Institution
Texas A&M University
Education•College Station, Texas, United States•
About: Texas A&M University is a education organization based out in College Station, Texas, United States. It is known for research contribution in the topics: Population & Finite element method. The organization has 72169 authors who have published 164372 publications receiving 5764236 citations.
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TL;DR: In this paper, it is argued that uniquely valuable synergy might be created where differences (versus similarities) exist between resources in the acquiring and target firms, and tests of these competing hypotheses confirmed that differences contributed significantly to performance in the merged firm.
522 citations
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TL;DR: This Review discusses the theoretical foundations of the development of acceleration methods for iterative convergence of discrete-ordinates simulations, the important results that have been accomplished, and remaining open questions.
521 citations
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TL;DR: Metal homeostasis is governed by the formation of specific protein-metal coordination complexes used to effect uptake, efflux, intracellular trafficking within compartments, and storage, and is the focus of this review.
Abstract: The transition or d-block metal ions manganese, iron, cobalt, nickel, copper, zinc, and to a more specialized degree, molybdenum, tungsten and vanadium, have been shown to be important for biological systems. These metal ions are ubiquitously found in nature, nearly exclusively as constituents of proteins.1 The unique properties of metal ions have been exploited by nature to perform a wide range of tasks. These include roles as structural components of biomolecules, as signaling molecules, as catalytic cofactors in reversible oxidation-reduction and hydrolytic reactions, and in structural rearrangements of organic molecules and electron transfer chemistry.1 Indeed, metal ions play critical roles in the cell that cannot be performed by any other entity, and are therefore essential for all of life. However, an individual metal ion is capable of performing only one or a few of these functions, but certainly not all; as a result, nature has evolved mechanisms to effectively distinguish one metal from another. The coordination chemistry of metal ion-protein complexes is fundamental to this biological discrimination, and is largely the focus of this review.
1.1. Metal Ion Homeostasis
Extensive regulatory and protein-coding machinery is devoted to maintaining the “homeostasis” of biologically required metal ions and underscores the essentiality of this process for cell viability. Homeostasis is defined as the maintenance of an optimal bioavailable concentration, mediated by the balancing of metal uptake and intracellular trafficking with efflux/storage processes so that the needs of the cell for that metal ion is met, i.e., the “right” metal is inserted into the “right” macromolecule at the appropriate time.2,3 Just as a scarcity of a particular metal ion induces a stress response that can lead to reprogramming of cellular metabolism to minimize the consequences of depletion of a particular metal ion, e.g., zinc in ribosome biogenesis4 or Cu vs. Fe in photosynthesis by Synechocystis,5 too much of the same metal ion can also be toxic to a cell or organism.
Metal homeostasis is governed by the formation of specific protein-metal coordination complexes used to effect uptake, efflux, intracellular trafficking within compartments, and storage (Figure 1). How metal ions move to and from their target destinations in the active site of a metalloenzyme or as a structural component of biomolecules also contributes to intracellular metal homeostasis (Figure 1). Metal transporters move metal ions or small molecule-metal chelates across otherwise impermeable barriers in a directional fashion, and most of these are integral membrane proteins embedded in the inner or plasma membrane (Figure 1). Specialized protein chelators designated metallochaperones traffic metals within a particular cellular compartment, e.g., the periplasm or the cytosol, and function to “hold” the metal in such a way that it can be readily transferred to an appropriate acceptor protein. This intermolecular transfer is known or is projected to occur through transiently formed, specific protein-protein complexes that mediate coordinated intermolecular metal ligand exchange. Metallochaperones have been described for copper,6-9 nickel10 and iron-sulfur protein biogenesis,11 and recent work suggests that the periplasmic Zn(II) binding protein, YodA, has characteristics consistent with a role as a zinc chaperone in E. coli (Figure 1).12 Salient features of these chaperones are discussed in more detail in the context of acquisition and efflux of individual metal ions (Section 2). Finally, specialized transcriptional regulatory proteins, termed metalloregulatory or metal sensor proteins, control the expression of genes encoding these proteins that establish metal homeostasis in response to either metal deprivation or overload (Section 3).
Figure 1
Schematic metal homeostasis models for iron, zinc and manganese, copper, nickel and cobalt, shown specifically in gram-negative bacteria. Homeostasis of molybdate and tungstate oxyanions are not shown, due primarily to a lack of knowledge of these systems, ...
A hypothesis that emerges is that in order to effect the cellular homeostasis of a particular metal ion, each component of the homeostasis machinery (Figure 1) must be selective for that metal ion under the prevailing conditions, to the exclusion of all others.13 Furthermore, individual systems must be “tuned” such that the affinity or sensitivity of each component is well-matched, either to coordinate gene expression by pairs of metal sensor proteins that coordinately shut off uptake and up-regulate efflux or detoxification systems, or to facilitate vectorial transport from metal donor to metal acceptor target protein in a metal trafficking pathway in the cell (Figure 1).14-16
521 citations
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TL;DR: In this article, the authors examine how excessive concern over current stock price can motivate managers to use observable investment decisions to manipulate the market's inferences about the firm's stock price, and find that firms with high/persistent informational asymmetries between managers and shareholders will tend to favor contracts that focus on long-run stock returns (both current and future) over contracts focusing on near-term stock returns alone.
521 citations
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TL;DR: In this article, a beam element based on first-order shear deformation theory is developed to study the thermoelastic behavior of functionally graded beam structures, and the stiffness matrix has super-convergent property and the element is free of shear locking.
521 citations
Authors
Showing all 72708 results
Name | H-index | Papers | Citations |
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Yi Chen | 217 | 4342 | 293080 |
Scott M. Grundy | 187 | 841 | 231821 |
Evan E. Eichler | 170 | 567 | 150409 |
Yang Yang | 164 | 2704 | 144071 |
Martin Karplus | 163 | 831 | 138492 |
Robert Stone | 160 | 1756 | 167901 |
Philip Cohen | 154 | 555 | 110856 |
Claude Bouchard | 153 | 1076 | 115307 |
Jongmin Lee | 150 | 2257 | 134772 |
Zhenwei Yang | 150 | 956 | 109344 |
Vivek Sharma | 150 | 3030 | 136228 |
Frede Blaabjerg | 147 | 2161 | 112017 |
Steven L. Salzberg | 147 | 407 | 231756 |
Mikhail D. Lukin | 146 | 606 | 81034 |
John F. Hartwig | 145 | 714 | 66472 |