Presented by: Bishnu Adhikari

Evolution of Photosynthesis

2016.4.26

Contents Introduction Geological Evidence for Photosynthesis Mechanisms of Evolution Evolution of Cofactors Evolution of Protein Complexes Photosynthetic Reaction Centers Electron Transport Chains Summary

Introduction

• The energy gradient that maintains our biosphere is provided by photosynthesis.

Light• CO₂+ H2O (CH2O)6 + O2

• Atmosphere provides the basis of the energy gradient that sustains life

close to the Earth’s surface.

• The dominant group of photosynthetic organisms generates O2 through the decomposition of water..

• Electrons liberated in this process can be used to reduce inorganic carbon to form organic molecules to build cellular components.

• First photosynthetic organisms evolved early in the evolutionary history of life used reducing agents such as H2 or H2S as sources of electrons, rather than water.

• Use of water as an electron donor for the evolution of life is of particular importance.

• Photosynthesis, ancient process evolved via a complex path to produce the distribution of types of photosynthetic organisms and metabolisms.

• C3 , C4 and CAM are three major metabolic pathways of photosynthesis. C3 photosynthesis is the oldest and most common form.

• Oxidation of water in the process of photosynthesis lead to the formation of oxygen, critical importance for aerobic life forms.

• Photosynthesis using electron donors others than water is carried out in non cyanobacterial photosynthetic bacteria which generally operates under anaerobic conditions.

• Anoxygenic photosynthesis uses protein complexes may derive from the same ancestor from which oxygenic photosynthesis evolved.

Geological Evidence for Photosynthesis

• Meteorites may have provided large amounts of organic molecules.

• Chemical analysis of meteorites shows substantial amounts of organic materials.

• Carbon-based life forms incorporated into inorganic carbon when abiotic source of organic carbon was used up.

• Understanding of the emergence of life and photosynthesis has resulted from advances in the analysis of ancient rocks.

Oxygen

• Solid indicator for cyanobacterial-type photosynthesis.

• Ability of organisms to carry out oxygenic photosynthesis may have preceded the accumulation of oxygen in the atmosphere.

• Oxygen was very inefficient at first, organisms slowly developing the needed defenses against oxygen. Dissolved buffers prevented oxygen from escaping.

• Prominent buffer ferrous iron largely stop forming oxygen in the atmosphere.

• The increase of oxygen in the atmosphere comes from the nitrogen–oxygen redox cycle.

Carbon in Ancient Rocks

• Due to advances in the analysis of ancient rocks, great amount of progress in the understanding of life and photosynthesis.

• Early life may reach as far back as the oldest rocks but metamorphic events that may have changed ancient rocks.

Fossil Record

• The mineralized imprints of organisms provide another measure for the occurrence of life.

• The fossil record covers the diversification of vascular plants and the earlier eukaryotes.

Chemical Indicators• Different organic molecules derived from distinctive cellular components

used as biomarkers for specific organism groups.

• Oxygen-producing photosynthesis enabled the synthesis of biological molecules whose biogenesis is oxygen-dependent.

Genetic Evidence

• The presence of oxygen triggered a revolution in cellular metabolism.

• Oxygen can be generated from nitric-oxiden indicates that oxygen-dependent pathways may have been operational before the emergence of oxygenic photosynthesis.

Mechanisms of Evolution

Molecular Evolution

• Evolution occurs on a molecular level through changes in DNA that create novel proteins offering novel metabolic opportunities.

• Within an organism, gene or genome duplication may provide a “sand box” for molecular innovation.

• RC evolution is a case of gene duplication in which a single gene coding for a homodimeric protein is duplicated to derive heterodimeric RCs.

• Gene fusion and splitting are also the likely mechanisms behind the fused RC core and antennas.

• Lateral gene transfer enables the transfer of metabolic capabilities between organisms and is a likely present photosynthetic mechanism.

