Question

Does the crystal stucture of xanthine dehydrogenase exhibit quaternary structure? If yes, how many subunits do you recognize (what are they)?

Answer 1

Answer :-   Xanthine dehydrogenase/oxidase (XDH¹/XO) is a complex metallo-flavoprotein catalyzing the oxidative hydroxylation of purines, pyrimidines, pterines, and aldehyde substrates using NAD⁺ or molecular oxygen as electron acceptor. Mammalian XDH (EC 1.1.1.204) is a homodimer (290 kDa), with each subunit containing a single molybdenum cofactor (Moco) as well as two iron-sulfur clusters and an FAD molecule.

The enzyme is encoded by two genes, xdhA and xdhB, with the FeS clusters and FAD bound to the XDHA subunit and Moco bound to the XDHB subunit. A third protein, designated XDHC, was shown to be essential for XDH activity in R. capsulatus but was not found to be a subunit of active XDH (7).

The XDHB subunit it is cleaved into four fragments with molecular weights of 35,000, 30,000, 20,000 and 15,000, whereas the XDHA subunit remains unchanged. However, the quaternary structure and the activity of the enzyme remain unaltered, showing no increase in the activity with molecular oxygen as electron acceptor (data not shown).