FCH 532 Lecture 32

Chapter 31: PhotosynthesisQuiz on Friday(4/20): Ribonucleotide reductase

mechanismFriday (4/20): extra credit seminar, Dr. Jimmy

Hougland,145 Baker, 3-4PM.ACS exam has been moved to Monday (4/30)Quiz on Final is scheduled for May 4, 12:45PM-

2:45PM, in 111 Marshall

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Biosynthesis of of NAD and NADP+

Produced from vitamin precursors Nicotinate and Nicotinamide and from quinolinate, a Trp degradation product

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Biosynthesis of FMN and FAD from riboflavin

FAD is synthesized from riboflavin in a two-reaction pathway.

Flavokinase phosphorylates the 5’OH group to give FMN

FAD pyrophosphorylase catalyzes the next step (coupling of FMN to ADP).

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Biosynthesis of CoA from pantothenate

Photosynthesis• Life on Earth depends on solar energy.

• Plants and cyanobacteria use photosynthesis to fix CO2 to carbohydrates

• General reaction for oxygenic photosynthesis:

Light

CO2 + H2O (CH2O) + O2

General reaction for anoxygenic photosynthesis:

Light

CO2 + 2H2S (CH2O) + 2S + H2O

Figure 24-1 Chloroplast from corn.

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Figure 24-2 Electron micrograph of a section through the purple photosynthetic bacterium Rhodobacter sphaeroides.

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Choroplasts• Site for photosynthesis in eukaryotes• Part of organelles found in plants

(plastids).• Similar to mitochondria• Highly permeable outer membrane• Nearly impremeable inner membrane• Inner membrane encloses stroma-

contains enzymes, DNA RNA and ribosomes - similar to mitochondrial matrix.

• Stroma encloses a third membrane component - thylakoids

• Thylakoids is a single, highly folded vesicle that appears as a stack of discs called grana.

• The grana are interconnected by stromal lammelae.

• Each chloroplast has about 10-100 grana.

chloroplast

stroma

thylakoid membrane

DNA for chloroplast proteins can be in the nucleus or chloroplast genome

Buchannan et al. Fig. 4.4

Import of proteins into chloroplasts

Buchannan et al. Fig. 4.6

Biochemistry inside plastids

• Photosynthesis – reduction of C, N, and S• Amino acids, essential amino acid synthesis restricted to

plastids– Phenylpropanoid amino acids and secondary compounds start

in the plastids (shikimic acid pathway)– Site of action of several herbicides, including glyphosate– Branched-chain amino acids– Sulfur amino acids

• Fatty acids – all fatty acids in plants made in plastids

Lipids of the thylakoid membrane

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1O

OH

OHHO

OH

*2

3

4

5

6

O CH2

HC

HC

O

O

C R1

C R2

O

O

•Only ~10% phospholipids•~80% are mono-and digalactosyl diacylglycerols•~10% are sulfoquinovosyl diacylglycerol

digalactosyl diacylglycerol

Biochemistry inside plastids

• Carotenoids – source of vitamin A

• Thiamin and pyridoxal, B vitamins

• Ascorbic acid – vitamin C

• Tocopherol – vitamin E

• Phylloquinone (an electron accepttor in PS I – vitamin K)

Light and dark reactionsPhotosynthesis happens in 2 distinct phases:

• Light reactions use light energy to generate NADPH and ATP.

• Dark reactions (light independent reactions), use NADPH and ATP to make carbohydrate from CO2 and H2O.

• Light reactions take place in the thylakoid membrane

• Light reactions similar to electron transport in mitochondria and oxidative phosphorylation.

• Takes place in plasma of the inner membrane or invaginated structures called chromatophores.

• In eukaryotes, dark reactions occur in the stroma

6 CO2 + 6 H2O + 48 h C6H12O6 + 6 O2

Go = -1290 kcal/mol (-5398 kJ/mol). Keq = 10942 !

Synthesizing carbohydrates from CO2 and water presents a formidable thermodynamic problem:

Photosynthetic organisms use the energy of light to drive carbohydrate synthesis against this enormous gradient.

*An einstein is a mol of photons. N = Avogadro’s number (6x1023); h = Planck’s constant (6.63x10-34 J/s); = frequency (s-1).

Go = +679 kcal/mol (+2480 kJ/mol). Keq = 10-496

6 CO2 + 6 H2O C6H12O6 + 6 O2

The energy of red light (700 nm) is E = Nh = 41 kcal/einstein* (172 kJ/einstein)

Absorption of light• Main photoreceptor for photosynthesis is chlorophyll.• Cyclic tetrapyrolle, like heme groups of cytochromes and

globins.• Differs from these molecules in 4 ways1. Central metal ion is Mg2+ not Fe(II) or Fe(III).2. Has cyclopentenone ring, (Ring V), fused to pyrrole Ring III3. Pyrolle Ring IV is partially reduced in chlorophyll a (Chl a)

and chlorophyll b (Chl b). In bacteriochlorophyll Rings II and IV are partially reduced.

