Function and regulation of mitochondrial cytochrome c oxidase using mutant forms of the yeast enzyme

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Mitochondria are the major energy providers of our cells. They have a chain of proteins - the mitochondrial respiratory chain - embedded in their inner membrane that oxidise products derived from the metabolism of food and reduce oxygen to water. Cytochrome c oxidase (CcO) is the final component of this chain that catalyses the oxygen reduction reaction; in fact, it is this enzyme that is responsible for consumption of most of the oxygen that we breathe and it provides a significant fraction of our energy needs. Energy is released during its reaction with oxygen and CcO is able to trap it by creating a charge difference across the mitochondrial membrane in which it is located - rather equivalent to charging of a capacitor in electronics. This charge gradient is then be used by another enzyme, called ATP synthase, to make ATP. ATP is a stable chemical that can release energy when it is hydrolysed; it diffuses around the cell and supplies a large part of energy needed by the myriad of reactions required for healthy functioning of our cells and tissues.We already know a great deal about the atomic structure of the enzyme and the way in which it safely reacts with oxygen to release energy. We also have a very good idea of the general way that it uses the energy to make the charge gradient. However, in order to elucidate the specific details of the coupling mechanism we have had to rely heavily on studies of simpler bacterial forms of the enzyme, particularly aided by the ability to change individual parts of these bacterial enzymes by introducing mutations. However, such bacterial enzyme studies cannot address all aspects of how our own human, more complicated, CcO works. The structures of several different types of bacterial CcOs are known but, to date, the only mitochondrial CcO structure that has been resolved is that from cow hearts. Although all forms of CcO share a similar core structure around the region where oxygen in reduced, other structural differences between bacterial and mammalian CcOs have led to suggestions that the energy storage reactions are different from those seen in bacterial CcOs.. An even more striking difference is that mitochondrial CcOs have up to 10 additional subunits, none of which are found in bacterial CcOs and to date we have little understanding of their functions. There is also good evidence that, in contrast to bacterial CcOs, those in more complex organisms can occur in different forms and are regulated differently in the cellular environments of different tissues. It is important to understand these aspects because a range of human genetic diseases arise from malfunctioning of CcO. Unfortunately, studies of these uniquely mitochondrial CcO questions are severely hampered by the lack of a good system to introduce mutations.Remarkably, the CcO that is present in mitochondria of Baker's yeast cells is structurally extremely similar to human CcO mitochondria in terms of both its core structure and the structures of all of its additional subunits. Because of this, and because we can make alterations in parts of yeast CcO in the same way as has been done successfully with bacterial CcOs, we can now investigate just how these mitochondrial (and therefore human) CcOs function and can be controlled. The understanding of the unique aspects of the more complex mitochondrial enzymes will provide a platform for testing and understanding how the human enzyme functions and malfunctions in health and disease.