Computational Study on DNA Repair: The Roles of Electrostatic Interactions Between Uracil-DNA Glycosylase (UDG) and DNA

Uracil-DNA glycosylase (UDG) is one of the most important base excision repair (BER) enzymes involved in the repair of uracil-induced DNA lesion by removing uracil from the damaged DNA. Uracil in DNA may occur due to cytosine deamination or deoxy uridine monophosphate (dUMP) residue misincorporation during DNA synthesis. Medical evidences show that an abnormal expression of UDG is related to different types of cancer, including colorectal cancer, lung cancer, and liver cancer. Therefore, the research of UDG is crucial in cancer treatment and prevention as well as other clinical activities. Here we applied multiple computational methods to study UDG in several perspectives: Understanding the stability of the UDG enzyme in different pH conditions; studying the differences in charge distribution between the pocket side and non-pocket side of UDG; analyzing the field line distribution at the interfacial area between UDG and DNA; and performing electrostatic binding force analyses of the special region of UDG (pocket area) and the target DNA base (uracil) as well as investigating the charged residues on the UDG binding pocket and binding interface. Our results show that the whole UDG binding interface, and not the UDG binding pocket area alone, provides the binding attractive force to the damaged DNA at the uracil base.

StructureMan: A Structure Manipulation Tool to Study Large Scale Biomolecular Interactions

Studying biomolecular interactions is a crucial but challenging task. Due to their large scales, many biomolecular interactions are difficult to be simulated via all atom models. An effective approach to investigate the biomolecular interactions is highly demanded in many areas. Here we introduce a Structure Manipulation (StructureMan) program to operate the structures when studying the large-scale biomolecular interactions. This novel StructureMan tool provides comprehensive operations which can be utilized to study the interactions in various large biological systems. Combining with electrostatic calculation programs such as DelPhi and DelPhiForce, StructureMan was implemented to reveal the detailed electrostatic features in two large biological examples, the viral capsid and molecular motor-microtubule complexes. Applications on these two examples revealed interesting binding mechanisms in the viral capsid and molecular motor. Such applications demonstrated that the StructureMan can be widely used when studying the biomolecular interactions in large scale biological problems. This novel tool provides an alternative approach to efficiently study the biomolecular interactions, especially for large scale biology systems. The StructureMan tool is available at our website: http://compbio.utep.edu/static/downloads/script-for-munipulation2.zip.

Spike Proteins of SARS-CoV and SARS-CoV-2 Utilize Different Mechanisms to Bind With Human ACE2

The ongoing outbreak of COVID-19 has been a serious threat to human health worldwide. The virus SARS-CoV-2 initiates its infection to the human body via the interaction of its spike (S) protein with the human Angiotensin-Converting Enzyme 2 (ACE2) of the host cells. Therefore, understanding the fundamental mechanisms of how SARS-CoV-2 S protein receptor binding domain (RBD) binds to ACE2 is highly demanded for developing treatments for COVID-19. Here we implemented multi-scale computational approaches to study the binding mechanisms of human ACE2 and S proteins of both SARS-CoV and SARS-CoV-2. Electrostatic features, including electrostatic potential, electric field lines, and electrostatic forces of SARS-CoV and SARS-CoV-2 were calculated and compared in detail. The results demonstrate that SARS-CoV and SARS-CoV-2 S proteins are both attractive to ACE2 by electrostatic forces even at different distances. However, the residues contributing to the electrostatic features are quite different due to the mutations between SARS-CoV S protein and SARS-CoV-2 S protein. Such differences are analyzed comprehensively. Compared to SARS-CoV, the SARS-CoV-2 binds with ACE2 using a more robust strategy: The electric field line related residues are distributed quite differently, which results in a more robust binding strategy of SARS-CoV-2. Also, SARS-CoV-2 has a higher electric field line density than that of SARS-CoV, which indicates stronger interaction between SARS-CoV-2 and ACE2, compared to that of SARS-CoV. Key residues involved in salt bridges and hydrogen bonds are identified in this study, which may help the future drug design against COVID-19.

Revealing the mechanism of SARS-CoV-2 spike protein binding with ACE2

A large population in the world has been infected by COVID-19. Understanding the mechanisms of Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2) is important for the management and treatment of COVID-19. When it comes to the infection process, one of the most important proteins in SARS-CoV-2 is the spike (S) protein, which is able to bind to human Angiotensin-Converting Enzyme 2 (ACE2) and initializes the entry of the host cell. In this study, we implemented multiscale computational approaches to study the electrostatic features of the interfaces of the SARS-CoV-2 S protein receptor binding domain and ACE2. The simulations and analyses were performed on high-performance computing resources in the Texas Advanced Computing Center. Our study identified key residues on SARS-CoV-2, which can be used as targets for future drug design. The results shed light on future drug design and therapeutic targets for COVID-19.

