Notes

The medullary heart for lapping in rodents.
The results revealed a new mechanism for the transaminase reaction where hyperconjugation is key to reducing the energy barrier which finally provides a clear explanation of the Dunathan alignment. © 2020 Elsevier Inc. All rights reserved.Protein kinases transmit chemical signals by phosphorylating substrate proteins, thus regulating a multitude of cellular processes. cAMP-dependent protein kinase (PKA), a prototypical enzyme for the whole kinase family, has been the focus of research for several decades, however, the details of the chemical mechanism of phosphoryl group transfer have remained unknown. We used neutron crystallography to map key proton sites and hydrogen bonding interactions in the PKA catalytic subunit (PKAc) in a product complex containing adenosine diphosphate (ADP) and the phosphorylated high affinity protein kinase substrate (pPKS) peptide. To improve neutron diffraction, we deuterated PKAc allowing us to use very small crystals. In the product complex, the phosphoryl group of pPKS is protonated whereas the catalytic Asp166 is not. H/D exchange analysis of the main-chain amides and comparison with the NMR analysis of PKAc with inhibitor peptide complex revealed exchangeable amides that may distinguish the catalytic and inhibited states. © 2020 Elsevier Inc. All rights reserved.The carbonic anhydrases (CAs; EC 4.2.1.1) are a family of metalloenzymes that catalyze the reversible hydration of carbon dioxide (CO2) and bicarbonate (HCO3-). Since their discovery in 1933, CAs have been at the forefront of scientific discovery the understanding of enzymatic reactions, structural biology, molecular dynamics, drug discovery, and clinical medicine. These ubiquitous enzymes equilibrate the reaction between CO2, HCO3-, and protons. Hence, CAs have important roles in ion transport, acid-base regulation, gas exchange, photosynthesis, and CO2 fixation. In this chapter, we describe the protocols leading to, and the analysis of CA neutron crystal structures. This accumulation of structural knowledge adds to our understanding of the enzymatic mechanism and development of CA inhibitors. © 2020 Elsevier Inc. All rights reserved.HIV-1 protease is an essential therapeutic target for the design and development of antiviral inhibitors to treat AIDS. We used room temperature neutron crystallography to accurately determine hydrogen atom positions in several protease complexes with clinical drugs, amprenavir and darunavir. Hydrogen bonding interactions were carefully mapped to provide an unprecedented picture of drug binding to the protease target. We demonstrate that hydrogen atom positions within the enzyme catalytic site can be altered by introducing drug resistant mutations and by protonating surface residues that trigger proton transfer reactions between the catalytic Asp residues and the hydroxyl group of darunavir. When protein perdeuteration is not feasible, we validate the use of initial H/D exchange with unfolded protein and partial deuteration in pure D2O with hydrogenous glycerol to maximize deuterium incorporation into the protein, with no detrimental effects on the growth of quality crystals suitable for neutron diffraction experiments. © 2020 Elsevier Inc. All rights reserved.The rate of deposition of models determined by neutron diffraction, or a hybrid approach that combines X-ray and neutron diffraction, has increased in recent years. The benefit of neutron diffraction is that hydrogen atom (H) positions are detectable, allowing for the determination of protonation state and water molecule orientation. This study analyses all neutron models deposited in the Protein Data Bank to date, focusing on protonation state and properties of H (or deuterium, D) atoms as well as the details of water molecules. In particular, clashes and hydrogen bonds involving H or D atoms are investigated. BAY2927088 As water molecules are typically the least reproducible part of a structural model, their positions in neutron models were compared to those in homologous high-resolution X-ray structures. For models determined by joint refinement against X-ray and neutron data, the water structure comparison was also carried out for models re-refined against the X-ray data alone. The homologues have generally fewer conserved water molecules where X-ray only was used and the positions of equivalent waters vary more than in the case of the hybrid X-ray model. As neutron diffraction data are generally less complete than X-ray data, the influence of neutron data completeness on nuclear density maps was also analyzed. We observe and discuss systematic map quality deterioration as result of data incompleteness. © 2020 Elsevier Inc. All rights reserved.The use of neutron protein crystallography (NPX) is expanding rapidly, with most structures determined in the last decade. This growth is stimulated by a number of developments, spanning from the building of new NPX beamlines to the availability of improved software for structure refinement. The main bottleneck preventing structural biologists from adding NPX to the suite of methods commonly used is the large volume of the individual crystals required for a successful experiment. A survey of deposited NPX structures in the Protein Data Bank shows that about two-thirds came from crystals prepared using vapor diffusion, while batch and dialysis-based methods all-together contribute to most of the remaining one-third. This chapter explains the underlying principles of these protein crystallization methods and provides practical examples that may help others to successfully prepare large crystals for NPX. © 2020 Elsevier Inc. All rights reserved.This chapter aims to give an overview of the process of interactive model building in macromolecular neutron crystallography for the researcher transitioning from X-ray crystallography alone. The two most popular programs for refinement and model building, phenix.refine and Coot, respectively, are used as examples, and familiarity with the programs is assumed. Some work-arounds currently required for proper communication between the programs are described. We also discuss the appearance of nuclear density maps and how this differs from that of electron density maps. Advice is given to facilitate deposition of jointly refined neutron/X-ray structures in the Protein Data Bank. © 2020 Elsevier Inc. All rights reserved.
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