Engineering Escherichia coli for the synthesis of taxadiene, a key intermediate in the biosynthesis of taxol

Taxadiene, the key intermediate of paclitaxel (Taxol) biosynthesis, has been prepared enzymatically from isopentenyl diphosphate in cell-free extracts of Escherichia coli by overexpressing genes encoding isopentenyl diphosphate isomerase, geranylgeranyl diphosphate synthase and taxadiene synthase. In addition, by the expression of three genes encoding four enzymes on the terpene biosynthetic pathway in a single strain of E. coli, taxadiene can be conveniently synthesized in vivo, at the unoptimized yield of 1.3mg per liter of cell culture. The success of both in vitro and in vivo synthesis of taxadiene bodes well for the future production of taxoids by non-paclitaxel producing organisms through pathway engineering.

Crystal structure of precorrin-8x methyl mutase

BACKGROUND

The crystal structure of precorrin-8x methyl mutase (CobH), an enzyme of the aerobic pathway to vitamin B12, provides evidence that the mechanism for methyl migration can plausibly be regarded as an allowed [1,5]-sigmatropic shift of a methyl group from C-11 to C-12 at the C ring of precorrin-8x to afford hydrogenobyrinic acid.

RESULTS

The dimeric structure of CobH creates a set of shared active sites that readily discriminate between different tautomers of precorrin-8x and select a discrete tautomer for sigmatropic rearrangement. The active site contains a strictly conserved histidine residue close to the site of methyl migration in ring C of the substrate.

CONCLUSION

Analysis of the structure with bound product suggests that the [1,5]-sigmatropic shift proceeds by protonation of the ring C nitrogen, leading to subsequent methyl migration.

Multiple biosynthetic pathways for vitamin B12: variations on a central theme

The manner in which vitamin B12 is synthesized is detailed with emphasis on the different mechanisms for ring contraction encountered in aerobic and anaerobic organisms. The aerobic process utilizes two enzymes and is dependent on molecular oxygen, in stark contrast to the anaerobic mechanism which is controlled by cobalt and requires only one enzyme.

An improved synthesis of the dinucleotides pdCpA and pdCpdA

An improved route was developed for the preparation of the dinucleotide hybrid 5'-O-phosphoryl-2'-deoxycytidylyl-(3'--> 5')adenosine (pdCpA) 7. This simple and concise synthesis involves the successive coupling of 2-cyanoethyl N, N, N', N'-tetra- isopropylphosphorodiamidite with 4-N-benzoyl-5'-O-(4, 4'-dimethoxytrityl)-2'-deoxy-cytidine 1 and 6-N,6-N,2'-O,3'-O-tetrabenzoyladenosine 2 as the key step. Some dinucleotide derivatives bearing different protecting groups were also synthesized and the selective deprotection conditions were studied in detail. The utility and efficiency of this approach has been further demonstrated by its application to the synthesis of total DNA dinucleotide pdCpdA 17 and total RNA dinucleotide 21.

Implementing evidence-based psychiatry: whose responsibility?

Evidence-based medicine is fine in principle, but needs to demonstrate tangible benefits to clinicians and their patients in practice. The main obstacles to its implementation are: (a) identifying information needs; (b) delivering reliable and clinically useful information to the ward/clinic; and (c) ensuring the information is regularly updated. How can this be achieved in psychiatry?

A survey of methohexitone use by anesthetists in the clinical practice of ECT in Edinburgh

The dosage of methohexitone (methohexital) administered by anesthetic staff was surveyed in a consecutive series of 52 patients referred for electroconvulsive therapy (ECT) in routine clinical practice in Edinburgh. Patients were weighed before the first treatment, and the ratio of administered dose to weight in kilograms calculated. Anesthesia was administered by three consultant staff and six nonconsultant staff. In only one patient (2%) was the administered dose within the range recommended by the Royal College of Psychiatrists (0.75-0.9 mg/kg), and in only four patients (8%) were the doses within the range recommended by the American Psychiatric Association (0.75-1.0 mg/kg). In all other patients the dose exceeded these recommended ranges; the average dose was 1.5 (+/- 0.3) mg/kg. The possible implications of these findings are discussed.

Microvascular complications in cystic fibrosis-related diabetes mellitus: a case report

CONTEXT, The prevalence of cystic fibrosis-related diabetes mellitus is increasing and is associated with increased survival from cystic fibrosis. CASE REPORT, This study describes a case of the premature onset of disabling and widespread microvascular complications resulting from cystic fibrosis-related diabetes mellitus. Previously asymptomatic retinopathy was diagnosed on recognition of diabetic nephropathy. CONCLUSIONS, The treatment of pulmonary exacerbations has become more complex due to the nephrotoxic potential of intravenous aminoglycoside drugs which are frequently used to control chronic Pseudomonas infection in cystic fibrosis.

