Performance at MRCP(UK): when should trainees sit examinations?

This paper analyses candidate performance in the three components of the MRCP(UK) examination to establish when they are most likely to pass. Using data from the 2010 and 2011 MRCP(UK) examinations, pass rates of candidates who gained their primary medical qualification (PMQ) in 2005 or later were analysed. Results from a total of 22,827 candidates were included in the study: 12,517 (54.8%) from Part 1, 5,545 (24.3%) from Part 2 written and 4,765 (20.9%) from the Part 2 practical assessment of clinical examination skills (PACES). The results show that candidates are more likely to pass Part 1 and Part 2 written 12-24 months after graduation and to pass Part 2 PACES 25-36 months after graduation. When we consider the training programme for physicians in the UK, successful candidates are likely to be in foundation programmes or early core or specialty training when they achieve success. At the moment, some candidates are dissuaded from taking the examination during their foundation programme, but our data show that their likelihood of success is highest during this period of training. The analysis also shows that for candidates who fail their first attempt, delaying their next attempt by one diet significantly increases the likelihood of them passing at their next attempt.

Asymmetric ligand binding facilitates conformational transitions in pentameric ligand-gated ion channels

The anesthetic propofol inhibits the currents of the homopentameric ligand-gated ion channel GLIC, yet the crystal structure of GLIC with five propofol molecules bound symmetrically shows an open-channel conformation. To address this dilemma and determine if the symmetry of propofol binding sites affects the channel conformational transition, we performed a total of 1.5 μs of molecular dynamics simulations for different GLIC systems with propofol occupancies of 0, 1, 2, 3, and 5. GLIC without propofol binding or with five propofol molecules bound symmetrically, showed similar channel conformation and hydration status over multiple replicates of 100-ns simulations. In contrast, asymmetric binding to one, two or three equivalent sites in different subunits accelerated the channel dehydration, increased the conformational heterogeneity of the pore-lining TM2 helices, and shifted the lateral and radial tilting angles of TM2 toward a closed-channel conformation. The results differentiate two groups of systems based on the propofol binding symmetry. The difference between symmetric and asymmetric groups is correlated with the variance in the propofol-binding cavity adjacent to the hydrophobic gate and the force imposed by the bound propofol. Asymmetrically bound propofol produced greater variance in the cavity size that could further elevate the conformation heterogeneity. The force trajectory generated by propofol in each subunit over the course of a simulation exhibits an ellipsoidal shape, which has the larger component tangential to the pore. Asymmetric propofol binding creates an unbalanced force that expedites the channel conformation transitions. The findings from this study not only suggest that asymmetric binding underlies the propofol functional inhibition of GLIC, but also advocate for the role of symmetry breaking in facilitating channel conformational transitions.

Reversal of ion-charge selectivity renders the pentameric ligand-gated ion channel GLIC insensitive to anaesthetics

pLGICs (pentameric ligand-gated ion channels) are a family of structurally homologous cation and anion channels involved in neurotransmission. Cation-selective members of the pLGIC family are typically inhibited by general anaesthetics, whereas anion-selective members are potentiated. GLIC is a prokaryotic cation pLGIC and can be inhibited by clinical concentrations of general anaesthetics. The introduction of three mutations, Y221A (Y-3'A), E222P (E-2'P) and N224R (N0'R), at the selectivity filter and one, A237T (A13'T), at the hydrophobic gate, converted GLIC into an anion channel. The mutated GLIC (GLIC4) became insensitive to the anaesthetics propofol and etomidate, as well as the channel blocker picrotoxin. MD (molecular dynamics) simulations revealed changes in the structure and dynamics of GLIC4 in comparison with GLIC, particularly in the tilting angles of the pore-lining helix [TM2 (transmembrane helix 2)] that consequently resulted in different pore radius and hydration profiles. Propofol binding to an intra-subunit site of GLIC shifted the tilting angles of TM2 towards closure at the hydrophobic gate region, consistent with propofol inhibition of GLIC. In contrast, the pore of GLIC4 was much more resilient to perturbation from propofol binding. The present study underscores the importance of pore dynamics and conformation to anaesthetic effects on channel functions.

