ch20_biosintesis

8/11/2019 ch20_biosintesis

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CHAPTER 20

Carbohydrate Biosynthesis in Plants

20.1 Photosynthetic Carbohydrate Synthesis

- Plastids

- CO2 => CH2O (carbohydrates) 3 stagesCarbon-fixation

CO2 + ribulose 1,5 bisphosphate => 2 3-phosphoglycerate

3-PG => triose phosphates

Pentose Phosphate Pathway

20.2 Photorespiration and the C4 and CAM Pathways

20.3 Biosynthesis of Starch and Sucrose

20.4 Synthesis of Cell Wall Polysaccharides:

20.5 Integration of Carbohydrate Metabolism in the Plant Cell


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Introduction to Anabolic

Pathways

The previous chapters were mainly concerned with

catabolism: how to extract energy from biomolecules

This and the following chapters are concerned with

anabolism: how to build biomolecules

Plants are extremely versatile in biosynthesis

Can build organic compounds from CO2

Can use energy of sunlight to support biosynthesis

Can adopt to a variety of environmental situations


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Assimilation of CO2 by Plants


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CO2Assimilation Occurs in

Plastids

Organelles found in

plants and algae

Enclosed by a doublemembrane

Have their ownsmall

genome

The inner membrane is

impermeable to ions

such as H+, and to polar

and charged molecules


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Origin and Differentiation of

Plastids

Plastids were acquired during evolution by earlyeukaryotes via endosymbiosis of photosyntheticcyanobacteria

Plastids reproduce asexually via binary fission

The undifferentiated protoplastidsin plants candifferentiate into several types, each with adistinct function

Chloroplastsfor photosynthesis

Amyloplastsfor starch storage

Chromoplastsfor pigment storage

Elaioplasts for lipid storage

Proteinoplasts for protein storage


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CO2Assimilation

The assimilation of carbon dioxide occurs in the

stroma of chloroplastsvia a cyclic process known as

the Calvin cycle

The key intermediate, ribulose 1,5-bisphosphate is

constantly regenerated using energy of ATP

The key enzyme, ribulose 1,5-bisphosphate

carboxylase / oxygenase (Rubisco), is probably the

most abundant protein on Earth

The net result is the reduction of CO2with NADPH

that was generated in the light reactions of

photosynthesis


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The Calvin Cycle

CO2 fixation

Net reaction

3CO2 + 9ATP + 6NADPH >

GAP + 9ADP + 6Pi +6NADP+

3 5C sugars + 3 CO2 > 6 3C sugars

5 3C sugars > 3 5C sugars


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Rubisco (ribulose 1,5-bisphosphate carboxylase)

Rubisco is a large Mg++-containing enzyme that makes anew carbon-carbon bond using CO2as a substrate

Inefficient (kcat~ 3s-1)

Ru1,5P2

+ CO2> 2 3-phosphoglycerate (3PG)


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Synthesis of Glyceraldehyde-3

Phosphate (First Stage)

Three rounds of

the Calvin cycle

fixes three CO2moleculesand

produces one

molecule of 3-

phosphoglycerate


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Converted to

starch in the

chloroplast

Converted to

sucrose for

export

Recycled toribulose 1,5-

bisphosphate

Glyceraldehyde 3-

phosphate


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Isomerization

Phosphatase/Kinase

Carbon

backbone

rearrangements

Regenerates

ribulose 1,5-

bisphosphate

Stage 3: C3,

C4, C5, C7Rxns


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Calvin Cycle - Isomerization

reactions Triose phosphate isomerase

GAP DHAP

Ribose phosphate isomerase

R5P Ru5P

Phosphopentose epimerase

Xu5P Ru5P


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Calvin Cycle - CC bond

rearrangements Aldolase

GAP + DHAP > F1,6P2

E4P + DHAP > S1,7P2

Transketolase

F6P + GAP > E4P + Xu5P

S7P + GAP > R5P + Xu5P


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Calvin Cycle - Phosphatase

reactions Fructose bisphosphatase

F1,6P2 > F6P + Pi

Sedoheptulose bisphosphatase

S1,7P2 > S7P + Pi


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Fixation ofthree

CO2molecules

yields one

glyceraldehyde

3-phosphate

Nine ATP

molecules and

six NADPHmolecules are

consumed

Stoichiometry

and Energetics


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Photosynthesis: From Light and CO2

to Glyceraldehyde 3-phosphate

The photosynthesis of one

molecule of glyceraldehyde

3-phosphate requires the

capture of roughly 24

photons


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Enzymes in the Calvin Cycle are

Regulated by Light

Target enzymes are active when reduced

ribulose 5-phosphate kinase,

fructose 1,6-bisphosphatase,

seduloheptose 1,7-bisphosphatase, and

glyceraldehyde 3-phosphate dehydrogenase


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Photorespiration

So far, we saw that plants oxidize water to O2

and reduce CO2to carbohydrates during the

photosynthesis

Plants also have mitochondria where usual

respiration with consumption of O2occurs in thedark

In addition, a wasteful side reactioncatalyzed by

Rubisco occurs in mitochondria This reaction consumes oxygen and is called

photorespiration; unlike mitochondrial

respiration, this process does not yield energy


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Oxygenase Activity

of Rubisco

The reactive nucleophile in

the Rubisco reaction is the

electron-rich enediol form of

ribulose 1,5-bisphosphate The active sitemeant for

CO2also accommodates O2

Mg++also stabilizes the

hydroperoxy anion that

forms by electron transfer

from the enediol to oxygen


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Rubisco in C3Plants Cannot

Avoid Oxygen

Plants that assimilatedissolved CO2in the

mesophyllof the leaf into three-carbon 3-

phosphoglycerate are called the C3plants

Our atmosphere contains about 21% of oxygen

and 0.038% of carbon dioxide

The dissolved concentrations in pure water are

about 260 M O2and 11 M CO2(at the

equilibrium and room temperature)

The Kmof Rubisco for oxygen is about 350 M


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C4 plants

concentrate CO2 Many tropical plants avoid wasteful

photorespiration by a physical

separation of CO2capture and

Rubisco activity CO2is captured into oxaloacetate

(C4) in mesophyll cells

CO2is transported to bundle-sheath

cells where Rubisco is located

The local concentration of CO2in

bundle-sheath cells is much higher

than the concentration of O2


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Conversion of

stored fatty acids tosucrose in

germinating seeds


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Chapter 20: Summary

ATP and NADPH from light reactions are needed in order to

assimilate CO2into carbohydrates

Assimilations of three CO2molecules via the Calvin cycle

leads to the formation of one molecule of 3-phosphoglycerate 3-Phosphoglycerate is a precursor for the synthesis of larger

carbohydrates such as fructose and starch

The key enzyme of the Calvin cycle, Rubisco, fixes carbon

dioxide into carbohydrates

Low selectivity of Rubisco causes a wasteful incorporation of

molecular oxygen in C3plants; this is avoided in C4plants by

increasing the concentration of CO2 near Rubisco

In this chapter, we learned that:

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