Thursday, December 1, 2011

Science:

Give me red !

We learnt in school that green chlorophyll is necessary for photosynthesis. So what do red algae do for food? 

T.V. Jayan finds out
The Telegraph, Calcutta.

Team work: The researchers at Max Planck (from left) Matthias Stotz; Manajit Hayer-Hartl; Oliver Mueller-Cajar, Andreas Bracher, F. Ulrich Hartl=================================================
Science has finally pried open one of Nature’s long-kept secrets: how red algae, which include seaweeds, undertake photosynthesis, the most fundamental process without which life would not have been possible on earth. Though the process is fairly well known in green plants and blue-green algae, what regulates photosynthesis in red algae remained a mystery till date.
The findings have not just academic value but the potential to solve two critical problems facing humanity: food scarcity and global warming.
More than 50 years ago, scientists discovered that a key protein called Rubisco is responsible for photosynthesis. A complex molecule containing 16 sub-units, Rubisco bonds with carbon dioxide and converts it into sugar. While this sugar is the primary source of energy for living organisms, oxygen, which is a by-product of photosynthesis is released into the atmosphere.
However, despite Rubisco’s fundamental importance, the enzyme has many shortcomings. One of the problems is that it does not bind just with CO2, but also with oxygen. Its interaction with sugar molecules, including the natural substrate sugar ribulose-1,5-bisphosphate (RuBP), can also inactivate the Rubisco enzyme. For photosynthesis to take place, Rubisco has to be saved from the clutches of such inhibitors.

For a while now, scientists have known how the safeguarding process works in green plants — research has shown that a helper protein called Rubisco activase does this job. However, researchers failed to find a similar protein similar in red algae.

Now, two studies by a team of scientists at the Max Planck Institute of Biochemistry (MPIB) in Germany published last week — in Nature and Nature Structural and Molecular Biology — have resolved this puzzle. The team has not only discovered a mechanism that keeps Rubisco active in red algae, but also proved that the mechanism is distinctly different from that in green plants.

“A protein called CbbX is required to maintain Rubisco active during photosynthesis. This is quire similar to Rubisco activase found in green plants,” says Manajit Hayer-Hartl who heads the MPIB team. Her team has identified, for the first time, a protein that ensures photosynthesis — which uses water and carbon dioxide in presence of sunlight to produce sugar and oxygen — go unhindered in red algae and certain cyanobacteria.

Which brings us to this study’s Indian connection: Hayer-Hartl. Though she was born and brought up in Singapore, Hayer-Hartl’s parents are originally from a village near Goraya in Punjab.




“Rubisco is a relatively slow enzyme, fixing only two molecules of CO2 per second. Worse, it distinguishes poorly between CO2 and O2. Moreover, the enzyme is finicky about the sequence in which it binds with RuBP and CO2. It works fine if it binds CO2 first and then RuBP, but freezes if it binds the substrates in the reverse order,” says M.K. Mathew, professor at the National Centre for Biological Sciences, Bangalore.

“If the binding order is random then after some time all the enzyme will be inactivated. This is where the activase comes in. Green plants and green algae have an activase that uses ATP energy to pull off the RuBP from inactivated enzyme, allowing the cycle to start afresh. The corresponding enzyme in red algae had not been found so far,” Mathew says.

Hayer-Hartl says that the Rubisco activase in red algae repairs useless Rubisco proteins by pulling on one end of the protein, like someone opening a shoe string. In doing so, the helper protein opens the active centre of Rubisco and releases the inhibitory sugar. In green plants, on the other hand, the Rubisco activase works like an egg opener, squeezing the inactive Rubisco protein and forcing it to let go of the sugar molecules.

The scientists hope that the new insights they gained into Rubisco activase in red algae (which is twice as efficient as that in green plants) can help them design a modified Rubisco that can absorb CO2 from the atmosphere more effectively. If it happens, plants may be able to absorb more CO2 — which is predominantly responsible for global warming — helping the world fight climate change better. Similar, if the modified enzyme is introduced in food crops, it is possible to improve yield.

“Understanding the structure and function of the two activase helper proteins should facilitate efforts to generate plants and micro-organisms that are able to convert more CO2 into biomass than nature does,” she says. “This is especially important for the plant enzyme, which is thermolabile (changes in response to heat) and is expected to lose its activity as the weather becomes warmer.”
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