The recent discovery of isologs to the nuclear-coded subunits of cytochrome c oxidase has led to the hypothesis that these polypeptides play a role in the modulation of cytochrome c oxidase activity and that this modulation is affected by a differential expression of their genes in response to environmental (or developmental) signals. This hypothesis will be tested by studying the two isologs Va and Vb for subunit V of yeast cytochrome c oxidase. Preliminary studies have suggested that mutant cells carrying only Vb produce cytochrome c oxidase molecules whose in situ turnover numbers are higher than those produced in mutant cells carrying only Va. The effects of Va and Vb on the binuclear reaction center in holocytochrome c oxidase and the interactions of the enzyme with its substrates oxygen and cytochrome c will be examined. Do the holoenzymes that carry Va or Vb interact preferentially with one of the two cytochrome c isologs? An answer will be sought. Chimeric proteins will be contructed and the domains in Va and Vb that modulate the catalytic functions of the enzyme will be identified and located with respect to the inner mitochondrial membrane. How does intracellular levels of Va and Vb change in response to cell physiology? Cellular energy production is necessary for the growth and division of cells and cytochrome c oxidase is a key component in the respiratory chain, a multienzyme complex. Mitochondrial oxidative phosphoylation is capable of supplying more than 95% of the total ATP requirement in respiring encaryotic cells. The respiratory chain that drives it acts in series to affect the transfer of electrons from reduced substrates to water. Cytochrome c is the terminal member of this chain and seems to be the key enzyme in cellular energy production. It is very important to discover how eukaryotic cells alter their cytochrome c oxidase activity levels in response to energy demand. Dr. Poyton has done very significant work in this field and is certainly well suited to carry out this investigation.
