Two tandem repeats of the cystathionine beta-synthase (CBS pair) domains associated with the ACT domain
The putative Acetoin Utilization Protein (Acub) from Vibrio Cholerae contains a CBS pair domain. The acetoin utilization protein plays a role in growth and sporulation on acetoin or butanediol for use as a carbon source. Acetoin is an important physiological metabolite excreted by many microorganisms. It is used as an external energy store by a number of fermentive bacteria. Acetoin is produced by the decarboxylation of alpha-acetolactate. Once superior carbon sources are exhausted, and the culture enters stationary phase, acetoin can be utilised in order to maintain the culture density. The conversion of acetoin into acetyl-CoA or 2,3-butanediol is catalysed by the acetoin dehydrogenase complex and acetoin reductase/2,3-butanediol dehydrogenase, respectively. Acetoin utilization proteins, acetylpolyamine amidohydrolases, and histone deacetylases are members of an ancient protein superfamily.This cd contains two tandem repeats of the cystathionine beta-synthase (CBS pair) domains in the acetoin utilization proteins in bacteria. Acetoin is a product of fermentative metabolism in many prokaryotic and eukaryotic microorganisms. They produce acetoin as an external carbon storage compound and then later reuse it as a carbon and energy source during their stationary phase and sporulation. In addition these CBS domains are associated with a downstream ACT (aspartate kinase/chorismate mutase/TyrA) domain, which is linked to a wide range of metabolic enzymes that are regulated by amino acid concentration. Pairs of ACT domains bind specifically to a particular amino acid leading to regulation of the linked enzyme. 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).
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