Today’s post highlights another plenary talk from the Midwest Photosynthesis Meeting last weekend. Dr. James Moroney from our very own Louisiana State University presented work from his lab on investigating and improving the dark side of photosynthesis. Click here for a general review on the dark reactions (yeah, yeah they’re really just light-independent).
The carbon fixation reactions seem unnecessarily convoluted, but this is the only way biological systems fix carbon dioxide into organic compounds for energy storage. Beyond this, the process suffers from other inefficiencies. The enzymatic entry point for carbon dioxide into the cycle is Rubisco, which takes carbon dioxide and 1,5-RuBP and forms two molecules of 3-phosphoglycerate. The problem is that Rubisco isn’t a very good enzyme. It’s slow and has a very low turnover, and it often gets confused about its substrates. If it uses oxygen instead of carbon dioxide, it yields only one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate. Cells must then dedicate additional energy recycling the phosphoglycolate into 3-phosphoglycerate and carbon dioxide so these regenerated substrates can re-enter the carbon fixation cycle.
Preventing the oxygenase pathway is particularly important for photosynthetic organisms that are constantly making oxygen as a product of the light reactions. To do this and simultaneously combat the inherent enzymatic slowness of Rubsico, photosynthetic organisms have elaborate systems to raise the local concentration of carbon dioxide in the vicinity of Rubisco. These systems are collectively called the ‘Carbon Concentrating Mechanism’ or CCM. Some mechanisms include trapping carbon dioxide as bicarbonate inside cells. Whereas CO2 can traverse biological membranes easily, the charged bicarbonate molecule cannot. Among the photosynthesizers, cyanobacteria and algae have much better CCMs than plants. They have elaborate machinery for uptake of both CO2 and bicarbonate as well as a series of carbonic anhydrases strategically placed to interconvert CO2 and bicarbonate as necessary. These organisms also physically isolate large quantities of the Rubsico enzyme into specialized locations within the cell.
The Moroney lab is interested in figuring out how CO2/bicarbonate is taken up, partitioned among cellular compartments and eventually fed to Rubisco in both plant and algal systems. This kind of basic understanding of the systems is necessary to eventually improve upon them. For instance, can CCM components from cyanobacteria and algae work in plants to make plants more efficient at carbon fixation. However, before researchers can begin to put these systems together, they must first understand how the independent systems work. A critical limitation in understanding the algal CCM is a lack of available mutants in known or putative CCM components. Moroney described a genetic screen that was performed by his group to isolate the necessary mutants in the algal Chalmydomonas reinhardtii. By sampling tens of thousands of mutants, they were able to identify some known genes they needed mutants in as well as some additional interesting mutants based on phenotype. They have also developed a system for efficiently mapping the positions of the insertion mutations so that novel genes can be identified.
The carbon fixation reactions are the rate-limiting step for photosynthesis and represent the area with the largest margin for improvement. A number of different presentations at the Turkey Run meeting were focused on figuring out the CCM secrets of cyanobacteria and algae.