Photosynthesis

Chapter 22, p. 626-661

Voet & Voet, Second Edition, 1995


Introduction

Life depends on the sun. It is the energy source for photosynthesis. In this process both CO2 and H2O are reduced to yield carbohydrate and O2. Above 1011 tons of carbon are fixed annually representing the storage of 1018kJ of energy.

Early experiments

von Helmont

soil weight didn't change, willow tree weight did -- water (really CO2 also)

Priestly

oxygen produced

Lavoisier

combustion & respiration

Ingen-Housz

sunlight & green parts to purify air

Senebier

CO2 used

de Saissire

O2 produced is >CO2 used there fore H2O involved

Mayer

light energy converted to chemical energy

CO2 + H2O -----> (CH2O) + O2 in presence of light


I. Chloroplasts

The site of photosynthesis in eukaryotes is the chloroplast. There are 1 to 1k per cell. Similar to mitochondria in that they have highly permeable outer membranes and a nearly impermeable inner membrane. The inner membrane encloses the stroma . There are enzymes, DNA, RNA, and ribosomes in the organelle. The stroma surrounds a third membranous compartment, the thylakoid. The lipids of the thylakoid membrane are 80 % mon and diacyl glycerols, 10 % sulfolipids, and 10 % phospholipids.

Photosynthesis occurs in two phases

1. The light reactions - use light energy to generate NADPH and ATP - photophosphorylation.

2. Dark reactions - use NADPH and ATP to make carbohydrates from CO2 and H2O - carbon fixation.


II. Light Reactions

The Hill reaction light

H2O + CO2 ----------------> (CH2O) + O2

water is photolyzed (split by light)

A. Absorption of Light

Chlorophyll is the principal photoreceptor

1. It has a central Mg2+

2. Ring V is a cyclopentanone fused to a pyrrole ring III.

3. Pyrrole ring IV is partially reduced

4. The propionyl side chain is esterified.

See Fig. 22-3.

Interaction of light and matter

1. internal conversion

2. Fluorescence

3. Exciton transfer (resonance energy transfer)

4. Photooxidation

 

Light absorbed by antenna chlorophylls and accessory pigments is transferred to photosynthetic reaction centers

More chlorophylls than reaction centers

8 photons per O2

Most chlorophylls gather light

Light-harvesting antenna

>90% efficiency

Accessory pigments function to fill in the absorption capabilities

The x-ray structure of two antenna proteins are shown in fig. 22-8

B. Electron Transport in Photosynthetic Bacteria

Photosynthesis is a process in which electrons from excited chlorophyll molecules are passed through a series of acceptors that convert electronic energy to chemical energy.

The photosynthetic reaction center is a trans-membrane protein containing a variety of chromophores

The purple photosynthetic bacterium Rhodospirillum rubrum has a bacteriochlorophyll named P870.

Components

three hydrophobic subunits

H 258 residues, L 273 residues, and M 323 residues

two molecules of bacteriopheophytin b

one non-Fe-S Fe (II) ion

one ubiquinone

one menaquinone

X-ray structure determined - see fig 22-9

Two Bchl b molecules form a "special pair"

The most striking aspect of the reaction center is that its chromophoric prosthetic groups are arranged with nearly perfect twofold symmetry. See Fig 22-10a

Mg-Mg distance is 7 A

The electronic states of molecules undergoing fast reactions can be monitored by EPR and Laser spectroscopy

Turnover in milliseconds

Photon absorption rapidly photoxidizes the "special pair"

(a) The primary photochemical event of bacterial photosynthesis is absorption of a photon by the special pair.

(b) P870*

(c) Menaquinone

 

Electrons are returned to the photoxidized special pair via an electron-transport chain - see Fig. 22-11. This sequence of electron transfers is so efficient that its overall quantum yield is almost 100 %.

Photosynthetic electron transport drives the formation a proton gradient

Synthesis of ATP, a process called photophosphorylation, is driven by dissipation of the resulting pH gradient similar to oxphos.

C. Two-Center Electron Transport

Half reactions

O2 + 4e- + 4H+ <====> 2H2O

NADP+ + H+ + 2e- <====> NADPH

Overall 4 electron reaction

2NADP+ + 2H2O <====> 2NADPH + O2 + 2H+

If photosynthesis were 100% efficient - more than one photon would be required, 438 kJ/mol

8-10 photons required to produce one molecule of O2

 

Photosynthetic O2 production requires two sequential photosystems that react is a Z-scheme. The two systems are connected in series.

1. Photosystems I (PSI)

generates a strong reductant capable of reducing NADP+, and concomitantly, a weak oxidant.

2. Photosystem II (PSII)

generates a strong oxidant capable of oxidizing H2O, and concomitantly, a weak reductant.

see fig. 22-12

The weak reductant reduces the weak oxidant so that PSI and PSII form a two stage electron "energizer". Both photosystems must function for electron transfer from H2O to NADPH to occur.

O2-Producing photosynthesis is mediated by three trans-membrane protein complexes linked by mobile electron carriers

1. PSII

2. Cytochrome b6-f complex

3. PSI

Fig 22-15 shows the ETC of the thylakoid membrane

The mobile carrier is plastoquinone (Q) s ubiquinone analog - connects PSII & PSI

Plastocyanin (PC) is a mobile protein connecting cyt b & PSI above.

