IGEM:Stanford/2009/Photosynthesis

Project Summary

Attempt to increase rate/efficiency of photosynthesis by expressing non-native photosynthetic pigments in cyanobacteria. Our theory is if we can expand the wavelengths that cyanobacteria can absorb, then it can photosynthesize faster/more efficiently.

Original Presentation: Increase Absorption Range for Photosynthesis

What We Know

• Interesting general information sites:
  • Review of Photosynthesis
  • Overview of Photosynthetic Pigments
  • Photosynthetic Pigments: Structure and Spectroscopy

What We Don't Know (but need to know)

• How do we incorporate pigments into the photosystems?
• How do we measure photosynthesis rate or efficiency?

Experiment Ideas

I've been reading a chapter on Photosynthesis in the Berg Biochemistry book, and here are some of my ideas for an iGEM project in photosynthesis:

(Apologies beforehand; the notes aren't very coherent. They're mostly just musings at the moment.)

Cyanobacterial photosynthesis systems have only 4 bacteriochlorophyll b molecules bound, which is not as effective at absorbing photons and transferring the light energy carried by these photons to the reactions center (i.e. the chlorophyll a molecules in photosystem II absorb different wavelengths of light, not all of which are optimal, but as they drop back to ground state they re-emit a photon with a lower energy that may correspond to the optimal wavelength/energy; this photon would be taken up by the special pair P680). Engineer cyanobacteria to express a mutant photosynthesis system that binds more bacteriochlorophyll b? But photosynthetic bacteria do have light harvesting complexes (LHC). Or, to enable cyanobacteria to absorb more wavelengths of light, determine the proteins that reduce one of the pyrrole rings and add other small modifications to create the differences between bacteriochlorophyll b and plant chlorophyll a; can we knock out the genes for these proteins to see if the cyanobacteria produce chlorophyll a and absorb at the 680 nm wavelength?

Why do thylakoid membranes contain such high galactolipid contect (~40%)? Note that Glycolipids are found exclusively on the extra-cellular leaflet of lipid bilayers. Glycolipids include the sphingolipids, which are important to lipid raft formation. Are lipid raft analogues comprised of galactolipids present in thylakoid membranes to bring together photosystem II, cytochrom bf, and photosystem I?

Developing thylakoid membranes “bud off” of the inside chloroplast membrane; i.e. they are like cristae in mitochondria that have fully pinched off. Thus the lumen of the thylakoid is analogous to the intermembranous space of the mitochondria; both accumulate a high concentration of H+ during the generation of the H+ gradient needed for ATP synthesis (via F1-F0 ATP synthase in oxidative phosphorylation in mitochondria and via CF1-CF0 ATP synthase – chloroplast factor 1 and chloroplast factor 0 – in the light reactions in the thylakoid stacks/grana). Can we identify and stimulate the genes that cause this budding off, thus creating more thylakoid? We’d probably have to look at the genome of the chloroplast.

Note that bacteria, like mitochondria and chloroplasts, have two (or more) lipid bilayer membranes and that in the cyanobacteria the H+ accumulates in the periplasmic (intermembranous) space, just as in mitochondria and in the chloroplast analogue, the thylakoid lumen.

Cyanobacteria probably create a H+ gradient with a higher concentration of H+ in the periplasmic space because the cytoplasmic proteins are sensitive to pH.

The dark reactions occur in the stroma of the chloroplast. Note that peak solar radiation occurs at wavelengths between 450 and 650 nm.

UNRELATED: Can we kill bacteria by cause the integration of bacterial ATP synthase backwards, resulting in no ATP synthesis? Or could we even cause backward integration of cytochrome bc1 so that the cytoplasm drops in pH, denaturing the cytoplasmic proteins and killing the microbe? Antibacterial… We’d probably have to generate a virus that can mutate the integration gene for ATP synthase/cytochrome bc1.

Mark Y. Fang 03:48, 28 March 2009 (EDT)Mark

Important/Interesting Papers

• Chlorophyll b Expressed in Cyanobacteria Functions as a Light-harvesting Antenna in Photosystem I through Flexibility of the Proteins - This group expressed Chlorophyll B in Cyanobacteria and it incorporated itself into the photosystem!
• Other iGEM projects
  • A BioBrick toolkit for cyanobacteria - The team from Hawaii last year built some biobrick parts that work with Cyanobacteria
• see also Photosynthesis Links

Questions/Discussion

Before I forget about this, I remember reading somewhere that there is a protein called proteorhodopsin which can be put into E. coli so that the E. coli could potentially perform photosynthesis. This might be useful when thinking about how to design our project. -Leon

Jerome Bonnet: A good application for enhancing photosynthesis in cyanobacteria is increasing carbon sequestration:some folks have patented a method for "removing a carbon-containing compound from a flowing gas stream by interposing in the stream a membrane having photosynthetic microbes, such as algae and cyanobacteria, deposited thereon" [1].