Nα-Acetylation of the virulence factor EsxA is required for mycobacterial cytosolic translocation and virulence

The Mycobacterium tuberculosis virulence factor EsxA and its chaperone EsxB are secreted as a heterodimer (EsxA:B) and are crucial for mycobacterial escape from phagosomes and cytosolic translocation. Current findings support the idea that for EsxA to interact with host membranes, EsxA must dissociate from EsxB at low pH. However, the molecular mechanism by which the EsxA:B heterodimer separates is not clear. In the present study, using liposome-leakage and cytotoxicity assays, LC-MS/MS-based proteomics, and CCF-4 FRET analysis, we obtained evidence that the Nα-acetylation of the Thr-2 residue on EsxA, a post-translational modification that is present in mycobacteria but absent in Escherichia coli, is required for the EsxA:B separation. Substitutions at Thr-2 that precluded Nα-acetylation inhibited the heterodimer separation and hence prevented EsxA from interacting with the host membrane, resulting in attenuated mycobacterial cytosolic translocation and virulence. Molecular dynamics simulations revealed that at low pH, the Nα-acetylated Thr-2 makes direct and frequent "bind-and-release" contacts with EsxB, which generates a force that pulls EsxB away from EsxA. In summary, our findings provide evidence that the Nα-acetylation at Thr-2 of EsxA facilitates dissociation of the EsxA:B heterodimer required for EsxA membrane permeabilization and mycobacterial cytosolic translocation and virulence.

The Roles of Electrostatic Interactions in Capsid Assembly Mechanisms of Giant Viruses

In the last three decades, many giant DNA viruses have been discovered. Giant viruses present a unique and essential research frontier for studies of self-assembly and regulation of supramolecular assemblies. The question on how these giant DNA viruses assemble thousands of proteins so accurately to form their protein shells, the capsids, remains largely unanswered. Revealing the mechanisms of giant virus assembly will help to discover the mysteries of many self-assembly biology problems. Paramecium bursaria Chlorella virus-1 (PBCV-1) is one of the most intensively studied giant viruses. Here, we implemented a multi-scale approach to investigate the interactions among PBCV-1 capsid building units called capsomers. Three binding modes with different strengths are found between capsomers around the relatively flat area of the virion surface at the icosahedral 2-fold axis. Furthermore, a capsomer structure manipulation package is developed to simulate the capsid assembly process. Using these tools, binding forces among capsomers were investigated and binding funnels were observed that were consistent with the final assembled capsid. In addition, total binding free energies of each binding mode were calculated. The results helped to explain previous experimental observations. Results and tools generated in this work established an initial computational approach to answer current unresolved questions regarding giant virus assembly mechanisms. Results will pave the way for studying more complicated process in other biomolecular structures.

Painless delivery--a short experience

PURPOSE

To determine efficacy and safety a randomized comparison of continuous infusion versus intermittent injection of epidural bupivacaine for labor analgesia was performed in the Maternity Hospital, Thapathali Kathmandu.

METHODS

Twenty healthy parturient received a loading dose of 10 ml of epidural 0.1% bupivacaine with 25 mg of pethidine. They were then randomized to receive continuous infusion of 0.1% bupivacaine 10 ml/hour with the help of infusion pump or intermittent injection of 0.1% bupivacaine 10 ml hourly. For breakthrough pain 10 ml of 0.1% bupivacaine top ups given in both groups. The two groups were compared for analgesic efficacy, mode of delivery, patient assessment of analgesia, motor block and other complications. Data analyzed in Pentium III version with SPSS and statistical significance test is done with independent samples t-test.

RESULTS

The 10 patients in each group were comparable in age but not in parity. Analgesic efficacy was excellent in 10 cases and comfortable in another 10 cases [excellent/comfortable 6:4 with infusion and 4:6 with intermittent injection]. There were no statistically significant differences between groups in pain scores or duration of active first or second stage of labor. Fifteen women had spontaneous vaginal deliveries, one caesarian section (infusion group) and four instrumental deliveries (intermittent injection group). Four women in the infusion group had hypotension and motor block, but none in the intermittent injection group. APGAR scores in both groups were 7-8/10 at 1 minute and 9-10/10 at 5 minutes.

CONCLUSION

Both continuous infusion and intermittent injection of low dose bupivacaine are very good methods of relieving labor pain in our context. Analgesic efficacy was similar in both groups and there was no prolongation of second stage of labor.