Two tandem repeats of the cystathionine beta-synthase (CBS pair) domains present in archaea, repeat 2
This cd contains two tandem repeats of the cystathionine beta-synthase (CBS pair) domains found in Inosine monophosphate (IMP) dehydrogenases and related proteins including IMP dehydrogenase IX from Methanothermobacter. IMP dehydrogenase is an essential enzyme in the de novo biosynthesis of Guanosine monophosphate (GMP), catalyzing the NAD-dependent oxidation of IMP to xanthosine monophosphate (XMP). The CBS domain, named after human CBS, is a small domain originally identified in cystathionine beta-synthase and is subsequently found in a wide range of different proteins. CBS domains usually occur in tandem repeats. They associate to form a so-called Bateman domain or a CBS pair based on crystallographic studies in bacteria. The CBS pair was used as a basis for this cd hierarchy since the human CBS proteins can adopt the typical core structure and form an intramolecular CBS pair. The interface between the two CBS domains forms a cleft that is a potential ligand binding site. The CBS pair coexists with a variety of other functional domains and this has been used to help in its classification here. It has been proposed that the CBS domain may play a regulatory role, although its exact function is unknown. Mutations of conserved residues within this domain are associated with a variety of human hereditary diseases, including congenital myotonia, idiopathic generalized epilepsy, hypercalciuric nephrolithiasis, and classic Bartter syndrome (CLC chloride channel family members), Wolff-Parkinson-White syndrome (gamma 2 subunit of AMP-activated protein kinase), retinitis pigmentosa (IMP dehydrogenase-1), and homocystinuria (cystathionine beta-synthase).