O2 is generated in a five-stage water-splitting reaction mediated by a Mn-Containing protein complex

The oxidation of two molecules of H2O to form one molecule of O2 requires four electrons. Since transfer of a single electron from H2O to NADP+ requires two photochemical events, this accounts for the minimum of 8 to 10 photons absorbed per molecule of O2 produced. Each O2 molecule must be produced by a single photosystem. Four water-derived protons are released into the inner thylakoid space.

The PSII reaction center resembles that of photosynthetic bacteria

P680 - a chlorophyll a; pheophytin a - a chlorophyll a with 2 H+ in place of Mg2+

Electron transport through the cytochrome b6-f complex generates a proton gradient

Plastocyanin transports electrons from cytochrome b6f to PSI

PSI has both similarities to and major differences with PSII and the bacterial photosynthetic reaction center

PSI-activated electrons may reduce NADP+ or motivate proton gradient formation

Two alternate pathways

1. Noncyclic

2. Cyclic

PSI and PSII occupy different parts of the thylakoid membrane

1. PSI unstacked stroma lamellae

2. PSII almost exclusively between the closely stacked grana

see fig 22-23

The mobile carriers plastoquinone and plastocyanin are required.

Reduced plastoquinone activated a protein kinase that phosphorylates a specific thr in the light-harvesting complex. The LHC migrates closed to PSI.

D. Photophosphorylation

 

Chloroplast generate ATP work like mitochondria by coupling the dissipation of proton gradient to the enzymatic synthesis of ATP.

Chloroplast proton-translocating ATP synthase resembles that of mitochondria

CF0 and CF1

A difference is that chloroplast ATP synthase translocates protons out of the thylakoid space; mitochondrial ATP synthase conducts them into the matrix space.

Photosynthesis with noncyclic electron transport produces around 1.25 ATPs per absorbed photon

the pH gradient is ~3.5 pH units

~12 protons are translocated per O2 produced by noncyclic electron transport

4 ATPs per molecule of O2 evolved

The electrochemical gradient results almost entirely from the pH gradient.

The light reactions give rise to both ATP and NADPH. The energy source and reducing power needed for carbon fixation.


III. Dark Reactions

A. The Calvin Cycle

Calvin, Bassham & Benson traced the metabolic fate of 14CO2

Chlorella

Pathway Fig 22-25

The first stable labeled compound is 3-phosphoglycerate labeled in the COO group

Ribulose-5-phosphate is the carboxylation substrate

The Calvin cycle generates GAP from CO2 via a two-stage process

Stage 1 The production phase - 3 molecules of Ru5P react with 3 CO2 molecules to yield 6 GAP at the expense of 9ATPs & 6 NADPH - cyclic and results in the synthesis of 1GAP from 3CO2

Stage 2 The recovery phase - carbon atoms are shuffled to reform the 3 Ru5Ps

no ATP or NADPH required for stage 2

 

Most Calvin cycle reactions also occur in other metabolic pathways

RuBP carboxylase catalyzes CO2 fixation in an exergonic process

X-ray structure is known Fig 22-27

The world's most important enzyme mechanism in Fig 22-28

GAP is the precursor of glucose-1-phosphate and other biosynthetic products

3CO2 + 9ATP + 6NADPH ---> GAP + 9ADP + 8Pi + 6NADP

B. Control of the Calvin Cycle

During the day - both light & dark reactions

At night use nutritional resources to generate ATP and NADPH via glycolysis, oxidative phosphorylation, and the pentose phosphate pathway.

Plants must have light-sensitive control mechanisms

RuBP carboxylase responds to 4 light-dependent factors

1. pH - optimum near pH 8

2. Stimulated by Mg2+ - have Mg2+ efflux to the stroma

3. Allosterically activated by NADPH

4. Inhibited by 2-carboxyarabinitol-1-phosphate which many plants make only in the dark

Redox potential

thioredoxin

it activates FBPase and SBPase

 

C. Photorespiration and the C4 Cycle

Illuminated plants consume O2 and evolve CO2 in a pathway distinct from oxidative phosphorylation.

At low CO2 and high O2 levels, this photorespiration process can outstrip photosynthetic CO2 fixation.

The explanation was unexpected

O2 competes with CO2 as a substrate for RuBP carboxylase

O2 reacts with RuBP to form 3PG and 2-phosphoglycolate. The 2-phosphoglycolate is hydrolyzed to glycolate. See Fig 22-31

 

Photorespiration dissipates ATP and NADPH

The net result of the pathway is that some of the ATP and NADPH generated in the light reaction is uselessly dissipated.

Photorespiration limits the growth rate of plants

On a hot bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised the O2, the rate of photorespiration may approach that of photosynthesis. This is a major limiting factor in the growth of many plants,

C4 plants concentrate CO2

Certain species of plants

sugar cane, corn & most important weeds

have a metabolic cycle that concentrates CO2 in their photosynthetic cells thereby almost totally preventing photorespiration

 

C4 plants are largely tropical

 

CAM plants store CO2 through a variant of the C4 cycle

Cushman studies


02-12-97 FRL