BIOC 3310 - March 27th Lecture

Anabolic pathways - generally

reductive rather than oxidative.

Plants are

autotrophs

Carbohydrate Biosynthesis is a

dynamic state, usually around the same rate

Carbohydrate Biosynthesis mainly occurs in

plasmids

Triose phosphates are the main

energy currency

Starch is the

long term energy

Sucrose is inert and helps

transport C

Most biosynthetic activities occur in plastids. • Plastids reproduce by

binary fission. (self-producing)

Chloroplasts are the site of

CO2 assimilation. (getting CO2 from the atmosphere to useful currency)

Chloroplasts can become

proplastids.

Relative amount of each is dependent on

plant tissue type.

Amino acids produced in plasmids:

A, C, E, and K

Chloroplasts lack

starch

In amyloplasts, colour change is important, indicating

high starch component

Carbon Dioxide Assimilation • Stage 1 -

CO2 assimilation is the carbon-fixation reaction.

Stage 2 - 3-phosphoglycerate reduced to

triose phosphates

Stage 3 - 5/6 triose phosphate →

ribulose 1, 5 - bisphosphate (3).

ribulose 1, 5 - bisphosphate (3) is the

starting material.

Fructose-6-phosphate is the key intermediate in stage

3 of CO2 assimilation.

Hexose phosphate →

pentose bisphosphate

Hexose phosphate → pentose bisphosphate • Same set of reactions in

opposite direction

Fructose-6-phosphate is the branching point between

starch vs sucrose

Carbon Dioxide Assimilation is a

cyclic process

Photosynthetic organisms in 1940's (Calvin). • Plants that follow this incorporations are

C3 plants vs. C4 plants.

C4 plants

dont incorporate CO2

C3 plants

incorporate CO2 via the atmosphere

Incorporation of CO2 is

ribulose 1,5 - bisphosphate carboxylase.

• Rubisco

How molecules of 3-phosphoglycerate are formed?

2

Two distinct forms of rubisco

Form I

• Form II

Form I -

vascular plants, algae, and cyanobacteria.

Form II -

Certain photosynthetic bacteria.

form 2 has a slightly different shape, is

smaller

rubisco: how many subunits

8 identical subunits, 8 smaller units, 16 total

rubisco is slow, only 3

CO2 per second per molecule, not enough to keep the balance, is solved by the amount of rubisco produced

Mg 2+ helps to

orient the reactants in preparation for the reaction to occur, polarizing CO2, opening it to a nucleophilic attack

Mg forms ___ coordinates with 6 O

6

___ O comes from carbamoyl-Lys

1

____ O from Glu

2

____ O from substrate (1-5 biphosphate)

2

___ O from CO2

1

First stage of CO2 assimilation: rubisco's

carboxylase activity

The resulting six-carbon intermediate breaks down yielding

two molecules of 3-phosphoglycerate.

Rubisco is a common target for

regulation

Rubisco comes into pocket, radiolabeled CO2 comes in and is incorporated, building Beta-keta intermediate, which plants make to

shut down process, and is based off light levels, then 3-phosphoglycerate is released (not the radioactive one in this example)

Rubisco remains inactive until carbamoylated on the amino group of

Lys201 .

Ribulose 1,5 - bisphosphate inhibits

carbamoylation.

Rubisco activase overcomes the inhibition by promoting the release of

ribulose 1,5 - bisphosphate.

Regulatory mechanism by the

nocturnal inhibitor, 2-carboxyarabinitol 1-phosphate

2-carboxyarabinitol 1-phosphate, is the

nocternal inhibitor, broken down in light and activates in the dark

Through rubisco activase and ATP going to ADP, Ribulose 1,5 - bisphosphate overcomes the

tight interaction

3- phosphoglycerate formed in step 1 is converted to

glyceraldehyde 3-phosphate in 2 steps.

3- phosphoglycerate formed in step 1 is converted to glyceraldehyde 3-phosphate in 2 steps. • One exception!!

using NADPH, not NADH

Chloroplast stroma contains all the glycolytic enzymes except

phosphoglycerate mutase.

Step 1 -

stromal 3-phosphoglycerate kinase.

