Validation of molecular dynamics simulation

How the results obtained by computer simulation of molecular systems can be validated is considered. The overall quality of the simulated properties of a molecular system will depend on (i) the quality of the theory or model, (ii) the accuracy of the interatomic interaction function or force field, (iii) the degree of sampling, statistics and convergence reached in the simulation, (iv) the quality of the simulation software, and (v) how competently the simulation software is used. These five validation issues are discussed and illustrated with examples. Guidelines for different members of the scientific community are formulated which are aimed at enabling and improving the validation of simulation results in the literature. © 1998 American Institute of Physics

pKa Modulation of the Acid/Base Catalyst within GH32 and GH68: A Role in Substrate/Inhibitor Specificity?

Glycoside hydrolases of families 32 (GH32) and 68 (GH68) belong to clan GH-J, containing hydrolytic enzymes (sucrose/fructans as donor substrates) and fructosyltransferases (sucrose/fructans as donor and acceptor substrates). In GH32 members, some of the sugar substrates can also function as inhibitors, this regulatory aspect further adding to the complexity in enzyme functionalities within this family. Although 3D structural information becomes increasingly available within this clan and huge progress has been made on structure-function relationships, it is not clear why some sugars bind as inhibitors without being catalyzed. Conserved aspartate and glutamate residues are well known to act as nucleophile and acid/bases within this clan. Based on the available 3D structures of enzymes and enzyme-ligand complexes as well as docking simulations, we calculated the pKa of the acid-base before and after substrate binding. The obtained results strongly suggest that most GH-J members show an acid-base catalyst that is not sufficiently protonated before ligand entrance, while the acid-base can be fully protonated when a substrate, but not an inhibitor, enters the catalytic pocket. This provides a new mechanistic insight aiming at understanding the complex substrate and inhibitor specificities observed within the GH-J clan. Moreover, besides the effect of substrate entrance on its own, we strongly suggest that a highly conserved arginine residue (in the RDP motif) rather than the previously proposed Tyr motif (not conserved) provides the proton to increase the pKa of the acid-base catalyst

Assessment of nuclear totipotency of fetal bovine diploid germ cells by nuclear transfer.

Nuclear transfer was used to study nuclear reprogramming of fetal diploid bovine germ cells collected at two stages of the fetal development. In the first case, germ cells of both sexes were collected during their period of intragonadal mitotic multiplication at 48 days post coitum (d.p.c.). In the second case, only male germ cells were collected after this period, between 105 and 185 d.p.c. Isolated germ cells were fused with enucleated oocytes. Reconstituted embryos were cultured in vitro and those reaching the compacted morula or blastocyst stage were transferred into synchronous recipient heifers. Of 511 reconstituted embryos with 48 d.p.c. germ cells (309 males and 202 females), 48% (247/511 ) cleaved; 2.7% (14/511 ) reached the compacted morula stage and 8 of them the blastocyst stage (1.6%). No difference was observed between sexes. All 14 compacted morulae/blastocysts were transferred into 6 recipients and one pregnancy was initiated. This recipient was slaughtered at Day 35 and an abnormal conceptus (extended trophectoderm and degenerated embryo) was collected. Its male sex, genetically determined, corresponded to that of donor fetus. Of 380 reconstituted embryos with male 105 to 185 d.p.c. germ cells, 72.1% (274/380 ) cleaved, 2.1% (8 380 ) reached the compact morula stage and 7 of these the blastocyst stage (1.8%). Three blastocysts and one morula were transferred into 4 recipients. Two became pregnant at Day 21 but only one at Day 35 which aborted around Day 40. Our results show that the nucleus of diploid bovine germ cells of both sexes can be reprogrammed. However, in the absence of further development of these reconstituted embryos, nuclear totipotency of bovine diploid germ cells remains to be evidenced

Interlaboratory Evaluation of 3 Culture Media for Postimplantation Rodent Embryos

The first aim of the study was to compare the ability of rat serum, human serum, and a mixture of human and rat serum (4:1) to support in vitro development of rodent postimplantation embryos. The comparison was made in three laboratories using rat embryos and in one laboratory using mouse embryos. Batches of sera, initial developmental stage, duration of culture, and endpoints were identical in the laboratories. The second aim of the study was to evaluate if other variables that could not be standardized would significantly influence the results of the laboratories. No reproducible difference was observed among the culture media or among the laboratories except that growth and differentiation were slower in the laboratory using mouse embryos. Further experiments are needed to exclude small differences in performance of the media

Use of loan loss provisions for capital, earnings management and signalling by Australian banks

This study examines whether and to what extent Australian banks use loan loss provisions (LLPs) for capital, earnings management and signalling. We examine if there were changes in the use of LLPs as a result of the implementation of banking regulations consistent with the Basel Accord of 1988, which made loan loss reserves no longer part of Tier I capital in the numerator of the capital adequacy ratio. We find some evidence to indicate that Australian banks use LLPs for capital management, but we find no evidence of a change in this behaviour after the implementation of the Basel Accord. Our results indicate that banks in Australia use LLPs to manage earnings. Furthermore, listed commercial banks engage more aggressively in earnings management using LLPs than unlisted commercial banks. We also find that earnings management behaviour is more pronounced in the post-Basel period. Overall, we find a significant understating of LLPs in the post-Basel period relative to the pre-Basel period. This indicates that reported earnings might not reflect the true economic reality underlying those numbers. Finally, Australian banks do not appear to use LLPs for signalling future intentions of higher earnings to investors. Copyright (c) The Authors Journal compilation (c) 2007 AFAANZ.