• Some genomes contain compact clusters that include genes coding for RCs and the synthesis of photosynthetic pigments serve as a vehicle for transfer of capabilities between organisms.

• Lateral gene transfer between Bacteria and Archaea and Bacteria may account for the present distribution of rhodopsins.

Evolution of Cofactors

Protein complexes and molecules are utilized to perform functions. of photosynthetic systems

Hemes• Share part of the biosynthetic pathway with chlorophylls.

• Heme carrying proteins were postulated present in the last common ancestor of Bacteria and Archaea .

Quinones• Membrane-bound quinones are nearly ubiquitous in Archaea, Bacteria,

and Eukarya.

Chlorophylls

• Chlorophylls are the defining feature for charge-separating RCs.

• Chlorophylls provide the principal antenna pigments in all RC containing organisms

Chlorophyll diversity• All chlorophylls are circularized terapyrroles with a central magnesium.

• The biogensis of chlorophylls is of interest in understanding the evolution of photosynthesis

• “Original” chlorophyll help in reconstructing the evolution of photosynthetic machinery in different organisms.

Quinones in quinone-type reaction centers

• The Q-type RCs contain two quinones as electron acceptors.

• Single-electron acceptor QA, and the second QB accepts two electrons

Quinones in iron sulfur-type reaction centers• Cyanobacteria and their plastid progeny use phylloquinone as a

membrane-bound one-electron acceptor in Photosystem I.

• Phylloquinone and Menaquinone have identical naphthoquinone head group but different side chains synthesized by homologous biosynthetic pathways

Oxygen-dependent chlorophyll biogenesis steps• The presence of oxygen allowed the development of novel reaction

pathways that include Chlorophyll biosynthesis.

• Three of the enzymes involved in chlorophyll biosynthesis are different in aerobic and anaerobic phototrophs.

• Facultative organisms contain both copies of the enzymes, whereas strict anaerobes contain only the anaerobic versions and aerobes have the aerobic versions.

Evolution of Protein Complexes

Rhodopsins

• Appear to be a simple way of harvesting light energy.

• Proteins that are composed of seven transmembrane helices and catalyze the light-driven translocation of ions across the membrane.

• Display a broad, yet patchy distribution in Archaea, thought to be the result of lateral gene transfer and gene loss.

Rhodopsin distribution

• Photo converters in the last common ancestor of Archaea or in the last common ancestor of Bacteria and Archaea.

Rhodopsin autotrophy

• Currently, light-driven, autotrophic life isn’t dependent on rhodopsins as photo-converters.

• Chlorophyll RCs and rhodopsin-based photo-converters do not seem to be functional within a single organism at the same time.

Photosynthetic Reaction Center

• A complex of several proteins, pigments and other co-factors that together execute the primary energy conversion reactions of photosynthesis.

• Have a fundamentally common structure.

• Composed of an integral membrane protein complex of essentially a homodimeric or heterodimeric nature to which pigments and redox-active co factors are bound.

• The RC complex is at the heart of photosynthesis.

• Anoxygenic phototrophs have just one type, either type I or II, while all oxygenic phototrophs have one of each type.

• Light energy is absorbed primarily by antenna pigments, which harvest light and transfer it to reaction centre.

Photosynthetic reaction centers can be divided into two groups.

1. Photosystem II-type ( Non iron type).

2. Photosystem-I type (FeS cluster type).

• The electron transport pathway utilizes only one branch of the electron transport chain.

• The interquinone electron transfer direction is roughly parallel to the plane of the membrane.

• This functional preference imposes a heterodimeric structure on the Q-type RCs

of cyanobacteria and eukaryotes

• The RCs of purple bacteria and filamentous anoxygenic phototrophs consist of a dimeric 5 TMH domain.

Quinone-type reaction centers (Q-type RCs)/ Photosystem II

Iron sulfur-type reaction centers (FeS-type RCs)/Photosystem I

• Electron is transferred to a cytochrome b6f complex and then to plastocyanin, a blue copper protein and electron carrier.

