Allosteric interactions after 50 years

A retrospective on statistical mechanical models for hemoglobin allostery

Understanding allosteric interactions in proteins has become one of the major research areas in protein science. The original aim of the famous theoretical model of Monod, Wyman, and Changeux (MWC) was to explain the regulation of enzymatic activity in biochemical pathways. However, its first successful quantitative application was to explain cooperative oxygen binding by hemoglobin, often called the "hydrogen molecule of biology." The combination of its original application and the enormous amount of research on hemoglobin has made it the paradigm for studies of allostery, especially for multi-subunit proteins, and for the development of statistical mechanical models to describe how structure determines function. This article is a historical account of the development of statistical mechanical models for hemoglobin to explain both the cooperative binding of oxygen (called homotropic effects by MWC) and how oxygen binding is affected by ligands that bind distant from the heme oxygen binding site (called heterotropic allosteric effects by MWC). This account makes clear the many remaining challenges for describing the relationship of structure to function for hemoglobin in terms of a satisfactory statistical mechanical model.

Musings on mechanism: quest for a quark theory of proteins?

The give and take between biology and physics is an important part of the history of modern science, with this partnership perhaps now more intimate than ever. In this essay, I explore the ways in which these different fields can strengthen each other through their distinct outlooks on some of the most important questions being asked about the living world. In particular, I highlight three areas where the perspective from physics might lead to deeper insights into the workings of cells. First, I consider what it means for two ostensibly unrelated problems to be the same, and how such sameness can provide unexpected insights into apparently unrelated phenomena. Second, I consider different conceptions of what constitutes a mechanistic understanding of a given phenomenon with an emphasis on nonmolecular notions of mechanism. The third idea is the importance of "toy problems" as a way of providing foundational insights into the real problems. Though my focus here is primarily on ways in which physics approaches might prove interesting in biology, I close with an example of how biology might substantially alter physics by providing a forum and the tools to uncover a fundamental understanding of nonequilibrium phenomena.-Phillips, R. Musings on mechanism: quest for a quark theory of proteins?

Two classes of bacterial IMPDHs according to their quaternary structures and catalytic properties

Inosine-5'-monophosphate dehydrogenase (IMPDH) occupies a key position in purine nucleotide metabolism. In this study, we have performed the biochemical and physico-chemical characterization of eight bacterial IMPDHs, among which six were totally unexplored. This study led to a classification of bacterial IMPDHs according to the regulation of their catalytic properties and their quaternary structures. Class I IMPDHs are cooperative enzymes for IMP, which are activated by MgATP and are octameric in all tested conditions. On the other hand, class II IMPDHs behave as Michaelis-Menten enzymes for both substrates and are tetramers in their apo state or in the presence of IMP, which are shifted to octamers in the presence of NAD or MgATP. Our work provides new insights into the IMPDH functional regulation and a model for the quaternary structure modulation is proposed.

Chromatin fiber allostery and the epigenetic code

The notion of allostery introduced for proteins about fifty years ago has been extended since then to DNA allostery, where a locally triggered DNA structural transition remotely controls other DNA-binding events. We further extend this notion and propose that chromatin fiber allosteric transitions, induced by histone-tail covalent modifications, may play a key role in transcriptional regulation. We present an integrated scenario articulating allosteric mechanisms at different scales: allosteric transitions of the condensed chromatin fiber induced by histone-tail acetylation modify the mechanical constraints experienced by the embedded DNA, thus possibly controlling DNA-binding of allosteric transcription factors or further allosteric mechanisms at the linker DNA level. At a higher scale, different epigenetic constraints delineate different statistically dominant subsets of accessible chromatin fiber conformations, which each favors the assembly of dedicated regulatory complexes, as detailed on the emblematic example of the mouse Igf2-H19 gene locus and its parental imprinting. This physical view offers a mechanistic and spatially structured explanation of the observed correlation between transcriptional activity and histone modifications. The evolutionary origin of allosteric control supports to speak of an 'epigenetic code', by which events involved in transcriptional regulation are encoded in histone modifications in a context-dependent way.

An allosteric receptor by simultaneous "casting" and "molding" in a dynamic combinatorial library

Allosteric synthetic receptors are difficult to access by design. Herein we report a dynamic combinatorial strategy towards such systems based on the simultaneous use of two different templates. Through a process of simultaneous casting (the assembly of a library member around a template) and molding (the assembly of a library member inside the binding pocket of a template), a Russian-doll-like termolecular complex was obtained with remarkable selectivity. Analysis of the stepwise formation of the complex indicates that binding of the two partners by the central macrocycle exhibits significant positive cooperativity. Such allosteric systems represent hubs that may have considerable potential in systems chemistry.

Integration of methods in cheminformatics and biocalorimetry for the design of trypanosomatid enzyme inhibitors

Background: The enzyme GAPDH, which acts in the glycolytic pathway, is seen as a potential target for pharmaceutical intervention of Chagas disease. Results: Herein, we report the discovery of new Trypanosoma cruzi GAPDH (TcGAPDH) inhibitors from target- and ligand-based virtual screening protocols using isothermal titration calorimetry (ITC) and molecular dynamics. Molecular dynamics simulations were used to gain insight on the binding poses of newly identified inhibitors acting at the TcGAPDH substrate (G3P) site. Conclusion: Nequimed125, the most potent inhibitor to act upon TcGAPDH so far, which sits on the G3P site without any contact with the co-factor (NAD+) site, underpins the result obtained by ITC that it is a G3P-competitive inhibitor. Molecular dynamics simulation provides biding poses of TcGAPDH inhibitors that correlate with mechanisms of inhibition observed by ITC. Overall, a new class of dihydroindole compounds that act upon TcGAPDH through a competitive mechanism of inhibition as proven by ITC measurements also kills T. cruzi.

A novel equation for cooperativity of the allosteric state function

The MWC (Monod–Wyman–Changeux) allosteric model postulates concerted conformational changes between two states: the intrinsically more stable T state with relatively weak ligand binding and the R state with relatively strong ligand binding. The model distinguishes between Y¯ (the fractional occupation of the binding sites) and R¯ (the fraction of molecules in the R state). Cooperativity (measured by the Hill coefficient) has strikingly different properties for Y¯ and R¯. For the latter, cooperativity depends only on the relative affinities of the two states, not on their relative intrinsic stabilities, as demonstrated here with a simple new equation relating the Hill coefficient to R¯.

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A simple new equation relating the Hill coefficient to R¯ is presented.

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This equation shows that cooperativity (measured by the Hill coefficient) for R¯ depends only on the relative affinities of the two states, not on their relative intrinsic stabilities.

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The curves for R¯ may be characterized by Hill coefficients < 1, even under conditions of positive cooperativity.