Structure of the pentameric ligand-gated ion channel GLIC bound with anesthetic ketamine

Pentameric ligand-gated ion channels (pLGICs) are targets of general anesthetics, but a structural understanding of anesthetic action on pLGICs remains elusive. GLIC, a prokaryotic pLGIC, can be inhibited by anesthetics, including ketamine. The ketamine concentration leading to half-maximal inhibition of GLIC (58 μM) is comparable to that on neuronal nicotinic acetylcholine receptors. A 2.99 Å resolution X-ray structure of GLIC bound with ketamine revealed ketamine binding to an intersubunit cavity that partially overlaps with the homologous antagonist-binding site in pLGICs. The functional relevance of the identified ketamine site was highlighted by profound changes in GLIC activation upon cysteine substitution of the cavity-lining residue N152. The relevance is also evidenced by changes in ketamine inhibition upon the subsequent chemical labeling of N152C. The results provide structural insight into the molecular recognition of ketamine and are valuable for understanding the actions of anesthetics and other allosteric modulators on pLGICs.

Synthesis of taxane ABC tricyclic skeleton from lycoctonine

A new conversional synthesis of the ABC ring system of taxoids from the C19-diterpenoid alkaloid lycoctonine was developed in 6 steps with 2% overall yield. The distinctive features of the conversion include pinacol rearrangement, enlargement of ring B, and opening of a four-membered ring.

Structure-cardiac activity relationship of C19-diterpenoid alkaloids

Thirty three C19-diterpenoid alkaloids, twenty-two prepared from known C19-diterpenoid alkaloids and eleven isolated from Aconitum and Delphinium spp. were evaluated for their cardiac activity in the isolated bullfrog heart assay. Among them, eleven compounds exhibited cardiac activity, with average rate of amplitude increase in the range of 16-118%. Compound 7, mesaconine (17), hypaconine (25), and beiwutinine (26) exhibited strong cardiac activities relative to the reference drug. The structure-activity relationship data acquired indicated that an alpha-hydroxyl group at C-15, a hydroxyl group at C-8, an alpha-methoxyl or hydroxyl group at C-1, and a secondary amine or N-methyl group in ring A are important structure features necessary for the cardiac activities of the aconitine-type C19-diterpenoid alkaloids without any ester groups. In addition, an alpha-hydroxyl group at C-3 is also helpful for the cardiac activity of these alkaloids.

Synthesis of Taxane ABC Tricyclic Skeleton from Lycoctonine

A new conversional synthesis of the ABC ring system of taxoids from the C19-diterpenoid alkaloid lycoctonine was developed in 6 steps with 2% overall yield. The distinctive features of the conversion include pinacol rearrangement, enlargement of ring B, and opening of a four-membered ring.

Structure-Cardiac Activity Relationship of C19-Diterpenoid Alkaloids

Thirty three C19-diterpenoid alkaloids, twenty-two prepared from known C19-diterpenoid alkaloids and eleven isolated from Aconitum and Delphinium spp. were evaluated for their cardiac activity in the isolated bullfrog heart assay. Among them, eleven compounds exhibited cardiac activity, with average rate of amplitude increase in the range of 16-118%. Compound 7, mesaconine (17), hypaconine (25), and beiwutinine (26) exhibited strong cardiac activities relative to the reference drug. The structure-activity relationship data acquired indicated that an α-hydroxyl group at C-15, a hydroxyl group at C-8, an α-methoxyl or hydroxyl group at C-1, and a secondary amine or N-methyl group in ring A are important structure features necessary for the cardiac activities of the aconitine-type C19-diterpenoid alkaloids without any ester groups. In addition, an α-hydroxyl group at C-3 is also helpful for the cardiac activity of these alkaloids.

Structure of the pentameric ligand-gated ion channel ELIC cocrystallized with its competitive antagonist acetylcholine

ELIC, the pentameric ligand-gated ion channel from Erwinia chrysanthemi, is a prototype for Cys-loop receptors. Here we show that acetylcholine is a competitive antagonist for ELIC. We determine the acetylcholine–ELIC cocrystal structure to a 2.9-Å resolution and find that acetylcholine binding to an aromatic cage at the subunit interface induces a significant contraction of loop C and other structural rearrangements in the extracellular domain. The side chain of the pore-lining residue F247 reorients and the pore size consequently enlarges, but the channel remains closed. We attribute the inability of acetylcholine to activate ELIC primarily to weak cation-π and electrostatic interactions in the pocket, because an acetylcholine derivative with a simple quaternary-to-tertiary ammonium substitution activates the channel. This study presents a compelling case for understanding the structural underpinning of the functional relationship between agonism and competitive antagonism in the Cys-loop receptors, providing a new framework for developing novel therapeutic drugs.

The pentameric ligand gated ion channel from Erwinia chrysanthemi (ELIC) is similar in structure to the nicotinic acetylcholine receptor, a member of the Cys-loop receptor family. This study reports the crystal structure of ELIC bound to acetylcholine and shows that acetylcholine is a competitive antagonist of ELIC.