Prokaryotic Cell Module 4: Photosynthesis

Role of Photosynthesis

Provides energy for electrochemical gradient using membrane-bound photosystems

Oxygenic Photosynthesis

Water is the electron donor, generates oxygen, performed by cyanobacteria and plants

Anoxic Photosynthesis

H2S, S0, H2, or an organic molecule donates electrons, no oxygen generated, performed by purple/green sulfur and non-sulfar bacteria

Q-type (Type II) Reaction Centers

Present in all purple bacteria, green nonsulfur (gliding) bacteria, and oxygenic phototrophs; Quinone B is final electron carrier, carrying hydrogen to quinone pool (QP)

Iron-sulfur-cluster-type (Type I) Reaction Centers

Present in green suflur bacteria heliobacteria, and oxygenic phototrophs; Fe-S cluster is final electron carrier, carrying electron only to ferredoxin which directly transfers to NAD(P)+

Reaction Center Primary Donor (Special Pair) and Light Harvesting Pigments

Two types of photosynthetic pigments

Reaction Center Primary Donor (Special Pair)

Converts energy into redox potential change

Light harvesting pigments

Captures light energy and transfers to photosystem (no e- transfer)

Examples of Light Harvesting Pigments

Antenna pigments and carotenoids

Pigment absorption

Alteration of pigment composition due to environmental factors to change light harvesting wavelength

(Bacterio)chlorophyll pigments

Found in both RC and LH complexes, but not used in respiratory chains. Varies in redox potential unlike carotenoids, with primary donors (special pairs) having the highest.

Heme-type porphyrin ring with central Mg or Zn, additional ring, and lipophilic side-chain

(Bacterio)chlorophyll Structure

Cyanobacteria and plants

Organisms that use chlorophyll

Organisms that use bacteriochlorophyll

Purple and green bacteria

Bacteriopheophytin (Bph / Pheo)—not part of special pair

Additional chlorophyll-related-pigment seen in RCs in purple and green bacteria where Magseium is replaced by two hydrogens

Carotenoids

Perform LH, photoprotection, and anti-oxidant activities

Isoprenoid w/ conjugated DB w/ 6-member ring at each end

Carotenoid structure

Phycobilin

Role in light harvesting (LH) in cyanobacteria

Phycobilisome

Where is phycobilin found?

Phycobilin structure

Linear tetrapyrrole chromophores derived from heme, covalently attached to cysteine residues of proteins

Arrangements of photopigments in purple bacteria

LH and RC within invaginated cell membranes

Arrangements of photopigments in green bacteria

LH within monolayered chlorosomes inside cell, RC within cell membrane,

Arrangements of photopigments in cyanobacteria

LH within phycobilisome outside thylakoid; RC within thylakoid membrane

Purple Nonsulfur Metabolism

Mostly H2, low quantities of H2S as donor; chemoheterotroph if no light

Purple Sulfur Metabolism

H2S donor; releases S0

Anaerobic photoheterotrophs + photoautotrophs of gram-negative lineage

Purple Photosynthetic Bacteria

30:1

LH:RC ratio in purple photosynthetic bacteria

Chromatophores

Membrane invaginations in vesicles in purple photosynthetic bacteria that increases per-cell photosystem count

Anerobic photoautotrophs with some photoheterotrophs of gram-negative lineage

Green Photosynthetic Bacteria

Green gliding bacteria use a…

RCII / Q-type RC

Green sulfur bacteria use a…

RCI / Fe-S-type RC

Chlorosomes

Organelle-like structure in green photosynthetic bacteria with galactolipid monolayer surrounding LH pigments

1:1000

RC:LH ratio in green photosynthetic bacteria due to chlorosome

Septal channels

Connect multicellular filamentous green-gliding bacteria

Chlorosome mechanism

Protons are pumped out of cell via cyt bc1 complex in cell membrane; chlorosome rapidly transfers energy to RC while protecting photopigments from oxidative damage

Cyanobacteria metabolism

Photolithoautotrophs that mediate oxygenic photosynthesis

N2

Nitrogen source for cyanobacteria

Thylakoids

Intracellular membrane-bound sacs that harbor reaction centers, covered in phycobilisomes

