Unlike the respiration that we all know wherein we breathe in oxygen and breathe out carbon-dioxide, cellular respiration is a series of metabolic reactions that convert chemical energy from the food that we eat into ATP, the energy currency of the cell. ATP can then be used whenever cells need energy. A key group of reactions in this process is the citric acid cycle that produces energy.
The citric acid cycle is a series of chemical reactions that take place within the cells of every living organism (including humans) that consume oxygen for survival. It is the connecting link between all living organisms and one of the fundamental metabolic pathways. Therefore, it was thought that this series of reactions evolved before the origin of life, sometime around 4 billion years ago on early earth.
In a new study conducted recently and published January 8, 2018 in the journal Nature Communications, chemists identified simple chemicals that could have existed on early earth as precursors to the components of the citric acid cycle.
The chemistry that runs our cells could have evolved much before life itself originated but was composed of much more simpler versions of the chemicals that we can detect today. In essence, it is probable that simpler chemical reactions on early earth gradually evolved into the complex biochemistry that we see inside cells today.
The new study describes how two non-biological cycles, called the 4-hydroxy-2-ketoglutarate (HKG) cycle and the malonate cycle could have come together to kick-start a crude version of the citric acid cycle. The two cycles use reactions that perform the same fundamental chemistry of α-ketoacids and β-ketoacids as in the citric acid cycle. These shared reactions include aldol additions, which bring new source molecules into the cycles, as well as beta and oxidative decarboxylations, which release the molecules as carbon dioxide.
As they ran these reactions, the researchers found they could produce amino acids in addition to carbon-dioxide, which are also the end products of the citric acid cycle. The researchers think that as biological molecules like enzymes became available, they could have led to the replacement of non-biological molecules in these fundamental reactions to make them more elaborate and efficient.
Their experiments demonstrate that key chemical reactions that support life today could have been carried out with ingredients likely present on the planet four billion years ago.
- G. Springsteen, et al., Linked cycles of oxidative decarboxylation of glyoxylate as protometabolic analogs of the citric acid cycle. Nature Communications 9 (2018). doi:10.1038/s41467-017-02591-0