Perhaps, we are all very familiar with cyanide, the chemical which takes life out of us! But, have you heard of cytochrome oxidase - a key protein involved in respiration which gives us our life?

You may ask, what relation do the two of them have? It is precisely the interaction of these two players that we see in a suicide victim. Cyanide binds to cytochrome oxidase, rendering it useless for respiration by blocking its function as an electron transporter, thus squeezing the life out of us.

Cytochrome oxidase, a large transmembrane lipo-protein found in bacteria and the inner mitochondrial membrane of eukaryotic cells is a rate limiting enzyme in the electron transport chain. In higher animals it has a very complex dimeric structure having two identical parts - each a composite of 13 subunits. Interestingly, unlike most other proteins, 3 subunits each of cytochrome oxidase are coded by mitochondrial DNA.

Over 90 per cent of the world's oxygen utilisation by life is due to the function of this protein which turns oxygen into water in the process of burning food, thus producing energy in the form of ATP.

Leasing new life into taxonomy: In the recent years, cytochrome oxidase has found significance in biological systematics. Not the protein by itself, but the gene coding for one of its subunits - cytochrome oxidase subunit 1 (CO1) - is proving itself to be a remarkable tool in species identification and classification.

With millions of species and life stage transformations, taxonomists find it extremely difficult and challenging to identify and classify biological specimens. The classical species identification based on morphological features is quite difficult and laborious. Also, it has become too technical requiring specialisation, leaving very few experts available to work on a vast number of biological samples.

It was at this juncture that scientists hoped for an alternative which would make the task of species identification reliable, easier and available to all. Looking at the whole DNA is laborious and expensive, but sequencing a very short standard stretch of DNA should be easy, powerful, quick and cheap. The approach could be termed as DNA 'bar-coding', similar to the 'barcode' on products in a supermarket, giving identity and information on a catalogued species.

A 648 base pair region of CO1 gene has found itself as one of the ideal candidates in such an endeavour, due to its unique features. It is a mitochondrial gene inherited from mother, and lasts longer than nuclear DNA. It also accumulates mutations quicker than nuclear DNA.

It was Prof Paul Hebert, a zoologist at the University of Guelph, Canada who initiated and popularised the concept of species bar-coding. He had bar-coded hundreds of North American bird species using CO1 and showed the power of bar-coding in delineating specimens to subspecies and individual level.

Dr Daniel Janzen, an ecologist at the University of Pennsylvania, has recently used CO1 gene to barcode the lepidopteron insects in the Costa Rican rain forest. This remarkable molecular tool helped Jansen resolve closely related skipper butterfly species.

The issue was particularly interesting as one species of skipper butterfly showed incredible divergence in its larval coloration and pattern. What is intriguing is that each larval form is feeding on specific plant species. There could be no means by which they can be separated into different species as adults from all of them are similar, with no difference whatsoever, the classical taxonomy relies heavily upon.

However, it was obvious from the bar-coding that showed them to belong to distinct clusters. Those species have arguably diverged some four million years ago, driven by the shift in feeding preferences.

Thanks to CO1, we can now hope to identify species without getting lost in tons of dusted and faded taxonomy books. You must have been annoyed by those lousy flies falling on your cup every time you choose to sip one.
Soon you may at least know the species by bar-coding it using a small hand-held barcode machine before pondering on the next question - why does it want to share your coffee?

On the flip side...

Bar-coding using CO1 has some limitations. A recent study showed the limited ability of CO1 in delineating some fly species. It is not suitable for bar-coding plants because of its much slower rate of evolution. Scientists, however, have identified two other genes - a nuclear internal transcribed spacer (ITS) and plastid trnH-psbA intergenic spacer as the ideal candidates for bar-coding plants.