Phycobilisomes

LH complex attached to PSI/PSII in cyanobacteria with phycobilins of various wavelengths

Lumen

Interior of the thylakoid

Thylakoid membrane

PSI/PSII location

Outside thylakoid membrane

Phycobilisomes are found here

Protons are pumped into lumen; ATP made in exterior cytoplasm/stroma

Function and purpose of EC gradient in thylakoid

Cytoplasmic membrane in cyanobacteria

No chlorophyll or PS components but has aerobic respiratory chain

1st step of general photosynthesis mechanism

LH absorbs light energy, shifting energy of electrons of pigments

2nd step of general photosynthesis mechanism

Energy dissipated (not e-) is transferred to RC primary donor special pair (chl or bchl). The pair is excited and becomes more negative in redox potential.

3rd step of general photosynthesis mechanism

e- is transferred from special pair to electron acceptor, effluxing protons in the process in generating an electrochemical gradient

Electron donor P that lost its e- must be re-neutralized by an e-

must happen before subsequent PS rounds

Photopigment excitation (in LH complex) by a photon occurs…

when the photon energy (hv) matches energy required to excite an electron

After excitation of the RC special pair…

energy can be transferred to neighboring molecule, exciting its electron

Fluorescence emission

wasteful energy release during photosynthesis

4/3 ATP per 2e-

ATP generated by EC gradient in Type II RC (Q-type)

Not done by photosystems; coupled to oxidation of organic carbon or H2 gas

NAD(P)H production in organisms with exclusively Type II (Q-type) RCs

Reverse Electron Transport

Used to generate NAD(P)H in organisms with Type II (Q-type) RCs

H2S, S2O3^2-, S0, Fe2+

External electron donors for reverse electron transport

Quinone pool

More positive redox potential than NAD+, EC gradient is needed for energy input

Fe-S-cluster type donor redox potential

Slightly more negative than Q-type

Donor —> Fe-S-cluster-proton —> menaquinone pool or ferredoxin

Path of electron transport in Fe-S-cluster-type

Ferredoxin

Fe-S cluster protein, which helps photoreduce NADP+ to NADPH (external donor is needed to neutralize positive special pair if this path is chosen)

Menaquinone pool (in Fe-S-cluster Type I RC)

leads to bc1 complex similar to type II, generating EC gradient and reneutralizing P 840 +

P 680

Electron donor in PSII, starts at very positive redox potential and can accept electrons from water

In anoxygenic bacteria, neither RCI (Fe-S-cluster-type) nor RCII (Q-type) can use…

electrons from water

Oxygen Evolving Complex

splits water molecules into O2, protons, and electrons, which reduces P680

When energized, the OEC…

transfers electrons to Q-type site to plastoquinone pool to bc1 type cytochrome complex (b6f), generating electrochemical gradient.

P 700

Special pair for second reaction center in oxygenic photosynthesis (PSI)

PSI

Chlorophyll site, becomes energized by light, amazing donor to Fe-S type system to reduce NADP+ into NADPH

Red drop

Dramatic drop in oxygen evolution above 680 nm wavelength

PSII

Non-cyclic photosystem

Photofermentation by cyanobacteria

Nitrogenase makes hydrogen and carbon metabolism makes fermentation end products

Photofermentation mechanism

Hitting photosystems with light energy generates EC gradient which can make ATP (NADPH from PSI reduction); linked to the reduction of CO2 into carbon molecules for cell biomass and fermentation end products from GAP/G3P.

Generates EC gradient, minimizes energy need, maintains osmotic balance, controls cross-membrane ion excitability for phototaxis control and regulation

Function of retinal-based light driven complexes in archaea

Bacteriorhodopsin pumps…

protons outside the cell to generate EC gradient

Halorhodopsin pumps…

negative ions inside the cell; favorable for organisms and energy

Bacteriorhodopsin Function

Photon changes retinal confirmation from all-trans to cis, causing efflux of protons to extracellular space