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Dihydroorotate dehydrogenases (DHODs) are common to all life and catalyze the oxidation of dihydroorotate (DHO) to orotate the precursor of all pyrimidine nucleotides. The core structure of all DHODs has a TIM-barrel topology (the PyrD subunit or domain) that harbors an FMN cofactor that interacts with DHO. There are two classes of DHOD enzymes. Each has unique structures and oxidant substrates that conserve part of the energy available by coupling the reaction to ATP synthesis. The class 1 enzymes are soluble and divided into classes 1A and 1B. Class 1A has fumarate as the electron acceptor forming succinate and is the simplest form of DHOD, successively binding DHO and fumarate at the same active site locale. Class 1B uses NAD+ as the oxidant and this form of DHOD is heterodimeric having, in addition to the PyrD subunit, a subunit (PyrK) whose structure is like those of ferredoxin reductases. PyrK adds a second active site with a bound FAD that interacts with the NAD+ substrate and includes an Fe2S2 center that resides at the interface of the subunits, forming a conduit for electrons. Class 2 DHODs have ubiquinone (UQ) as the electron acceptor. This form of DHOD is membrane associated via an N-terminal domain that also forms a quinone binding site end-on to the FMN xylene moiety. This arrangement uses the flavin to mediate between the substrates and as a redox partition between water-soluble NAD+ and lipid soluble UQ10. In this review, we summarize the structure and mechanism of DHOD enzymes.
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This content will become publicly available on August 1, 2026
The binding modes of quinones in flavoprotein oxidoreductases
Flavoprotein quinone reductases regenerate quinols which serve metabolic and antioxidant roles. These enzymes catalyze the two-electron oxidation of substrates and the subsequent two electron reduction of quinones. Despite the net two electron transfer between substrates, the binding mode of quinones is typically end-on to the flavin, rather than stacked, dictating that the oxidative half reaction cannot proceed via hydride transfer and must instead occur by two successive single electron transfers. Here we present a review of six of the most well-studied flavoprotein quinone reductases to establish a framework for discussing this positional orientation for the quinone oxidant. There are two non-mutually exclusive rationalizations for this binding mode where the flavin isoalloxazine acts as a redox partition. The first is that energetics of the single electron transfer pathway create a kinetic barrier to the reverse reaction, trapping electrons in the quinone pool and countering the high ratio of quinol to quinone present in the membrane. The second is that the end-on binding allows the enzymes to utilize different binding sites for cytosolic and membrane associated substrates, avoiding the need to desorb substrates. These effects may be additive and serve to funnel electrons into the quinone pool as efficiently as possible.
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- Award ID(s):
- 2203593
- PAR ID:
- 10613610
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Archives of Biochemistry and Biophysics
- Volume:
- 770
- Issue:
- C
- ISSN:
- 0003-9861
- Page Range / eLocation ID:
- 110443
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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