” as a result ofoxygenase function of Rubisco.