Step 2 - Chloroplast specific isozyme:

glyceraldehyde 3-phosphate dehydrogenase.

Step 3 -

Dihydroxyacetone phosphate (DHAP)

For continuous flow of CO2 into carbohydrate, ribulose 1,5 - bisphosphate must be

constantly regenerated.

Step 3 - 2 carbon

ketol group (CH2OH-CO-)

Fructose 1,6-bisphosphate to fructose 6-phosphate is an

irreversible process, due to the release of the phosphate

Transketolase is an TPP factor, requiring

Mg, which allows it to shuttle electrons

when catalyzed with aldolase, allows for the process to be

reversable

Step 5: sedoheptulose 1,7-biphosphatase is the 2nd

irreversible step

Transketolase catalyzed reactions of the

Calvin cycle.

Top box - general reaction catalyzed by

transketolase.

Middle box - hexose and triose to

4 and 5 carbon sugars.

Bottom box - conversion of 7 carbon and 3 carbon sugars to

two pentoses.

with a ketose donor, aldose acceptor, and transketolase, it causes a

transfer of 2 Carbon groups

Stage III: TPP as a

cofactor

Thiamine pyrophosphate is an

electron sink, being a 2-carbon transfer/shuttle

Stage III: Regeneration of

Ribulose I,5 - bisphosphate

1. Three turns of the Calvin cycle → conversion of 3

CO2 and to one triose phosphate.

2. Consume 6 ATP and

6 NADPH in the second stage

3. One molecule of glyceraldehyde 3-phosphate is the net product of

carbon-assimilation pathway.

4. Stage 3 requires

3 ATP.

5. For every molecule of triose phosphate by the Calvin cycle requires

6 NADPH and 9 ATP.

6. NADPH and ATP are produced in

light-dependent reactions of photosynthesis.

7. Animals lack:

Rubisco, sedoheptulose 1,7 - bisphosphate, and ribulose 5 - phosphate kinase.

- can not assimilate CO2 molecules

If 1 Pi gets lost, it throws off ratio and need Pi back from

cytosol, as it cant go through the mb

Inner chloroplast membrane is impermeable to

most phosphorylated compounds.

The Pi moved to the chloroplast and is used in

photophosphorylation.

Triose phosphate moved into cytosol →

sucrose.

Sucrose in cytosol and starch in the

chloroplast

Secondary function: ATP and NADH can not cross the

chloroplast membrane.

Pi gets moved into the membrane through

DHAP

ATP and NADH can not cross the chloroplast membrane. • DHAP formed in stroma is transported to

cytosol

Reductive assimilation of CO2 requires a lot of

ATP and NADPH.

Enzymes have adapted to be more efficient in

light conditions.

Fructose 1,6 - bisphosphate requires

Mg2+ and is dependent on pH.

Enzyme efficiency increases due to a

higher pH

4 enzymes are further regulated by light:

• Ribulose 5 - phosphate kinase

• Fructose 1,6 - bisphosphatase

• Sedoheptulose 1,7 - bisphosphatase

• Glyceraldehyde 3 - phosphate dehydrogenase.

Thioredoxin is driven by

The disulfide bond, when electrons are moved, it breaks the bond and activates it

1. Photosynthesis in vascular plants takes place in

chloroplasts.

2. Rubisco condenses CO2 ribulose 1,5 - bisphosphate, forming an unstable

hexose bisphosphate, splitting into 2 molecules of 3-phosphoglycerate.

3. Stromal isozymes of the glycolytic enzymes catalyze the reduction of

3 - phosphoglycerate to glyceraldehyde 3 - phosphate.

4. Stromal enzymes (transketolase, aldolase) rearrange the carbons of

triose phosphates.

5. Fixing three CO2 into one triose phosphate is

9 ATP and 6 NADPH.

6. Antiporter in the inner chloroplast membrane exchanges Pi in the cytosol for

3 - phosphoglycerate or DHAP produced by CO2 assimilation.

7. Four enzymes of the Calvin cycle are activated indirectly by

ight and inactive in the dark.

Production of carbohydrates during

bright light.

Synthesis of sucrose and starch occur in

different locations.