The pathway to pyrimidines: The essential focus on dihydroorotate dehydrogenase, the mitochondrial enzyme coupled to the respiratory chain

Abstract

This paper is based on the Anne Simmonds Memorial Lecture, given by Monika Löffler at the International Symposium on Purine and Pyrimidine Metabolism in Man, Lyon 2019. It is dedicated to H. Anne Simmonds (died 2010) - a founding member of the ESSPPMM, since 2003 Purine and Pyrimidine Society - and her outstanding contributions to the identification and study of inborn errors of purine and pyrimidine metabolism. The distinctive intracellular arrangement of pyrimidine de novo synthesis in higher eukaryotes is important to cells with a high demand for nucleic acid synthesis. The proximity of the enzyme active sites and the resulting channeling in CAD and UMP synthase is of kinetic benefit. The intervening enzyme dihydroorotate dehydrogenase (DHODH) is located in the mitochondrion with access to the ubiquinone pool, thus ensuring efficient removal of redox equivalents through the constitutive activity of the respiratory chain, also a mechanism through which the input of 2 ATP for carbamylphosphate synthesis is balanced by Oxphos. The obligatory contribution of O₂ to de novo UMP synthesis means that DHODH has a pivotal role in adapting the proliferative capacity of cells to different conditions of oxygenation, such as hypoxia in growing tumors. DHODH also is a validated drug target in inflammatory diseases. This survey of selected topics of personal interest and reflection spans some 40 years of our studies from tumor cell cultures under hypoxia to in vitro assays including purification from mitochondria, localization, cloning, expression, biochemical characterization, crystallisation, kinetics and inhibition patterns of eukaryotic DHODH enzymes.

Keywords:

• Pyrimidine de novo synthesis
• dihydroorotate
• mitochondria
• hypoxia
• inborn errors
• drugs

Acknowledgments

The composition of this and our previous reviews would have been not possible without the expert collaborative input by Elizabeth Carrey. I value greatly her idiomatic Britisch English and her contribution to the science described in this survey. Elke Zameitat (PhD 2005) and Wolfgang Knecht (PhD 1998) – as representatives of all enthousiastic Diploma and PhD/MD students - were awarded for their extraordinary achievements by Philipps-University Marburg. Support and grants for our projects have been acknowledged in the publications referred to in this article. This Lecture was generously sponsored by the Purine Metabolic Patients’ Association of the UK. Anne Simmonds was involved in the establishment of PUMPA and instrumental in its work with great devotion and professional skill. Finally, it is a pleasure to recognize that in the history and development of purine and pyrimidine investigation – also documented by the activites of the Purine and Pyrimidine Society - prominent figures like Gertrude Elion, Anne Simmonds and Mary-Ellen Jones have led the way for many female scientists, influencing research into enzymes of purine and pyrimidine metabolism from the 20^th to the 21^st century.

Disclosure statement

The authors declare that there is no conflict of interests.