The Role of O-acetylserine Sulfhydrylase in Cysteine Biosynt
The Role of O-acetylserine Sulfhydrylase in Cysteine Biosynthesis in Salmonella tryphimurium:A structural and functional AnalysisPyridoxial 5'-phosphate (PLP) acts as a cofactor in many enzymes involved in diverse aspects of amino acid metabolism such as transamination, β/γ-elimination, β/γ-replacement and racemization. In all PLP-dependant enzymes the carbonyl group of the PLP coenzyme binds to an ε-amino group of a lysine residue in the active site, forming an internal aldimine. O-Acetylserine sulfahydrylase (OASS), isolated from Salmonella typhimurium, belongs to the β -family of PLP-dependant enzymes and catalyzes the last step in the cysteine biosynthesis pathway via β-replacement, converting O-acetylserine (OAS) to cysteine, upon exchanging acetate in the OAS side chain for sulfide (Fig. 1). The structural and functional framework underlying the reaction mechanism for OASS has been characterized extensively by kinetic studies, site-directed mutagenesis, UV-visible fluorescence and phosphorescence spectroscopy and x-ray structural determination. Three conformationally distinct "open", "closed", and "inhibited" states were elucidated.
(1996) used site-directed mutagenesis to replace the catalytic Lys41 of the active site to an alanine (mutant K41A). The first step of the reaction is catalyzed by the enzyme serine acetyltransferase (SAT), which involves acytelation of the β-hydroxyl group by acetyl-coA to yeild OAS (Kedrich and Tomkins, 1966). Reaction Mechanismβ-replacement reactions can be broken down into two halves (Miles, 1986). This is achieved by allosteric interaction with SAT (Ki of 1μM). , 1998), a closed conformation induced by substrate binding, (Burkhard et al. Evidence that the external aldimine linkage between PLP and Met can be reduced by sodium [H3] borohydride in the native conformation and not in the mutant K41A supports the hypothesis that the reactive intermediate is protected from bulk solvent in the closed state (Strambini et al. This subdomain is part of the N-terminus domain and comprises β-strand 4, α-helix 3, β-strand 5 and α-helix 4. [Note that in the crystal structure of the inhibited form, illustrated in figure 13, a sulfate ion was bound to mimic the substrate because it is involved with similar ionic interactions as L-Met in the closed conformation. Before OASS crystals were obtained and three-dimensional structures were determined, UV-visible fluorescence and phosphorescence studies suggested different conformations were sampled along the OASS reaction pathway (McClure and Cook, 1994; Woehl et al. OASS is thought to follow the pattern outlined in figure 6, both from kinetic studies, which describe a ping-pong mechanism (Tie et al. The comparison supports that when the protein is bound by Cl- at the allosteric site it cannot achieve the fully closed conformation. OASS Topology Understanding the organization of secondary, tertiary and quaternary structural elements is crucial to characterizing a protein's function. Inhibited Conformation Resulting from Local and Global Conformational Changes.
Common topics in this essay:
Proximity Pro36,
Protein Folding,
Conformational Changes,
N-terminus C-terminus,
Gly178 Thr180,
McClure Cook,
Dynamic AOSS,
Concluding Remarks,
Topology Understanding,
SH- Catalytic,
et al,
burkhard et al,
active site,
burkhard et,
et al 1996,
al 1996,
closed conformation,
schiff base,
conformational changes,
free energy,
amino acid,
inhibited conformation,
schiff base linkage,
et al 2000,
et al 1993,
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