General Information
Coenzyme A (CoASH) replaces glutathione as the major low-molecular weight thiol in Staphylococcus aureus it is maintained in the reduced state by coenzyme A-disulfide reductase (CoADR), a homodimeric enzyme similar to NADH peroxidase, but containing a novel Cys43-SSCoA redox center.[1]
Coenzyme A-Disulfide Reductase, is an enzyme that is proposed to be involved in the robust oxygen-defense systems of aerobic and facultatively anaerobic organisms. [2]
The maintenance of low intracellular levels of cysteine in organisms has been attributed to the avoidance of the hydrogen peroxide produced during the rapid O2-dependent oxidation to cystine, necessitating the use of other small molecular mass thiols such as glutathione for the maintenance of internal redox levels. The results presented above are consistent with a role for CoA in maintaining a reducing environment or serving as a pool of reducing equivalents at the very high temperatures and high concentrations of metals found in the natural environment of 'Pyrococcus' one of the bacteria CoADR is involved with. The pyrococcal CoADR described in this work is able to efficiently utilize both NADPH and NADH, a result which is consistent with the unusual utilization of reduced nucleotide coenzymes by Pyrococcus. The central metabolism of this organism uses an unusual NADPH-dependent sulfide dehydrogenase which is capable of both the NADPH-dependent reduction of elemental sulfur and the NADP+-dependent oxidation of ferredoxin. [2]
phCoADR is stable for months at both −80 °C and −20 °C, and has half-lives of > 100 and 39 h at 85° and 95 °C, respectively. [2]
Structure
is a complex, multidomain protein composed of two chains, (,). It also has three different types of associated ligands; two , two and two
CoADR's ligands have been shown to follow four conserved sequence motifs. Among the four conserved sequence motifs identified by Dym and Eisenberg as being shared by all NAD(P)H-dependent members of the GR1 subfamily [which includes all of the PNDOR enzymes], one represents part of the βαβ Rossmann NAD(P)H dinucleotide-binding motif (). This specific motif has most recently been applied in the description and functional analysis of the “two dinucleotide binding domains” flavoproteins superfamily containing both FAD- and NAD(P)H-binding motifs.[3]
The crystal structure of CoADR reveals one symmetrical dimer (chains A and B) in the asymmetric unit, consistent with the quaternary structures observed with nearly all PNDOR enzymes. Both polypeptide chains are clearly defined in the electron density and have similar average B-factors of 17 Å2 and 18 Å2, respectively. Less well-defined regions of the protein involve two different segments with solvent-exposed loops, residues 51–61 in chain A with an average B-factor of 34 Å2 and residues 361–378 in chain B with an average B-factor of 24 Å2.[3][2] These two regions are better ordered in the corresponding complementary subunits with average B-factors of 21 Å2 and 14 Å2, respectively, owing in large part to crystal contacts with symmetry mates. These variations are therefore attributed to different crystal-packing environments rather than to intrinsic structural differences between the A and B chains. The CoADR monomer consists of three domains, the two-part FAD-binding domain (residues 1–114 and 242–319), the NADPH-binding domain (residues 115–241), and the C-terminal Interface domain (residues 323–438). As recently analyzed, this subfamily includes functionally diverse proteins such as glutamate synthase, adrenodoxin reductase, and cyclohexanone monooxygenase. [3]
The FAD- and NADPH-binding domains of CoADR both have canonical Rossmann folds ; each consists of a central five-stranded parallel β-sheet, with a three-stranded antiparallel β-sheet packed on one side of the central sheet and several α-helices on the opposite side. CoAS- is associated with a cleft at the dimer interface; this cleft is formed by portions of the FAD-binding domain of chain A and the Interface domain of chain B, which contains a large five-stranded antiparallel β-sheet with three short α-helices at the C-terminus.[3]
Medical Application
The human pathogen Staphylococcus aureus does not utilize the thiol/disulfide redox system based on glutathione and glutathione reductase (GSR)1 found in eukaryotes and Gram-negative bacteria. Instead, S. aureus appears to use a redox system based on CoA and coenzyme A disulfide reductase (CoADR) CoADR is a dimeric flavoprotein that specifically catalyzes the NADPH dependent reduction of oxidized CoA thereby contributing to the high ratio of CoA/oxidized CoA (<450) and the intracellular reducing environment. [4]
Additionally, the disparate disulfide specificities of CoADR and its presumed human counterpart, GSR, identified this enzyme as a possible target for the design of selective inhibitors that would interrupt the thiol metabolism of 'S. aureus' and function as anti-staphylococcal agents.[4]
