Adaptation of mitochondria to live within cells

(Mains GS 3 : Achievements of Indians in Science & Technology; Indigenization of Technology and Developing New Technology.)

Context:

• Biologists from the Centre for Cellular and Molecular Biology, Hyderabad (CCMB) found a clue from an organism that has been around from 2 billion years ago as to how mitochondria became an inseparable part of animal and plant cells.

Unexpected biochemistry

• The researchers identify two key transformations, one in the molecule known as DTD for short and another in the transfer-RNA (tRNA).
• Researchers observed some unexpected biochemistry of eukaryotic DTD that could be explained based on the endosymbiotic origin of complex eukaryotic cell organelles.”
• Endosymbiosis is an intense form of symbiosis when one of the organisms is captured and internalized by the other.

About mitochondria:

• Mitochondria are membrane-bound cell organelles that generate most of the chemical energy needed to power the cell's biochemical reactions.
• Chemical energy produced by the mitochondria is stored in a small molecule called adenosine triphosphate (ATP).
• Mitochondria contain their own small chromosomes. Generally, mitochondria, and therefore mitochondrial DNA, are inherited only from the mother.

Arisen from bacterial endosymbionts:

• Today, mitochondria are well known to be integral parts of the eukaryotic cell but they were not always part of the animal and plant cells.
• Once, about two billion years ago, a prokaryotic organism (without a nucleus) called archaea captured a bacterial cell and the bacterial cell learnt to live within the archaea as an endosymbiont.
• In the late 19th century, microscopists observed that organelles like chloroplast [and later mitochondria] undergo division inside eukaryotic cells that resembles bacterial division, which led them to suspect that these organelles might have arisen from bacterial endosymbionts.

Emergence of mitochondria:

• By studying an organism known as jakobid, which has been around since before animals and fungi branched off from plants and algae in the process of evolution, the researchers have identified two adjustments that had to take place to facilitate the integration of the two organisms.
• These adjustments were made in the process of optimisation when the two organisms merged together, evidently for compatibility.
• The researchers show that these changes, in a protein (DTD) and a tRNA (carrying an amino acid glycine for protein synthesis) are crucial for the successful emergence of mitochondria.

Course of evolution:

• Amino acids come with two types of handedness i.e. left-handed and right-handed; thus, their names have a prefix of L or D. All life forms function with only the L-amino acids, in addition to achiral glycine, in proteins.
• Performing the role of a proofreader, the protein DTD removes D-amino acids from entering protein synthesis.
• Before it got incorporated into the eukaryotes, when it was part of the bacterial cell, DTD would not act on glycine, which is essential for protein synthesis.
• However, Eukaryotic DTD has changed its recognition code preference in order to avoid untoward removal of glycine so that it would be compatible with the eukaryotic cell.
• Study shows that this switch in the recognition code is important without which DTD will be toxic to the eukaryotic cell.

Switch in base:

• The other change identified by the researchers is that mitochondrial tRNA(Gly) has changed its critical nucleotide base from U73 to A73, in order to be compatible with eukaryotic DTD.
• This switch in the so-called discriminator base of mitochondrial tRNA(Gly) is important for avoiding removal of glycine and thus stopping protein synthesis in mitochondria  as it can be toxic.
• This means that before the change took place in the nucleotide base, glycine would be removed, which would have been toxic for the cell as protein synthesis would not take place without glycine.

Conclusion:

• The researchers work shows how such molecular optimisation strategies are essential, when derived from different ancestors like archaea and bacteria, for the successful emergence of mitochondria and hence all of eukaryotic life as we see today including humans.