| Autor: |
Hester, Gerko, Stark, Wilhelm, Moser, Markus, Kallen, Jörg, Marković-Housley, Zora, Jansonius, Johan N. |
| Zdroj: |
JMB Online (Journal of Molecular Biology); February 1999, Vol. 286 Issue: 3 p829-850, 22p |
| Abstrakt: |
Phosphoserine aminotransferase (PSAT; EC 2.6.1.52), a member of subgroup IV of the aminotransferases, catalyses the conversion of 3-phosphohydroxypyruvate to l-phosphoserine. The crystal structure of PSAT from Escherichia colihas been solved in space group P212121using MIRAS phases in combination with density modification and was refined to an R-factor of 17.5 % (Rfree=20.1 %) at 2.3 Å resolution. In addition, the structure of PSAT in complex with α-methyl-l-glutamate (AMG) has been refined to an R-factor of 18.5 % (Rfree=25.1 %) at 2.8 Å resolution. Each subunit (361 residues) of the PSAT homodimer is composed of a large pyridoxal-5′-phosphate binding domain (residues 16–268), consisting of a seven-stranded mainly parallel β-sheet, two additional β-strands and seven α-helices, and a small C-terminal domain, which incorporates a five-stranded β-sheet and two α-helices. A three-dimensional structural comparison to four other vitamin B6-dependent enzymes reveals that three α-helices of the large domain, as well as an N-terminal domain (subgroup II) or subdomain (subgroup I) are absent in PSAT. Its only 15 N-terminal residues form a single β-strand, which participates in the β-sheet of the C-terminal domain. The cofactor is bound through an aldimine linkage to Lys198 in the active site. In the PSAT-AMG complex Ser9 and Arg335 bind the AMG α-carboxylate group while His41, Arg42 and His328 are involved in binding the AMG side-chain. Arg77 binds the AMG side-chain indirectly through a solvent molecule and is expected to position itself during catalysis between the PLP phosphate group and the substrate side-chain. Comparison of the active sites of PSAT and aspartate aminotransferase suggests a similar catalytic mechanism, except for the transaldimination step, since in PSAT the Schiff base is protonated. Correlation of the PSAT crystal structure to a published profile sequence analysis of all subgroup IV members allows active site modelling of nifsand the proposal of a likely molecular reaction mechanism. |
| Databáze: |
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