The effect of repeated bilateral electroconvulsive therapy on seizure threshold

Seizure threshold was measured by empirical titration in 28 patients referred for bilateral electroconvulsive therapy to treat depressive illness at the outset of treatment and after another six treatments. No patient was given antiepileptic drug treatment, and anesthetic technique and concomitant psychotropic drug treatment were fixed. The average (+/- SD) initial seizure threshold measured by set charge was 79.5 mC (+/- 33.4 mC), and this increased to 95.5 mC (+/- 37.9 mC). The average percentage increase was 22.8% (95% confidence interval, 13.7% to 31.8%). The seizure threshold measured by set charge did not change in 15 patients (54%), and there was no significant relation between change in seizure threshold and patient sex, change in seizure duration measured by cuff technique, or global clinical improvement during the course of treatment.

Variation in rates of electroconvulsive therapy use among consultant teams in Edinburgh (1993-1996)

BACKGROUND

Critics of electroconvulsive therapy (ECT) have expressed concern about variations in ECT use among consultant teams within the same hospital. The aim was to establish whether or not there was a significant variation in rates of ECT use among consultant teams in the same hospital when in-patient workload was taken into account.

METHODS

A computerised database was used to calculate annual and aggregate rates of ECT use by consultant team, expressed as the number of individual in-patients treated per 100 in-patients discharged between 1993 and 1996.

RESULTS

The variation in aggregate rates of ECT use varied approximately 18-fold among the 11 general adult psychiatric teams (P<0.001), and twofold among the three sector old-age psychiatric teams (P<0.05).

CONCLUSIONS

Substantial variation in the rates of ECT use was confirmed, but only among general adult psychiatric teams.

LIMITATIONS

The extent to which findings from one teaching hospital can be generalised was unknown. Possible explanations of the variations were not assessed.

Evidence-based psychiatry--do psychiatrists want it and can they do it?

OBJECTIVE

To examine (a) psychiatrists' attitudes to evidence-based psychiatry, (b) whether psychiatrists have identifiable clinical information needs, (c) if such information is practically obtainable, and (d) how psychiatrists respond to the information obtained.

DESIGN

We surveyed senior psychiatrists to enquire about their attitudes and to request up to three clinical questions they would like answered. We attempted to find evidence to answer the five most frequently asked questions using recommended strategies and timed how long it took us. We fed our answers back to those who had asked one of these questions and asked if they found the answers useful.

SETTING

Specialist registrars, senior registrars and consultants in south-east Scotland.

RESULTS

Ninety three (76%) of those surveyed returned usable questionnaires. Respondents thought that only 40% of their practice is evidence-based. They reported that 'insufficient time' was the biggest barrier to implementing evidence-based psychiatry. The most frequently asked questions concerned the treatment of major psychiatric disorders. It took us--three experienced researchers, trained in critical appraisal, with excellent local facilities--between 15-60 minutes to answer each question. Most of those who had asked the questions (15/22) thought they would not have been able to answer them and stated that such a 'question answering service' as piloted here would be valuable.

CONCLUSIONS

An evidence-based psychiatry appears to be desirable and possible, but impractical for the individual clinician. There is a need for short accessible evidence-based summaries of optimal treatment in psychiatry.

Holo-sterol carrier protein-2. (13)C NMR investigation of cholesterol and fatty acid binding sites

Although sterol carrier protein-2 (SCP-2) stimulates sterol transfer in vitro, almost nothing is known regarding the identity of the putative cholesterol binding site. Furthermore, the interrelationship(s) between this SCP-2 ligand binding site and the recently reported SCP-2 long chain fatty acid (LCFA) and long chain fatty acyl-CoA (LCFA-CoA) binding site(s) remains to be established. In the present work, two SCP-2 ligand binding sites were identified. First, both [4-(13)C]cholesterol and 22-(N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol) binding assays were consistent with a single cholesterol binding site in SCP-2. This ligand binding site had high affinity for NBD-cholesterol, K(d) = 4.15 +/- 0.71 nM. (13)C NMR-labeled ligand competition studies demonstrated that the SCP-2 high affinity cholesterol binding site also bound LCFA or LCFA-CoA. However, only the LCFA-CoA was able to effectively displace the SCP-2-bound [4-(13)C]cholesterol. Thus, the ligand affinities at this SCP-2 binding site were in the relative order cholesterol = LCFA-CoA > LCFA. Second, (13)C NMR studies demonstrated the presence of another ligand binding site on SCP-2 that bound either LCFA or LCFA-CoA but not cholesterol. Photon correlation spectroscopy was consistent with SCP-2 being monomeric in both liganded and unliganded states. In summary, both (13)C NMR and fluorescence techniques demonstrated for the first time that SCP-2 had a single high affinity binding site that bound cholesterol, LCFA, or LCFA-CoA. Furthermore, results with (13)C NMR supported the presence of a second SCP-2 ligand binding site that bound either LCFA or LCFA-CoA but not cholesterol. These data contribute to our understanding of a role for SCP-2 in both cellular cholesterol and LCFA metabolism.

