Saturday, 4 February 2012

On The Origin Of The First Biotic Sugars

The 3.8 billion year old rocks from the Isua Greenstone Belts in Greenland
contain  many organic molecules, including sugars, which are vital to life
Sugars are one of the most vital groups of chemicals to the existence for life. Nearly all organisms use glucose as their main source of energy. All use deoxyribonucleic acid, the complex nucleotide which contains the sugar ribose, as their genetic encoder. Sugars would have been vital for abiogenesis to occur. Therefore they must have been present in the primordial soup from which the first living organisms came. However there is a problem with this theory: there is just one chemical path in nature which forms the necessary products.

This pathway, known as the formose reaction, begins with formaldehyde, thought to be a common constituent of the primordial soup. Yet completing the reaction requires some very complex and messy chemistry, producing only a small amount of ribose and other sugars completely useless to life forms. This posed a real problem for chemists trying to reconstruct the origins of life and its metabolic processes.

Then in 2007, Albert Eschenmoser, an organic chemist who recently retired from the Scripps Research Institute, proposed a new way that organic sugars could form in nature. The reaction begins with glyoxylate reacting with dihydroxyfumerate. This initiates a cascade of reactions, leading to the formation of sugars, including a fair amount of ribose. Eschenmoser and his colleague, Ramanarayanan Krishnamurthy, also from the Scripps Research Institute began to test this theory through a series of experiments.

The results showed that, in contrast to the formose pathway, which produces only a small amount of ribose, this new reaction, dubbed the glyoxylate pathway, produces far more of the sugar. In fact the reaction was so efficient, transforming nearly 100 % of the reactants into the desired end products, that the researchers doubted the accuracy of their data. 'Prebiotic reactions are usually pretty messy, so when we saw how clean this was we were really pleasantly surprised' said Krishnamurthy.

Apart from the sheer efficiency of the pathway, it is also likely that the starting product, glyoxylate, was also common in the primordial soup as it can be assembled relatively easily in nature from carbon monoxide. The sugars used by living organisms would have formed without problems, ready to become incorporated into yet more complicated molecules such as DNA.