Carbon Fixation: The C3 and C4 Pathways

The net reaction of three turns of the Calvin cycle can be written as:

(3 CO2) + 3 RuBP + (6 NADPH) + (9 ATP) + 6 H2O →(1 GAP) + 3 RuBP + 6 NADP+ + 9 ADP + 9 Pi

CAM Pathway During the day

, the CO2 is released, allowing the Calvin Cycle to fix the CO2 into carbohydrate.

3 CO2 + 3 RuBP + 6 NADPH + 9 ATP + 6 H2O →

1 GAP + 3 RuBP + 6 NADP+ + 9 ADP + 9 Pi

Light regulates Calvin cycle enzymes by two primary mechanisms?

1) increased Rubisco activity in response to elevated pH and Mg2+ in the stroma, and 2) thioredoxin-mediated reduction of disulfide bonds.

Transketolase transfers a

2-C fragment

Transaldolase transfers a

3-C fragment.

In stage 2, 3-phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase convert

3-PGA to GAP

The C3 pathway converts CO2 and RuBP to

3-phosphoglycerate. This reaction is the first step of the Calvin cycle and is catalyzed by the enzyme Rubisco.

cost of c4 pathway

Note that two high energy phosphate bonds are required to convert pyruvate to phosphoenolpyruvate in mesophyll cells (PPi --> 2 Pi).

The most abundant enzyme on the planet?

Rubisco Rubisco is a SLOW enzyme: only 3 molecules of CO2 are fixed per second per molecule of Rubisco at 25 °C.

Wasteful photorespiration: Rubisco is also an oxygenase

Rubisco also catalyzes a oxygenase reaction that combines RuBP with O2 to generate one molecule of 3-phosphoglycerate (C3) and one molecule of 2-phosphoglycolate

photorespiration

Rubisco oxygenation reaction, and the glycolate pathway, are together called photorespiration because O2 is consumed and CO2 is released.

What is GAP used for?

Starch synthesis, stromal glycolytic pathway and export to plant tissues. Glycotyic pathway

the Calvin Cycle is the most active during the

daylight hours when ATP and NADPH are plentiful.

In the final stage of the Calvin cycle, a series of enzyme reactions convert

five C3 molecules (GAP or DHAP) into three C5 molecules (RuBP) to replenish supplies of this CO2 acceptor molecule.

The product of the carbon fixation is

glyceraldehyde-3P (GAP) which is converted to hexose sugars for use as chemical energy at night.

At night, plant cells rely on what to generate ATP for cellular processes

glycolysis and mitochondrial aerobic respiration

Plants such as sugarcane and crabgrass thrive under high temperature conditions by

having very low levels of photorespiration.

Plants in hot, sunny, climates are especially susceptible to photorespiration due to

high O2:CO2 ratios under these conditions. O2 is more soluble than CO2 at high temperatures, which raises the dissolved O2:CO2 ratio causing more photorespiration.

Light activation of photosynthetic electron transport system causes stromal pH to

increase from pH 7 to pH 8 due to proton pumping.

The Calvin Cycle generates triose phosphates, which are used to

make hexose sugars 1) sucrose for transport to other plant tissues 2) starch for energy stores within the cell 3) cellulose for cell wall synthesis 4) pentose phosphates for metabolic intermediates

The C4 and CAM carbon fixation pathways minimize the effects of

photorespiration by increasing biochemical concentrations of CO2.

If glycolysis, the pentose phosphate pathway, and the Calvin cycle were all active at the same time, then simultaneous starch degradation and carbohydrate biosynthesis would

quickly deplete the ATP and NADPH pools in the stroma

In the absence of light, Calvin cycle enzymes have

reduced activity and flux through the Calvin cycle is decreased dramatically.

These particular Calvin Cycle enzymes are active in the

reduced state, i.e., when thioredoxin is reduced by photosynthetic electron transport system.

However, when the sun goes down,

spontaneous oxidation leads to their inactivation

An additional 3 ATP are used in

stage 3 to regenerate these 3 RuBP molecules

One GAP is used to make

sugar by gluconeogenesis

bundle sheath cells use CO2 released from

the C4 intermediate malate to carry out the Calvin cycle reactions.

This "separation in space" between the two cell types essentially eliminates

the oxygenase reaction in rubisco and thereby blocks photorespiration.

Stage 1: CO2 fixation to RuBP to form

two 3-PGA. For every 3 CO2 that are fixed by carboxylation of 3 RuBP molecules, 6 3-phosphoglycerate are generated by aldol cleavage

The C4 pathway in tropical plants such as sugarcane that utilize

two separate cell types to reduce photorespiration during the day.

Why is photo-respiration wasteful?

unlike mitochondrial respiration, photorespiration requires energy input and is therefore considered a wasteful pathway in photosynthetic cells.

The rubisco reaction is

very exergonic (ΔG⁰' = -35.1 kJ/mol), with the aldol cleavage step being a major contributor to the favorable change in free energy

Photorespiration is a

wasteful side reaction of Rubisco that uses O2 to generate 2-phosphoglycolate instead of CO2 to produce GAP.

CAM Pathway During the night

when the stomata are open, CO2 is captured by the mesophyll cells and incorporated into OAA by PEP carboxylase. Calvin cycle is inactive.

Hatch-Slack pathway,

which involve the carboxylation of phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase to form oxaloacetate (OAA), a four carbon (C4) intermediate that serves as a transient CO2 carrier molecule.

Two variations of the "Hatch-Slack" pathway

C4 pathway in tropical plants , The CAM pathway in succulents

Mesophyll cells are responsible for

CO2 capture

Since photophosphorylation and NADPH production by the photosynthetic electron transport system is shut down in the dark, it is crucial that the

Calvin cycle only be active in the light.

Thioredoxin functions as

a redox protein that interconverts disulfide bridges and sulfhydryls in cysteine residues. In the dark, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatese, RuBP kinase and glyceradehyde-3P dehydrogenase, all contain disulfide bridges that inhibit enzyme activity.

What is the energy usage in stage 3?

All together, 6 ATP and 6 NADPH are required in stage 2 for every 3 CO2 that are converted to 1 GAP

What happens to glyceraldehyde 3-phosphate?

GAP fates: Converted to DHAP Combined with DHAP to form F-1,6,BP (starch) Converted to DHAP for export from chloroplasts Cytosolic DHAP used for glycolysis or sucrose

Calvin Cycle stage 3

Lastly, and five GAPs are used to replenish three RuBP (stage 3).

Calvin Cycle stage 2

Reduction of six 3-PGA makes six GAPs

Calvin Cycle stage 1

The Rubisco enzyme combines three RuBPs with three CO2 to form six 3-PGA(3-Phospho-glycerate)

Melvin Calvin

To identify the metabolic intermediates in carbon fixation, Calvin and his colleagues used radioactive labeling with 14CO2 to follow carbon fixation in photosynthetic algae cells grown in culture. They found that cells very quickly accumulated 14C-labeled 3-phosophoglycerate.

Increase in ph causes

an efflux of Mg2+ to the stroma to balance the charge. Rubisco and FBPase activities are maximal at pH 8 and high Mg2+.

The primary enzymes involved in interconverting C3, C4, C6 and C7 molecules

are transketolase and transaldolase

The CAM pathway in succulents such as the saguaro cactus

captures CO2 at night in the form of malate and uses it during the day.