Scientists Make Enzyme That Could Help Explain Origins of Life

News Brief by Frederick Lu 

"When I start to tell people about this, they sometimes wonder if we're merely suggesting the possibility of such an enzyme -- but no, we actually made it" - Gerald F. Joyce, professor in The Scripps Research Institute's Departments of Chemistry and Cell and Molecular Biology.

By imitating early earth conditions in test tubes, the laboratory team led by Gerald F. Joyce has created cross-chiral ribozymes that could give new understanding to molecular evolution. Chiral molecules are have mirrored structures that are identical yet non-superimposable. Human hands serve as a good metaphor for this idea – no matter what, it is impossible to get all fingers to stack on top of each other when the palms are facing the same direction.

Molecules on Earth evolved so that the majority of molecules are right-handed, making interactions more efficient. However, it is unlikely that RNA molecules in early Earth conditions would only be of one chirality. 30 years ago, Joyce published a paper in Nature showing that early ribozymes, enzymatic molecules made of RNA that are believed by some scientists to be the first self-replicating molecules, would have difficulty evolving in an environment with both left and right handed RNA molecules. Ribozymes like the one developed by Joyce and his team make new strands of RNA based off its own sequence as a temple but with one change – the end result is a mirrored molecule due to the use RNA molecules of the opposite chirality. In turn, the mirrored molecule would then make a copy of the original ribozyme. This solves an evolutionary hurdle faced by ribozymes that don’t make mirrored copies; eventually, they would incorporate a molecule of the opposite handedness and accidently block further replication. Scientists have also realized that RNA’s ability to form base pairs with other RNA molecules impairs its ability to make a copy of itself. This issue could also be resolved if ribozymes worked solely on opposite handed RNA with which it would be impossible to form base pairs with.

With this information in mind, Joyce and his team sought to create a cross-chiral ribozyme. Using a method developed by postdoctoral fellow Jonathan T. Sczepansk called “test tube evolution”, the team mixed roughly a quadrillion “right handed” RNA molecules that could catalyze “left handed” RNA molecules that could then be removed from solution and amplified. After 10 rounds of selection and amplification, the researchers had a candidate molecule that they refined to form a 83-nucleotide long ribozyme that could function even when same handed RNA molecules are present. So far, this molecule has succeed in catalyzing 11 segments of opposite handed RNA that are capable of making copies of the original molecule. Now, the researchers hope to continuing developing the ribozyme so it can work with a larger selection of RNA to become a more generalized replicator – one that could allow for molecular evolution in the primordial soup of early Earth.

Scripps Research Institute. "Scientists make enzyme that could help explain origins of life." ScienceDaily. ScienceDaily, 29 October 2014. .