Substrate (chemistry)

About: Substrate (chemistry) is a research topic. Over the lifetime, 35902 publications have been published within this topic receiving 740722 citations. The topic is also known as: enzyme substrate.

Classification of beta-lactamases: groups 2c, 2d, 2e, 3, and 4.

Abstract: Classification of many important P-lactamases has been accomplished based on functional behavior (9). A large group of cephalosporinases not inhibited by clavulanic acid is characterized elsewhere (10). Also included are the classical penicillinases and the broad-spectrum enzymes, all of which could be inhibited by clavulanic acid. In the tables in this paper, other P-lactamases are described, including enzymes with hydrolytic capacity against carbenicillin or cloxacillin, cephalosporinases inhibited by clavulanic acid, metallo-3-lactamases, and penicillinases not inhibited by clavulanic acid. The criteria for classification are given elsewhere (9, 10). Within group 2c are those P-lactamases that hydrolyze carbenicillin at rates at least 75% as fast as benzylpenicillin and that are inhibited by clavulanic acid (Table 1). Included in these are the PSE-1, PSE-3, and PSE-4 P-lactamases, enzymes that tend to hydrolyze cephalosporins much more slowly than penicillins and that generally have slow rates of hydrolysis for cloxacillin. Although PSE-2 might appear to be a candidate for inclusion in this group, hydrolysis of cloxacillin is a more discriminating characteristic that places this enzyme in the next group, 2d. Group 2c enzymes are usually inhibited well by clavulanic acid and have poorer affinities for cloxacillin or aztreonam. Isoelectric points tend to fall below neutrality. None of these enzymes has yet had any sequence studies published, although one would predict that this group would also belong to the molecular class A ,-lactamases. Group 2d includes ,-lactamases that hydrolyze cloxacillin faster than benzylpenicillin and that are generally inhibited by clavulanic acid (Table 2). It is interesting to note that many OXA enzymes hydrolyze cephaloridine at least as well as benzylpenicillin, coupled with a strong hydrolysis of cephalothin. An exception to the cloxacillin-hydrolyzing criterion is OXA-4 (36). These enzymes are not as well inhibited by clavulanic acid as the previous class 2 ,lactamases. However, they also have poor affinities for aztreonam and cloxacillin. All the OXA and the PSE-2 1-lactamases are inhibited by 100 mM sodium chloride. Isoelectric points tend to be within the range of 6.1 to 7.7. The amino acid sequences of OXA-2 and PSE-2 appear to be different enough from those of the other 3-lactamases that a new molecular class, class D, has been established (26). Group 2e includes a group of cephalosporinases separate from those described in group 1 (10), those enzymes that are inhibited by low concentrations of clavulanic acid (Table 3). In 1968, Sawai et al. described a group of inducible cephalosporinases from Proteus species that reacted immunologically more like penicillinases than like other cephalosporinases (52). Unfortunately, full profiles are lacking for many of the Proteus P-lactamases described in the earlier literature. The ,-lactamase described from Proteus penneri (21) is another candidate for this category of cephalosporinases, as are several other inducible cephalosporinases that are inhibited better by clavulanic acid than by aztreonam. It is predicted that these P-lactamases, originally assigned to the Richmond-and-Sykes class lc, will eventually be shown to belong to molecular class A. Saino et al. (45) indicated that the L2 cephalosporinase from Pseudomonas maltophilia resembles the Proteus P-lactamases. Although this enzyme has also been reported to contain zinc (7), its inhibition profile with boronic acids suggests that it is a serine enzyme and rightfully belongs in group 2. Group 3 P-lactamases include the metalloenzymes, enzymes that are inhibited by EDTA with activity restored upon addition of a divalent cation, usually Zn2+ (Table 4). None of the enzymes described to date is inhibited by clavulanic acid, nor does any of them appear to contain an active site serine residue. Noteworthy among this class are the Flavobacterium and Li ,-lactamases with their strong hydrolysis of imipenem. Although Bacillus cereus P-lactamase II has been studied extensively as a prototype of this group of enzymes, it is possible that this enzyme may be the only member of molecular class B. At this time, it appears that each metallo-p-lactamase has very different molecular properties, and a common heritage is unlikely. Group 4 contains those unusual penicillinases not inhibited by clavulanic acid (Table 5). Four of these enzymes exhibit high rates of hydrolysis with carbenicillin and/or cloxacillin. Several exhibit unusual behavior with respect to metal ion involvement. Whether these enzymes represent another molecular class of P-lactamase is not known. It is hoped that the groupings proposed in this study and elsewhere (10) will be helpful in evaluating a variety of P-lactamases. It is probable that some of the enzymes presented may ultimately be examined more thoroughly and assigned to a different group based on new information. However, as a result of this tabulation, evaluations of novel P-lactamases in the future may become more standardized, thereby permitting more valid comparisons to be made with previously described enzymes.

Electroless nickel plating on AZ91 Mg alloy substrate

Abstract: Electroless Ni-plating on Mg alloy AZ91 has been studied to understand the effect of the substrate microstructure and roughness on the deposition rate, nucleation, coating microstructure, and mechanical property of the coatings. Experimental results indicate that the growth of Ni deposit in the early stage was influenced by the substrate microstructure and roughness. The electroless Ni-plating on the abrasive blasted AZ91 (rough) substrate showed a higher deposition rate than that on the finely polished one, indicating that the mechanical roughening enhances the nucleation and coalescence of Ni crystallites. Scratching tests showed that higher coating adhesion is achieved on the roughened AZ91 substrate. Wear tests, however, showed that the Ni plating on the rough substrate has a higher friction coefficient than that on the polished surface. The hardness and adhesion property of Ni coatings before and after heat treatment were also characterised.

Enhancement of in vitro Capillary Tube Formation by Substrate Nanotopography

Abstract: Vascular engineering remains a key thrust in advancing the field of tissue engineering of highly vascularized, complex, metabolic organs. A wide variety of strategies have been employed to control the formation of organized vascular structures in vitro and in vivo. Some of these methods include, but are not limited to, controlled growth factor delivery,[1] filamentous scaffold geometry,[2] protein micropatterning,[3] and enhanced scaffold biomaterials.[4] Many of these approaches are motivated by biomimicry of the in vivo microenvironment. Extracellular matrix (ECM) proteins, both in vitro and in vivo, provide mammalian cells with biophysical cues including specific surface chemistry and rich three-dimensional surface topography[5] with features on the nanometer length scale.[6] ECM substrates provide chemical and physical external cues that dictate a variety of cell responses. Therefore, it is not only the milieu of soluble, diffusible factors, but also the adhesive, mechanical interactions with scaffolding materials, both natural and synthetic, that control select cell functions including cell attachment, migration, proliferation, differentiation, and regulation of genes.[7–9] We hypothesized that physical features on nanofabricated substrates could promote the organization of endothelial cell lineages into well-defined vascular structures in vitro by inducing the contact guidance phenomenon, which is known to affect the morphology of endothelial cells.[10–12]We found that endothelial progenitor cells (EPCs) responded to ridge-groove grating of 1200 nm in period and 600 nm in depth through alignment, elongation, reduced proliferation, and enhanced migration. Although endothelial- specific markers were not significantly altered, EPCs cultured on substrate nanotopography formed supercellular band structures after 6 d. Furthermore, an in vitro Matrigel assay led to enhanced capillary tube formation and organization.