Overexpression, purification, and characterization of the thermostable mevalonate kinase from Methanococcus jannaschii

We report here the first overexpression and characterization of a thermostable mevalonate kinase from an archae, Methanococcus jannaschii, a strict anaerobe, which produces methane and grows at pressure of 200 atm and an optimum temperature near 85 degrees C. PCR-derived DNA fragments containing the structural gene for mevalonate kinase were cloned into an expression vector, pET28a, to form pETMVK. The mevalonate kinase was overexpressed from Escherichia coli pETMVK/BL21(DE3) (15-20% of total soluble protein) when induced with isopropyl beta-d-thiogalactopyranoside. The protein was purified by heat treatment (to denature E. coli proteins), followed by metal-affinity chromatography on Talon metal-affinity resin column. The purified protein had a dimeric structure composed of identical subunits, and the M(r) of the enzyme determined by gel chromatography was 68K. Based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the subunit M(r) was 36, 000. The pI for mevalonate kinase was 7.8. The Michaelis constant (K(m)) for (RS)-mevalonate was 68.5 microM and was 92 microM for ATP. The V(max) was 387 units mg(-1). The optimal temperature for mevalonate kinase activity was 70-75 degrees C.

Genetic engineering of Escherichia coli for the production of precorrin-3 in vivo and in vitro

The construction of a new recombinant strain of Escherichia coli in which two vitamin B12 biosynthetic genes, cobA and cobI, from Pseudomonas denitrificans are simultaneously overexpressed has resulted in the in vivo synthesis and accumulation of Factor III, an isobacteriochlorin not normally synthesized in E. coli. A lysate of the new strain can take the place of two lysates normally required to provide uroporphyrinogen III methyltransferase (cobA) and precorrin-2 methyltransferase (cobI) in an anaerobic five-enzyme synthesis of the early B12 intermediate, precorrin-3 (the reduced form of Factor III) from delta-aminolevulinic acid.

Rates of electroconvulsive therapy use in Edinburgh (1992-1997)

BACKGROUND

The number of ECT treatments given each year is a topic of interest for psychiatrists, users and politicians, but there are major methodological problems in reported studies of rates of ECT use in the British Isles. The aim was to establish whether or not the use of ECT had fallen between 1992 and 1997.

METHOD

A computerised database of ECT treatments in Edinburgh and relevant population data were used to calculate annual indices of ECT usage. These indices were calculated separately for the population aged 18-64 years and those 65 years or older.

RESULTS

In the general adult population, the rate of ECT use fell progressively and significantly (p < 0.01) from 2.90 to 1.37 treatments per 1000 population. This fall was commensurate with the falls in other indices of ECT use. Among the elderly population, the rate of ECT use was three times that in the general adult population. There was no significant change, although the number of courses of treatment fell by 40% (p = 0.06).

CONCLUSIONS

There continues to be a progressive fall in the use of ECT in general adult psychiatry. ECT is used substantially more often in the elderly. Reports of ECT use ought to include numbers of both treatments and patients treated.

LIMITATIONS

There was no investigation of the possible causes of the fall in ECT use. The extent to which these findings can be generalised to the rest of the British Isles will require further study.

Initial seizure threshold in the clinical practice of bilateral electroconvulsive therapy in Edinburgh, Scotland

Initial seizure threshold was measured by empirical titration in 137 patients referred for bilateral electroconvulsive therapy (ECT) for the treatment of depressive illness. Treatment was given by an Ectron Series 5A ECT machine. The median and modal thresholds were 75 mC and the range was 50-200 mC. The only statistically significant relationship among initial threshold and age or gender was the correlation of age with threshold in men (rho = 0.46, p < 0.01). All patients (n = 22) younger than 30 years had an initial threshold < 100 mC, but age and gender did not predict threshold accurately for older men or women.

Chemoenzymatic synthesis of an unnatural tetramethyl cobalt corphinoid

The chemoenzymatic synthesis and structural characterization by 13C NMR of a tetramethyl cobalt-corphinoid produced by methylation of cobalt-precorrin-3 using CbiF are described.

