1,720,998 research outputs found
Insights into DNA catalysis from structural and functional studies of the 8-17 DNAzyme
No full textDNAzymes (deoxyribozymes) are single-stranded DNA molecules endowed with catalytic activity, obtained by in vitro selection. In the past 25 years, dozens of DNAzymes have been identified and employed for applicative purposes, yet our knowledge of the structural and mechanistic basis of DNA catalysis remains very limited. The RNA-cleaving 8-17 DNAzyme, which depends on divalent metal ions for function, is possibly the most studied catalytic DNA in terms of mechanism. It is very efficient, implying that it adopts a combination of distinct catalytic strategies, but until recently it was uncertain which strategies are at play and how they are implemented. Recently, however, new functional studies and the attainment of high-resolution X-ray structures of an 8-17 construct, have offered a great opportunity for a more detailed understanding of its mechanism. This review examines the functional information gathered on 8-17, in the light of the available crystal structures, pointing out the congruences and possible inconsistencies between the functional and structural data. We will analyze separately three aspects of the DNAzyme function: the structural requirements for catalysis, the role of metal ions and the influence of pH on activity. Ultimately, we will contrast the experimental data with a model for the 8-17 mechanism proposed in the crystallographic study, whereby one specific G residue (G14) acts as a general base and a metal-coordinated water molecule acts as a general acid. Throughout this analysis we will signal the most outstanding mechanistic issues that remain to be addressed, with implications for the broader field of DNA catalysis
Using Steady-State Kinetics to Quantitate Substrate Selectivity and Specificity: A Case Study with Two Human Transaminases
Full text linkWe examined the ability of two human cytosolic transaminases, aspartate aminotransferase (GOT1) and alanine aminotransferase (GPT), to transform their preferred substrates whilst discriminating against similar metabolites. This offers an opportunity to survey our current understanding of enzyme selectivity and specificity in a biological context. Substrate selectivity can be quantitated based on the ratio of the kcat/KM values for two alternative substrates (the ‘discrimination index’). After assessing the advantages, implications and limits of this index, we analyzed the reactions of GOT1 and GPT with alternative substrates that are metabolically available and show limited structural differences with respect to the preferred substrates. The transaminases’ observed selectivities were remarkably high. In particular, GOT1 reacted ~106-fold less efficiently when the side-chain carboxylate of the ’physiological’ substrates (aspartate and glutamate) was replaced by an amido group (asparagine and glutamine). This represents a current empirical limit of discrimination associated with this chemical difference. The structural basis of GOT1 selectivity was addressed through substrate docking simulations, which highlighted the importance of electrostatic interactions and proper substrate positioning in the active site. We briefly discuss the biological implications of these results and the possibility of using kcat/KM values to derive a global measure of enzyme specificity
Biological catalysis: Lessons from the comparison of RNA and protein enzymes
No full textRNA enzymes, or ribozymes, provide a distinct perspective on long-standing queslion.s of biological catalysis. The differences observed between RNA and protein enzymes have taught us about aspects of RNA and proteins that are distinct, whereas the common features have helped us understand the aspects that are fundamental to biological catalysis. RNA enzymes, like protein en /vines, are able to use binding interactions away from the site of chemical transformation to facilitate that transformation by positioning substrates with respect to catalytic functional groups and by inducing electrostatic ground state destabilization. These observations allow the concept of 'intrinsic bind ing energy', put forth by Jencks, to be extended to RNA enzymes, generalized and strengthened. In addition, RNA enzymes appear to be highly amenable to energetic dissection of function. These analyses have general implications for understanding the role of binding interactions in catalysis and for understanding the evolution of catalysts. Results with the RNA enzymes will be placed in a conceptual framework for enzymatic catalysis. This framework will further be used to analyze catalytic contributions from enzymatic groups that interact at the site of bond transformation. It is suggested that the enzymatic environment can enhance the ability of hydrogen bonds to preferentially stabilize the- transition stale, thereby providing rate enhancement relative to the corresponding solution reaction
Single crystal polarized absorption microspectrophotometry of aromatic L-amino acid decarboxylase
No full textpolarized absorption microspectrophotometry of aromatic L-amino acid decarboxylas
Structural characterization of human O-phosphoethanolamine phospho-lyase
No full textHuman O-phosphoethanolamine phospho-lyase (hEtnppl; EC 4.2.3.2) is a pyridoxal 5′-phosphate-dependent enzyme that catalyzes the degradation of O-phosphoethanolamine (PEA) into acetaldehyde, phosphate and ammonia. Physiologically, the enzyme is involved in phospholipid metabolism, as PEA is the precursor of phosphatidylethanolamine in the CDP-ethanolamine (Kennedy) pathway. Here, the crystal structure of hEtnppl in complex with pyridoxamine 5′-phosphate was determined at 2.05Å resolution by molecular replacement using the structure of A1RDF1 from Arthrobacter aurescens TC1 (PDB entry 5g4i) as the search model. Structural analysis reveals that the two proteins share the same general fold and a similar arrangement of active-site residues. These results provide novel and useful information for the complete characterization of the human enzyme
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Il corpo e l'handicap: ascoltando esperienze di operatori e volontari nei servizi per disabili
No full textLe esperienze di tirocinio si presentano come esperienze di conoscenz
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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