Serine hydroxymethyltransferase (SHMT), a member of the ␣-class of pyridoxal phosphate-dependent enzymes, catalyzes the reversible conversion of serine to glycine and tetrahydrofolate to 5,10-methylene tetrahydrofolate. We present here the crystal structures of the native enzyme and its complexes with serine, glycine, glycine, and 5-formyl tetrahydrofolate (FTHF) from Bacillus stearothermophilus. The first structure of the serine-bound form of SHMT allows identification of residues involved in serine binding and catalysis. The SHMT-serine complex does not show any significant conformational change compared with the native enzyme, contrary to that expected for a conversion from an "open" to "closed" form of the enzyme. However, the ternary complex with FTHF and glycine shows the reported conformational changes. In contrast to the Escherichia coli enzyme, this complex shows asymmetric binding of the FTHF to the two monomers within the dimer in a way similar to the murine SHMT. Comparison of the ternary complex with the native enzyme reveals the structural basis for the conformational change and asymmetric binding of FTHF. The four structures presented here correspond to the various reaction intermediates of the catalytic pathway and provide evidence for a direct displacement mechanism for the hydroxymethyl transfer rather than a retroaldol cleavage.Serine hydroxymethyltransferase (SHMT; 1 EC 2.1.2.1) is a PLP-dependent enzyme that plays a central role in the onecarbon metabolism. It catalyzes the reversible inter-conversion of serine and tetrahydrofolate to glycine and 5,10-methylene tetrahydrofolate, a key intermediate in the biosynthesis of purine, thymidine, choline, and methionine (1, 2). In addition to this physiological reaction, SHMT has also been shown to catalyze THF-independent aldolytic cleavage, decarboxylation, racemization, and transamination reactions (3). The importance of SHMT in DNA synthesis coupled with the observed high level of enzyme activity in rapidly proliferating cells has focused attention on SHMT as a potential target for the development of anticancer and antimicrobial agents (4 -6).Several mechanisms have been proposed for the hydroxymethyl transfer, the most favored being the retroaldol cleavage (7,8). The crystal structures of human liver SHMT (hcSHMT) and rabbit liver SHMT (rcSHMT) and Escherichia coli SHMT (eSHMT) as well as murine cytoplasmic SHMT (mcSHMT) have been reported (9 -12). The structure of a reduced form of rcSHMT representing a gem diamine equivalent has also been reported (10). Although these structures have provided a wealth of information regarding the architecture of the enzyme, active site, and residues involved in substrate binding and catalysis, several aspects of SHMT catalytic mechanism remain uncertain (7, 13). A detailed comparison and analysis of several structures of the enzyme corresponding to different intermediate steps and in complex with various substrates, substrate analogs, and product analogs are required to unravel the finer molecular details of ...
Most peer-to-peer lookup schemes keep a small amount of routing state per node, typically logarithmic in the number of overlay nodes. This design assumes that routing information at each member node must be kept small so that the bookkeeping required to respond to system membership changes is also small, given that aggressive membership dynamics are expected. As a consequence, lookups have high latency as each lookup requires contacting several nodes in sequence. In this paper, we question these assumptions by presenting a peer-to-peer routing algorithm with small lookup paths. Our algorithm, called "OneHop," maintains full information about the system membership at each node, routing in a single hop whenever that information is up to date and in a small number of hops otherwise. We show how to disseminate information about membership changes quickly enough so that nodes maintain accurate complete membership information. We also present analytic bandwidth requirements for our scheme that demonstrate that it could be deployed in systems with hundreds of thousands of nodes and high churn. We validate our analytic model using a simulated environment and a real implementation. Our results confirm that OneHop is able to achieve high efficiency, usually reaching the correct node directly 99 percent of the time.
A mild, efficient and rapid domino reaction involving the Bestmann-Ohira reagent (BOR) and α,β-unsaturated aldehydes has been developed for the synthesis of densely functionalized vinylpyrazoles. This reaction demonstrates the dual reactivity of BOR as a homologation reagent as well as a cycloaddition partner, thus constituting a domino reaction in an operationally simple procedure. The application of this efficient synthesis of pyrazoles has been demonstrated for the synthesis of phosphonyl analogues of pyrrolopyrazole alkaloids.
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