Snapshot Issue 59 September 2009
There is no life without energy. Much in the way a car needs petrol to run, we also need something essential to keep us going. And it is called adenosine tri-phosphate or ATP. ATP runs through every nook and cranny of our body to keep our heart pumping, our fingers moving and our thoughts alive. But – like petrol – we do not get it for free. We have to make it. So, in the great majority of our cells, we have powerhouses – known as mitochondria – that spend their time synthesizing ATP and distributing it where need be. Not surprisingly, hordes of proteins are involved in this process, one of which has been known for decades: cytochrome C.
Cytochrome C has been around for a very long time – almost as long as life really – which has made it an ideal candidate for evolutionary studies. In fact, Linus Pauling and Emile Zuckerkandl were the first to imagine the concept of a molecular clock based on comparisons of the sequences of haemoglobin and cytochrome C. The protein was discovered in the early part of the 20th century and besides becoming a paradigm in evolutionary research, cytochrome C has been a choice model in studies of electron transport, protein folding, molecular immunology, and at the heart of pioneering studies of site-directed mutagenesis.
If cytochrome C has been so popular in the study of electron transport, it is because that is what it does. It delivers electrons into the respiratory pathway – which is lodged in the mitochondrion – so that it can drive a proton pump which in turn will synthesize ATP. How ? By way of electrostatic interactions; the protein bounces between two respiratory chain complexes transferring electrons from one to the other. It can do this thanks to its haem which is harboured within its centre and protrudes just enough to grab hold of an electron for transfer.
Cytochrome C’s main role is to participate in the making of ATP. However, it is also involved in a number of other instances, one of which is apoptosis, or programmed cell death – thus giving the protein something of a Dr Jekyll and Mr Hyde quality. In the event of apoptosis, it is seen to leave the mitochondrion and collaborate with factors which are themselves directly involved in cell apoptosis. What is more, the growing loss of cytochrome C within the mitochondrion will also eventually interrupt the synthesis of ATP, and hence the fuel the cell needs to survive. So cytochrome C is yet another protein which has shed light on many biological processes and pioneered numerous studies… besides tiptoeing along the line between life and death.
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