Cathy Clarke, Ph.D.

Introduction – Ubiquinone (coenzyme Q or Q) functions in cells as a redox-active coenzyme of mitochondrial and plasma membrane electron transport, as well as an essential lipid soluble antioxidant. Human dietary supplementation with Q appears to have beneficial effects in slowing the progression of neuro- and muscle-degenerative diseases. However, Q is also involved in generating reactive oxygen species, through the adventitious reduction of dioxygen to superoxide by the ubisemiquinone radical, normally generated during mitochondrial electron transport. Thus, it is not clear how dietary supplementation with Q impacts the Dr. Jekyll/Mr. Hyde aspects of Q function. Cells are capable of synthesizing Q, but much remains to be learned about the sites of its synthesis, mechanisms of inter- and intra-cellular transport, and the regulation and enzymology of its biosynthesis. Research in my laboratory identified eight of the eleven polypeptides required for Q biosynthesis. The goals of my research are to characterize the Coq polypeptides responsible for production of Q and to determine how their activity can be modulated for optimal health.

Research Overview –

(1) We discovered a new Q biosynthetic pathway and showed that yeast are able to synthesize Q from a new aromatic ring precursor, para-aminobenzoic acid (pABA) [77]. This pathway operates in addition to the classic Q biosynthetic pathway that emanates from 4-hydroxybenzoic acid (4HB). We suggest a mechanism where Schiff base mediated deimination forms DMQ₆ quinone, thereby eliminating the nitrogen contributed by pABA. This scheme results in the convergence of the 4HB and pABA pathways in eukaryotic Q biosynthesis and has implications regarding the action of pABA-based antifolates. This work is funded by a grant from NSF.

(2) The findings we made in the yeast model have shed much light on diseases resulting from Q deficiency [82, 85]. We showed that human Coq8 and Coq6 polypeptide homologues function to rescue the respective yeast coq mutants [80, 81]. We discovered that yeast Coq8p, and its human homologue ADCK3 function in the phosphorylation of Coq3p, Coq5p, and Coq7p [80]. The kinase function of Coq8p is proposed to be important in the maintenance or formation of a high molecular mass Coq polypeptide complex, which is required for Q biosynthesis.

(3) Over-expression of Coq8p in many of the yeast coq null mutants restores steady state levels of Coq polypeptides. This has been a particularly important discovery, because the yeast coq null mutants (coq3-coq9) previously accumulated the same early Q-intermediate. Coq8 over-expression has enabled us to detect late-stage Q-intermediates that were previously elusive [86]. Finally, we used synthetic analogues of 4-hydroxybenzoic acid to bypass deficient biosynthetic steps and showed that 2,4-dihydroxybenzoic acid is able to restore Q₆ biosynthesis and respiratory growth in a coq7 null mutant over-expressing Coq8. The over-expression of Coq8 and the use of analogues of 4-HB represent new tools to elucidate the Q biosynthetic pathway. These recent discoveries are depicted in Figure 1.