1,721,003 research outputs found
Radicals from S-adenosylmethionine and their application to biosynthesis
No full textThe radical SAM superfamily of enzymes catalyzes a broad spectrum of biotransformations by employing a common obligate intermediate, the 5?-deoxyadenosyl radical (DOA). Radical formation occurs via the reductive cleavage of S-adenosylmethionine (SAM or AdoMet). The resultant highly reactive primary radical is a potent oxidant that enables the functionalization of relatively inert substrates, including unactivated C–H bonds. The reactions initiated by the DOA are breathtaking in their efficiency, elegance and in many cases, the complexity of the biotransformation achieved. This review describes the common features shared by enzymes that generate the DOA and the intriguing variations or modifications that have recently been reported. The review also highlights selected examples of the diverse biotransformations that ensue.<br/
Thiamine biosynthesis in Escherichia coli: in vitro reconstitution of the thiazole synthase activity
No full textThe biosynthesis of thiamine in Escherichia coli requires the formation of an intermediate thiazole from tyrosine, 1-deoxy-D-xylulose-5-phosphate (Dxp), and cysteine using at least six structural proteins, ThiFSGH, IscS, and ThiI. We describe for the first time the reconstitution of thiazole synthase activity using cell-free extracts and proteins derived from adenosine-treated E. coli 83-1 cells. The addition of adenosine or adenine to growing cultures of Aerobacter aerogenes, Salmonella typhimurium, and E. coli has been shown previously to relieve the repression by thiamine of its own biosynthesis and increase the expression levels of the thiamine biosynthetic enzymes. By exploiting this effect, we show that the in vitro thiazole synthase activity of cleared lysates or desalted proteins from E. coli 83-1 cells is dependent upon the addition of purified ThiGH-His complex, tyrosine (but not cysteine or 1-deoxy-D-xylulose-5- phosphate), and an as yet unidentified intermediate present in the protein fraction from these cells. The activity is strongly stimulated by the addition of S-adenosylmethionine and NADPH
Effect of iron-sulfur cluster assembly proteins on the expression of Escherichia coli lipoic acid synthase
No full textLipoic Acid Synthase (LipA) can accommodate a [4Fe-4S] cluster that is thought to be essential for the insertion of sulfur into an octanoyl substrate during the biosynthesis of lipoic acid. With the objective of improving soluble holo-LipA expression, a series of multi-cistronic plasmids were constructed carrying lipA in combination with one of the three systems: groE/SL, trxA, or the isc operon. Co-expression of lipA with the isc operon approximately trebled the isolated yield of soluble LipA and resulted in efficient assembly of the Fe-S cluster. This strategy may be helpful in the soluble expression of a wide range of Fe-S cluster-dependent proteins
Radical S-adenosylmethionine enzymes: mechanism, control and function
No full textThe radical SAM superfamily of enzymes use an iron sulfur cluster to reduce S-adenosylmethionine, which leads to the formation of a highly reactive intermediate, usually the 50-deoxyadenosyl radical. This potent oxidant is able to functionalize relatively inert substrates, including unactivated C–H bonds. This reactivity is evidently useful, as radical SAM enzymes are widely distributed throughout metabolism and catalyze some of the most complex and elegant biotransformations. In the first part of this review, the focus is on the mechanism of radical formation, including the features shared across the family, followed by a discussion of recent evidence for variations in cluster binding motifs and the mechanism of radical formation. In the second part, we survey how radical SAM chemistry has been applied to biosynthesis
A real-time assay for CpG-specific cytosine-C5 methyltransferase activity
No full textA real-time assay for CpG-specific cytosine-C5 methyltransferase activity has been developed. The assay applies a break light oligonucleotide in which the methylation of an unmethylated 5?-CG-3? site is enzymatically coupled to the development of a fluorescent signal. This sensitive assay can measure rates of DNA methylation down to 0.34 ± 0.06 fmol/s. The assay is reproducible, with a coefficient of variation over six independent measurements of 4.5%. Product concentration was accurately measured from fluorescence signals using a linear calibration curve, which achieved a goodness of fit (R2) above 0.98. The oligonucleotide substrate contains three C5-methylated cytosine residues and one unmethylated 5?-CG-3? site. Methylation yields an oligonucleotide containing the optimal substrate for the restriction enzyme GlaI. Cleavage of the fully methylated oligonucleotide leads to separation of fluorophore from quencher, giving a proportional increase in fluorescence. This method has been used to assay activity of DNMT1, the principle maintenance methyltransferase in human cells, and for the kinetic characterization of the bacterial cytosine-C5 methyltransferase M.SssI. The assay has been shown to be suitable for the real-time monitoring of DNMT1 activity in a high-throughput format, with low background signal and the ability to obtain linear rates of methylation over long periods, making this a promising method of high-throughput screening for inhibitors. <br/
Lecture capture: early lessons learned and experiences shared
No full textLecture capture has been on the minds of university level teachers for some time. The ability to record teaching sessions for delivery online has a number of potential impacts, not all of them positive. The technology now exists to make it feasible and relatively affordable to deliver entire lectures online. But should we do it just because we can? This article aims to share our experiences in recording a series of organic chemistry lectures, and the findings of the evaluation that followed
Thiamine biosynthesis in Escherichia coli: isolation and initial characterisation of the ThiGH complex
No full textIn Escherichia coli, two of the proteins required for the biosynthesis of the thiazole moiety of thiamine (vitamin B-1) are ThiG and ThiH, encoded as part of the thiCEFSGH operon. In this study, a C-terminally hexahistidine-tagged ThiH (ThiH-His) was expressed in E. coli as a soluble protein from thiGH-His-tag and thiFSGH-His-tag-bearing plasmids. When isolated under anaerobic conditions, ThiG and ThiH-His co-purify as a large multimeric non-covalent complex. Electron paramagnetic resonance and UV-visible spectroscopy together with iron and sulfide analyses revealed the presence of an iron-sulfur cluster within this complex
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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