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| {{STRUCTURE_2vgv| PDB=2vgv | SCENE= }} | | {{STRUCTURE_2vgv| PDB=2vgv | SCENE= }} |
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| '''CRYSTAL STRUCTURE OF E53QBSSHMT OBTAINED IN THE PRESENCE OF L-ALLO-THREONINE'''
| | ===CRYSTAL STRUCTURE OF E53QBSSHMT OBTAINED IN THE PRESENCE OF L-ALLO-THREONINE=== |
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| ==Overview==
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| Serine hydroxymethyltransferase (SHMT) belongs to the alpha-family of pyridoxal 5'-phosphate-dependent enzymes and catalyzes the reversible conversion of L-Ser and tetrahydrofolate to Gly and 5,10-methylene tetrahydrofolate. 5,10-Methylene tetrahydrofolate serves as a source of one-carbon fragment in many biological processes. SHMT also catalyzes the tetrahydrofolate-independent conversion of L-allo-Thr to Gly and acetaldehyde. The crystal structure of Bacillus stearothermophilus SHMT (bsSHMT) suggested that E53 interacts with the substrate, L-Ser and tetrahydrofolate. To elucidate the role of E53, it was mutated to Q and structural and biochemical studies were carried out with the mutant enzyme. The internal aldimine structure of E53QbsSHMT was similar to that of the wild-type enzyme, except for significant changes at Q53, Y60 and Y61. The carboxyl of Gly and side chain of L-Ser were in two conformations in the respective external aldimine structures. The mutant enzyme was completely inactive for tetrahydrofolate-dependent cleavage of L-Ser, whereas there was a 1.5-fold increase in the rate of tetrahydrofolate-independent reaction with L-allo-Thr. The results obtained from these studies suggest that E53 plays an essential role in tetrahydrofolate/5-formyl tetrahydrofolate binding and in the proper positioning of Cbeta of L-Ser for direct attack by N5 of tetrahydrofolate. Most interestingly, the structure of the complex obtained by cocrystallization of E53QbsSHMT with Gly and 5-formyl tetrahydrofolate revealed the gem-diamine form of pyridoxal 5'-phosphate bound to Gly and active site Lys. However, density for 5-formyl tetrahydrofolate was not observed. Gly carboxylate was in a single conformation, whereas pyridoxal 5'-phosphate had two distinct conformations. The differences between the structures of this complex and Gly external aldimine suggest that the changes induced by initial binding of 5-formyl tetrahydrofolate are retained even though 5-formyl tetrahydrofolate is absent in the final structure. Spectral studies carried out with this mutant enzyme also suggest that 5-formyl tetrahydrofolate binds to the E53QbsSHMT-Gly complex forming a quinonoid intermediate and falls off within 4 h of dialysis, leaving behind the mutant enzyme in the gem-diamine form. This is the first report to provide direct evidence for enzyme memory based on the crystal structure of enzyme complexes.
| | The line below this paragraph, {{ABSTRACT_PUBMED_17651438}}, adds the Publication Abstract to the page |
| | (as it appears on PubMed at http://www.pubmed.gov), where 17651438 is the PubMed ID number. |
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| | {{ABSTRACT_PUBMED_17651438}} |
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| ==About this Structure== | | ==About this Structure== |
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| [[Category: Shmt]] | | [[Category: Shmt]] |
| [[Category: Transferase]] | | [[Category: Transferase]] |
| ''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Sun May 4 18:47:17 2008'' | | |
| | ''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Tue Jul 29 05:12:37 2008'' |