Conformation in solution and dynamics of a structurally constrained linear insect kinin pentapeptide analogue

The preferred conformations of the active diuretic insect kinin pentapeptide analogue Phe-Phe-Aib-Trp-Gly-NH2 were studied using nmr spectroscopy and molecular modeling. Structure sets consistent with rotating frame nuclear Overhauser effect spectroscopy distance constraints obtained by restrained simulated annealing in vacuo indicate a predominant population of a type II beta-turn involving the Phe1-Trp4 region. An equilibrium between this type II and a type I beta-turn formed by residues Phe2 and Gly5 was observed in a 5 ns restrained molecular dynamics simulation using the implicit generalized Born solvent accessible surface area (GB/SA) solvation model. When subjected to 500 ps dynamics with explicit water both beta-turn folds were conserved throughout the simulations. The results obtained with implicit and explicit solvation models are compared, and their consistency with the nmr observations is discussed. The behavior of the linear pentapeptide in this study is in agreement with an earlier report on the consensus conformation of the insect kinin active core derived from analysis of cyclic active analogues.

Detection of nascent polyproline II helices in solution by NMR in synthetic insect kinin neuropeptide mimics containing the X-Pro-Pro-X motif

The conformations of three synthetic peptide analogs containing the dPro-dPro-dXaa motif (dXaa = dThr, dGlu, dAsn) in aqueous solution were studied by a combination of NMR and molecular modeling simulations. The three compounds were identified from a random D-amino acid tripeptide library on the basis of their ability to either mimic or block the diuretic activity of neuropeptides of the insect kinin family. TOCSY and ROESY correlations, as well as abnormal secondary chemical shifts for protons on the D-proline residues were employed to obtain conformational ensembles consistent with the experimental NMR data for the three analogs using an in vacuo simulated annealing protocol. Similar secondary structures were found for the three molecules after refinement, in agreement with the similarities observed between their NMR spectra. Unrestrained molecular dynamics simulations with explicit water representation indicate that the structural motifs found in vacuo are stable in aqueous solution. The three analogs can be considered initiators of right-handed poly D-proline II helices, mirror images of the poly L-proline II left-handed helical motifs normally found in proline-rich proteins. The role of these secondary folds on binding of the analogs to the kinin receptors is discussed.

Definition of the redox states of cobalt-precorrinoids: investigation of the substrate and redox specificity of CbiL from Salmonella typhimurium

The enzyme CbiL from the facultative anaerobe Salmonella typhimurium exhibits a high degree of homology to CobI from the aerobe Pseudomonas denitrificans (29% identity; 51% conservation obtained by a Blastp search of the ncbi database). As CobI catalyzes the third methylation in the aerobic pathway to vitamin B12 it is proposed that CbiL catalyzes the analogous step in the anaerobic pathway. Potential metallo and metal-free substrates were characterized and their redox states defined by a combination of physicochemical techniques (MALDI-MS, NMR, UV/vis, IR, and EPR) and then used to investigate the function of CbiL. CbiL exhibited an absolute requirement for the presence of a metal ion (Co(II), Ni(II), or Zn(II)) within the tetrapyrrole substrate. CbiL had no preference for the redox state of its cobalt tetrapyrrole substrate, methylating both the reduced form, Co(II) 2, 7-dimethyl-dipyrrocorphin (Co(II)-precorrin-2), and the oxidized form, Co(III) 2,7-dimethyl-isobacterioclorin (Co(III)-factor-II). In contrast CbiL had a marked preference for the oxidized Ni(II) and Zn(II)-2,7-dimethyl-isobacteriochlorin (Ni(II) and Zn(II)-factor-II). Removal of the metal ion from a product of CbiL (Zn(II)-factor-III) allowed characterization by 13C NMR, identifying the tetrapyrrole as 2,7,20-trimethyl-isobacteriochlorin (factor-3), indicating that CbiL methylates at C20, the same site as that methylated by CobI. Competition experiments, utilizing isotopic labeling to distinguish otherwise identical mass substrates and products, revealed that oxidized Co(III) or Ni(II)-factor-II were equally good substrates, whereas Co(II)-precorrin-2 was much preferred over Ni(II)-precorrin-2. Excess Ni(II)-precorrin-2 did not decrease CbiL methylation of Co(II)-precorrin-2, implying that CbiL has a low affinity for Ni(II)-precorrin-2. These results are interpreted on the basis of tetrapyrrole ruffling occurring on the optimization of the metallo-N bond distances. The greater flexibility of the reduced precorrin-2 ring system allows greater deformation on accommodating the bound metal ion, the distortions imposed by bound Ni(II) or Zn(II) ions being larger than Co(II). The resulting distortions imposed on the precorrin ring could then decrease catalysis by causing a departure from the optimal substrate conformation required for CbiL. On oxidation of the Ni(II) or Zn(II)-precorrin-2, the increased stiffness of the ring could then constrain the metallo-factor-II conformation toward that of the usual substrate, allowing greater methylation by CbiL. In contrast to its counterpart CobI in the aerobic pathway of B12 biosynthesis, which methylates the metal-free precorrin-2, these studies show CbiL to be the first methylase unique to the anaerobic pathway, methylating a metallo-precorrin-2 substrate. Implications of CbiL specificity for the mechanism of the anaerobic B12 pathway are discussed.