Metabolons,enzyme-enzyme assemblies that mediate substrate channeling,and their roles in plant metabolism

Metabolons are transientmulti-protein complexes of sequential enzymes that mediate substrate channeling.They differ from multi-enzyme complexes in that they are dynamic,rather than permanent,and as such have considerably lower dissociation constants.Despite the fact that a huge number of metabolons have been suggested to exist in plants,most of these claims are erroneous as only a handful of these have been proven to channelmetabolites.We believe that physical protein-protein interactions between consecutive enzymes of a pathway should rather be called enzyme-enzyme assemblies.In this review,we describe how metabolons are generally assembled by transient interactions and held together by both structural elements and non-covalent interactions.Experimental evidence for their existence comes fromprotein-protein interaction studies,which indicate that the enzymes physically interact,and direct substrate channelingmeasurements,which indicate that they functionally interact.Unfortunately,advances in cell biology and proteomics have far outstripped those in classical enzymology and flux measurements,rendering most reports reliant purely on interactome studies.Recent developments in co-fractionation mass spectrometry will likely further exacerbate this bias.Given this,only dynamic enzyme-enzyme assemblies in which both physical and functional interactions have been demonstrated should be termed metabolons.We discuss the level of evidence for the manifold plant pathways that have been postulated to contain metabolons and then list examples in both primary and secondary metabolism for which strong evidence has been provided to support these claims.In doing so,we pay particular attention to experimental and mathematical approaches to study metabolons as well as complexities that arise in attempting to follow them.Finally,we discuss perspectives for improving our understanding of these fascinating but enigmatic interactions.

Oxbow Lakes as Geological Archives of Historical Changes in Channel Substrate, Swan Creek, Toledo, Ohio (USA)

Efforts to restore urban rivers require an understanding of human-influenced changes in channel substrates. This study uses three naturally-occurring oxbows in a 3.5 km reach of Swan Creek, flowing through the City of Toledo, Ohio (USA) to reconstruct historical changes in channel substrate. Human impacts in the watershed were: 1) land clearance for agriculture (peaking in 1900-1920) and for suburban housing tracts (peaking in 1945-1970), followed by 2) the post-1940 creation of more efficient urban run-off systems from streets, parking lots, housing developments, and shopping centers. Historical aerial photographs and maps from 1935, 1940, 1950, 1963, 1974, and 1994 were georeferenced using ground control points, input to ArcGIS, and have root mean square error (RMSE) ranging from 0.19 - 0.77 m (average RMSE = 0.47 ± 0.20 m) when compared to the 2006 digital ortho quarter-quadrangle (DOQQ) image used as the basis for comparison. Results showed that channel sinuosity continually increased from 1.88 (1935) to 1.99 (2006). Two oxbows probably formed in 1913, and the third formed in 1940. Sediment cores and trenches were used to recognize historical channel substrates. Age control was provided by 14C geochronology and labels on food packaging materials found in flood layers. Grain-size analysis of channel substrates shows a historical coarsening-upward trend: the largest clast size interval (f5) changes from +0.78f in pre-1935 channels, to -1.15f in pre-1940 channels, to -1.69f in the 2006 channel. These results indicate recent urban runoff created fluvial pavements and increasing channel mobility as the stream removes legacy sediment from intrabasinal sediment storage.

Conjugated microporous polymers-scaffolded enzyme cascade systems with enhanced catalytic activity

Enhancing catalytic activity of multi-enzyme in vitro through substrate channeling effect is promis-ing yet challenging.Herein,conjugated microporous polymers(CMPs)-scaffolded integrated en-zyme cascade systems(I-ECSs)are constructed through co-entrapping glucose oxidase(GOx)and horseradish peroxidase(HRP),in which hydrogen peroxide(H_(2)O_(2)) is the intermediate product.The interplay of low-resistance mass transfer pathway and appropriate pore wall-H_(2)O_(2) interactions facilitates the directed transfer of H_(2)O_(2),resulting in 2.4-fold and 5.0-fold elevation in catalytic activ-ity compared to free ECSs and separated ECSs,respectively.The substrate channeling effect could be regulated by altering the mass ratio of GOx to HRP.Besides,I-ECSs demonstrate excellent stabili-ties in harsh environments and multiple recycling.

Facilitation of cascade biocatalysis by artificial multi-enzyme complexes——A review

Multi-enzyme complexes are the results of natural evolution to facilitate cascade biocatalysis.Through enzyme colocalization within a complex,the transfer efficiency of reaction intermediates between adjacent cascade enzymes can be promoted,resulting in enhanced overall reaction efficiency.Inspired by nature,a variety of approaches have been developed for the assembly of artificial multi-enzyme complexes with different spatial organizations,aiming at improving the catalytic efficiency of enzyme cascade.A recent trend of this research area is the creation of enzyme complexes with a controllable spatial organization which helps with the mechanistic studies and bears the potential to further increase metabolic productivity.In this review,we summarize versatile strategies for the assembly of artificial multi-enzyme complexes,followed by an inspection of the mechanistic studies of artificial multi-enzyme complexes for their enhancement of catalytic efficiency.Furthermore,we provide some highlighted in vivo,ex vivo,and in vitro examples that demonstrate the ability of artificial multi-enzyme complexes for enhancing the overall production efficiency of value-added compounds.Recent research progress has revealed the great biotechnological potential of artificial multi-enzyme complexes as a powerful tool for biomanufacturing.

Backgating effect in GaAs FETs with a channel–semi-insulating substrate boundary

This study focuses on modeling the effects of deep hole traps, mainly the effect of the substrate（backgating effect） in a GaAs transistor MESFT. This effect is explained by the existence, at the interface, of a space charge zone. Any modulation in this area leads to response levels trapping the holes therein to the operating temperature. We subsequently developed a model treating the channel substrate interface as an N–P junction, allowing us to deduce the time dependence of the component parameters of the total resistance R ds, the pinch-off voltage V P, channel resistance, fully open R co and the parasitic series resistance R S to bind the effect trap holes H1and H0. When compared with the experimental results, the values of the R DS（t S/ model for both traps show that there is an agreement between theory and experiment; it has inferred parameter traps, namely the density and the time constant of the trap. This means that a space charge region exists at the channel–substrate interface and that the properties can be approximated to an N–P junction.