• Ferredoxin is a soluble protein containing a 2Fe-2S cluster coordinated by four cysteine residues.

• FeS-type RCs are 11 TMH dimers, 5 TMH electron transport core and the 6 TMH core antennae

• The FeS-type RCs are homodimeric; that is, the dimer is formed from two identical

11 TMH proteins.

Fig. 1. a: Schematic diagram indicating the transmembrane helical composition of photosynthetic reaction centers (RCs). b: Arrangement of electron transport cofactors involved in the charge separation and stabilization of Q-type and FeS-type RCs.

Quinone-type versus iron sulfur-type reaction centers

• FeS-type RCs are homodimeric, and can be assumed to be the original form. FeS-type RCs represent the ancestral form.

• Two FeS-cluster complexes that are the defining part of all FeS-type RCs are housed in subunits not found in Q-type RCs.

• FeS-cluster proteins may have different evolutionary origins, as the green sulfur and cyanobacterial subunits appear not to be closely related.

• Quinones in green sulfur bacteria and heliobacteria appear as potential status as vestigial cofactors.

The Ancestral Reaction Center

• Structural similarities of the type I and type II reaction centers provide a convincing argument for a common evolutionary.

• The function of the original reaction centre was probably the generation of ATP rather than that of reducing equivalents.

• Purple non-sulfur bacteria and green bacteria and heliobacteria fit the bill of potentially being close to the photosynthetic ancestor.

• Purple bacteria, green bacteria and heliobacteria have a cyclic electron transfer pathway.

Evolution of Photosynthetic Reaction Centers

A scenario for the early evolution of photosynthetic reaction centers

• Original reaction center was a protein monomer developed the ability to dimerize producing a homodimeric complex.

• This requires that a single gene reaction center existed at some time, which has since been replaced by the two gene reaction center.

• Gene duplication and subsequent divergence permitted the development of a heterodimeric complex.

• Purple bacteria RC is closely coupled dimer of bacteriochlorophylls that make up the primary electron donor of the complex.

• This pigment dimer was lacking in a reaction center consisting of only one protein subunit.

Fig 2. Scheme for evolutionary development of photosynthetic reaction centers.

Origin of the linked reaction centers in oxygen evolving organisms

• Linked photosystems found in oxygen-evolving organisms is some sort of genetic fusion event took place between two bacteria.

• One with a pheophytin-quinone reaction center and the other with an FeS reaction center.

• This produced a chimeric organism with two unlinked photosystems.

• Subsequently, the two photosystems were linked, and the oxygen evolving system added.

Fig. 3. Scenario for evolution of photosynthetic reaction centers

Electron Transport Chains

• Primary photochemistry and other secondary electron transfer reactions take place within the RC complex.

• Additional electron transfer processes are necessary before the process of energy storage is complete.

• Cytochrome bc1 and b6f complexes oxidize quinols produced by photochemistry in type II RCs or via cyclic.

• All phototrophic organisms have a cytochrome bc1 or b6f complex of generally similar architecture.

Fig. 4. Electron transport diagram indicating the types of Photosynthetic RCs and electron transport pathways found in different groups of photosynthetic organisms.

Summary

• The process of photosynthesis originated early in Earth’s history.

• Evolved to its current mechanistic diversity and phylogenetic distribution by a complex, nonlinear process.

• The evolutionary history of photosynthetic organisms is further complicated .

• Lateral gene transfer that involved photosynthetic components as well as by endosymbiotic events.

• Photosynthesis originated and developed, from primitive cells through anoxygenic photosynthetic bacteria, through cyanobacteria and eventually to chloroplasts.

• The evidences suggest that the earliest photosynthetic organisms were anoxygenic.

• All photosynthetic RCs have been derived from a single source.

• Ancestor RCs have been a multimeric composite of a limited number of one-helix proteins that have fused together.

THANK YOU!