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, 187 (9), 3201-5

The Active Site of O-acetylserine Sulfhydrylase Is the Anchor Point for Bienzyme Complex Formation With Serine Acetyltransferase

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The Active Site of O-acetylserine Sulfhydrylase Is the Anchor Point for Bienzyme Complex Formation With Serine Acetyltransferase

Bin Huang et al. J Bacteriol.

Abstract

The biosynthesis of cysteine in bacteria and plants is carried out by a two-step pathway, catalyzed by serine acetyltransferase (SAT) and O-acetylserine sulfhydrylase (OASS; O-acetylserine [thiol] lyase). The aerobic form of OASS forms a tight bienzyme complex with SAT in vivo, termed cysteine synthase. We have determined the crystal structure of OASS in complex with a C-terminal peptide of SAT required for bienzyme complex formation. The binding site of the peptide is at the active site of OASS, and its C-terminal carboxyl group occupies the same anion binding pocket as the alpha-carboxylate of the O-acetylserine substrate of OASS. These results explain the partial inhibition of OASS by SAT on complex formation as well as the competitive dissociation of the complex by O-acetylserine.

Figures

FIG. 1.
FIG. 1.
The reactions catalyzed by SAT and OASS. The feedback inhibition of SAT is also depicted.
FIG. 2.
FIG. 2.
(A) Stereoview superposition of HiOASS with peptide (yellow), StOASS with methionine in external aldimine linkage (cyan), and StOASS apoenzyme (gray). The atoms corresponding to PLP at the active site of HiOASS are depicted. (B) Space-filling diagram of dimeric HiOASS in complex with the peptide (yellow) and PLP (green) viewed parallel to the dimeric twofold axis. Figures were prepared with PyMOL (7).
FIG. 3.
FIG. 3.
(A) Unbiased Fo − Fc electron density corresponding to the last four residues of the C-terminal peptide of SAT bound to the active site of HiOASS. The map was calculated before fitting the peptide to the electron density. The contour level is 3.0 rms units. (B) Superposition of the active sites of HiOASS in complex with the C-terminal four residues of HiSAT (yellow bonds) and StOASS in external aldimine linkage with methionine (cyan bonds). Hydrophilic interactions are indicated by dotted lines and illustrate the shared interactions of OASS with the C-terminal carboxylate of the peptide and the α-carboxylate group of the methionine ligand bound to PLP. The locations of PLP, the methionine of StOASS linked to PLP, and the C-terminal residue of the peptide (Ile-P267) are labeled. (C) Stereoview of the active site of the HiOASS peptide complex. The peptide (pink), PLP (green), and water molecules (cyan) are depicted as are the amino-terminal residues of helix α2 that interact with the peptide carboxyl.

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