4. Propionyl side chain of Ring IV is esterified to tetraisoprenoid alcohol. In Chl a and b and Bchlb it is phytol.

The photochemically reactive pigments are chlorins or bacteriochlorins, which are structurally related to hemes

N

N

N

NFe

Hemes

symmetrical systems;

absorb blue light

Chlorophylls

asymmetrical systems;

absorb blue & red light

Bacteriochlorophylls

more asymmetrical systems;

absorb blue, orange & near-IR light

N

N N

N

Mg

ON

N N

O

N

Mg

O

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Absorption of light• Molecules have numerous electronic quantum states of

differing energies.• Absorption of light by a molecule promotes an electron from

its ground (lowest energy)state molecular orbital to one of higher energy.

• A given molecule can only absorb photons of certain wavelengths-conservation of energy.

• The energy difference between the two states must exactly match that of the absorbed photon.

Absorption of light• Amount of light absorbed by a substance at a given wavelength is described by the Beer-

Lambert Law:

A = log = clI

I0

A = absorbance

I0 = intensity of incident light

I = intensity of transmitted light

c = molar concentration of sample

l = length of the light path

= molar extinction coefficient

The electromagnetic spectrum

PAR = photosynthetically available radiation

Different pigments absorb light differently

Figure 24-5 Absorption spectra of various photosynthetic pigments.

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Different pigments absorb light differently

When light raises a molecule to an excited electronic state, the molecule becomes a stronger reductant

LIGHT

Incr

easi

ng

En

erg

y

A* BA B

HOMO

LUMO

LUMO

HOMO

electrons

Absorption of light• Peak molar extinction coefficient of chlorophylls >105 M-1 cm-1

• Small chemical differences (structure) affect their abs specta• The electronically excited molecule can dissipate excitation

energy in a number of ways.

Figure 24-4 Energy diagram indicating the electronic states of chlorophyll and their most important modes of interconversion.

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Short wavelength abs

Long wavelength abs

Dissipation of excitation energy• Internal conversion-electronic energy is converted to heat

(molecular motion). Occurs very rapidly (<10-11s) and molecules returned to ground state.

• Excitation energy of a chlorophyll molecule that abs a short-wavelength band (2nd excited state) is no different than if photon was absorbed in its less energetic long-wavelength band (1st excited state).

Figure 24-4 Energy diagram indicating the electronic states of chlorophyll and their most important modes of interconversion.

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Short wavelength abs

Long wavelength abs

Dissipation of excitation energy• Fluorescence-electronic energy is reduced to ground state

by emitting a photon Occurs slower than internal conversion (~10-8s).

• Emitted photon has a longer wavelength (lower energy) than the initially absorbed photon.

• Accounts for 3-6% of light energy absorbed-usually causes red fluorescence.

Figure 24-4 Energy diagram indicating the electronic states of chlorophyll and their most important modes of interconversion.

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Short wavelength abs

Long wavelength abs

Dissipation of excitation energy• Exciton transfer (resonance energy transfer)-electronic

energy is directly transferred to nearby unexcited molecules with similar electronic properties

• Funnels the light to photosynthetic reaction centers• Photooxidation-light-excited donor molecule is oxidized by

transferring an electron to an acceptor molecule.

Most of the pigments in photosynthetic cells do not participate in the electron-transfer reactions of photosynthesis. Instead, they serve as an antenna that increases the absorption of light.

R. Emerson & W. Arnold measured the amount of O2 formed when they excited algae with short flashes of light.

Light absorbed (photons/Chl)

O2/Chl

0

0.0004

0.0002

0 0.004 0.008

At high light intensity, the maximum O2 released per flash was about 1 O2 per 2400 Chls.

At low light intensity, 1 O2 is formed for ~each 8 photons absorbed (yellow dashed line).

Light absorbed is transferred to Photosynthetic Reaction Centers

• Large excess of chlorophyll molecules don’t all participate in photochemical reactions.

• Most chlorophyll act as light harvesting antennas (antenna chlorophyll).

• They pass their energy until they reach a reaction center.• Transfer occurs <10-10 s with an efficiency of 90%.• RC intercepts only 1 photon per second.• Important as light harvesting complexes (LHCs)

Figure 24-7a Flow of energy through a photosynthetic antenna complex. (a) Diagram of random photon migration by exciton

transfer.

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Figure 24-7b Flow of energy through a photosynthetic antenna complex. (b) The excitation is trapped by the RC chlorophyll.

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When the antenna is excited with light, excitations are transferred to the reaction center within ~40 ps

view normal to the membrane

1 ps = 10-12 s

LH2

LH2

LH2

RC

Smaller “LH2” antenna complexes transfer energy rapidly to LH1

LH1

35 ps

0.1 - 0.2 ps

1.2 psantenna BChls are green and blue in this figure.