Metabolism of Aromatic Ring Precursors in Coenzyme Q Biosynthesis

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Scientific Merit:ATP, the cell's energy currency, is largely produced in mitochondria, membrane-bound bodies within cells that couple ATP formation to the controlled burn of carbohydrates or lipids (fat). Coenzyme Q (also known as ubiquinone or CoQ) is a complex organic molecule essential for mitochondrial energy production. CoQ is also a crucial antioxidant that can protect membrane lipids from oxidative damage. The CoQ molecule has two parts: (1) a long "tail" that anchors it in the membrane, and (2) an aromatic ring structure (benzoquinone) with properties that allow it to ferry electrons and protons essential to ATP formation. Cells are generally thought to synthesize the benzoquinone ring from the precursor 4-hydroxybenzoic acid (4-HB). Although nine genes essential for CoQ biosynthesis have been identified in yeast, plants and animals, none are required for 4-HB synthesis. Therefore, major questions remain unanswered regarding the generation of the aromatic benzoquinone component of CoQ. 4-HB has been believed to be synthesized in bacteria and yeast via coumarate from the phenylpropanoid biosynthesis pathway, a pathway used by plants to produce a wide variety of metabolites, or in animals by using the amino acid tyrosine. Recent observations indicate that yeast can also use 4-aminobenzoic acid (pABA) as a ring precursor in CoQ biosynthesis. This is surprising because pABA is a well-known precursor of folate, one of the B vitamins, but has not previously been connected with CoQ. The utilization of pABA as a ring precursor represents a novel biosynthetic pathway for the production of CoQ. This research utilizes yeast to define how pABA is converted to CoQ, how tyrosine is converted to 4-HB, and how coumarate is converted to 4-HB. This research, therefore, addresses the basic biochemistry required for energy generating processes in living cells. Results may also lead to applications in agriculture and medicine, as the enzymes involved are potential targets for both herbicides and antimicrobials.Broader Impacts:This project affords undergraduate students, particularly underrepresented minority students, the opportunity to do cutting edge research. UCLA undergraduate students generated the results identifying pABA as a ring precursor to CoQ biosynthesis in yeast. Construction and characterization of yeast deletion mutants, characterization of yeast growth, determination of CoQ content, isotopic labeling of yeast cultured in different types of minimal media, and preparation and HPLC analyses of yeast lipid extracts are all carried out by undergraduate students working on independent research projects. The PI is Chair of the Diversity and Leadership Committee in the UCLA Department of Chemistry and Biochemistry, and both the PI and Co-PI are active in advising and mentoring women and under-represented minority undergraduate and graduate students.