Predicting and avoiding the onset of milling chatter are desirable to reduce its harm to machine tools, workpieces, and cutters. This paper presents an updated method to complete the stability ...prediction for the milling process with multiple and distributed time delays. After the dynamic of the combination milling process with variable helix cutter (VHC) and variable spindle speed (VSS) is modeled as linear delay differential equations with multiple and distributed time delays, the presented method is applied to carrying out its stability prediction for the first time. By comparing with the existing researches and time-domain simulations, the effectiveness of the presented method has been validated. The influence and feasibility of the combination process on chatter suppression are explored and investigated for the associated one- and two-degree-of-freedom systems. Results show that the application of the combination process can realize a further suppression of milling chatter in practice. It can result in nearly 2-fold as high as the minimum depth of cut for the traditional milling or VSS milling and about 1.3-fold for VHC milling for some special domain, and can respectively lead to the average increase of stable area by 30.4%, 23.5%, and 1.5% for the adopted simulations. However, consider the contribution, the combination process is actually one process in which VHC plays an absolutely leading role but VSS plays an auxiliary role, in terms of milling stability.
The Gcn5-related N-acetyltransferases (GNAT) comprise one of the largest enzyme superfamilies, with over 10 000 known members represented in all kingdoms of life. ChloroacetylCoenzymeA was prepared ...and demonstrated to be a substrate for several GNAT members. ChloroacetylCoA (ClAcCoA) is used by the Hat1 histone acetyltransferase to correctly acetylate histone H4 in a mixture of histone proteins. Chloroacetylation can be assessed by the subsequent reaction of the chloroacetylated product with thiol-containing compounds, including those with fluorescent or affinity (His8) tags. The bacterial RimL N-acetyltransferase also uses ClAcCoA to chloroacetyl the α-amino group of its cognate substrate, the ribosomal L12 protein, and this reaction can be observed in crude extracts. ChloroacetylCoA is a reagent that can be used to identify the unknown substrate(s) for this large family of functionally uncharacterized enzymes.
Abstract
In order to study the grinding mechanism of quartz glass, a three-factor four-level orthogonal test under ultrasonic grinding and non-sonic grinding conditions was designed to investigate ...the laws of spindle speed, feed rate and depth of cut on the surface roughness of quartz glass. The extreme difference analysis and the analysis of influencing factors were carried out for ultrasonic grinding and non-ultrasonic grinding experiments. The experimental results show that the surface roughness of ultrasonic grinding quartz glass decreases with the increase of spindle speed, increases with the increase of feed speed and increases with the increase of depth of cut. The surface roughness of non-sonic grinding quartz glass increases with increasing spindle speed, increases with increasing feed rate, and decreases with increasing depth of cut. This study provides important implications for the study of the grinding mechanism of quartz glass.
Lipoamide dehydrogenase catalyzes the reversible NAD+-dependent oxidation of the dihydrolipoyl cofactors that are covalently attached to the acyltransferase components of the pyruvate dehydrogenase, ...α-ketoglutarate dehydrogenase, and glycine reductase multienzyme complexes. It contains two redox centers: a tightly, but noncovalently, bound FAD and an enzymic disulfide, each of which can accommodate two electrons. In the two-electron-reduced enzyme (EH2), the disulfide is reduced while the FAD cofactor is oxidized. In the four-electron-reduced enzyme (EH4), both redox centers are reduced. Lipoamide dehydrogenase can also catalyze the NADH-dependent reduction of alternative electron acceptors such as 2,6-dichlorophenolindophenol, ferricyanide, quinones, and molecular oxygen (O2). To determine the mechanism of these “diaphorase” reactions, we generated the EH2 and EH4 forms of Mycobacterium tuberculosis lipoamide dehydrogenase and rapidly mixed these enzyme forms with d,l-lipoylpentanoate, 2,6-dimethyl-1,4-benzoquinone, and O2, in a stopped-flow spectrophotometer at pH 7.5 and 4 °C. EH2 reduced d,l-lipoylpentanoate ≥100 times faster than EH4 did. Conversely, EH4 reduced 2,6-dimethyl-1,4-benzoquinone and molecular oxygen 90 and 40 times faster than EH2, respectively. Comparison of the rates of reduction of the above substrates by EH2 and EH4 with their corresponding steady-state kinetic parameters for kinetic competence leads to the conclusion that reduction of lipoyl substrates occurs with EH2 while reduction of diaphorase substrates occurs with EH4.
A new thiamin salvage pathway Begley, Tadhg P; Jenkins, Amy Haas; Schyns, Ghislain ...
Nature chemical biology,
08/2007, Volume:
3, Issue:
8
Journal Article
Peer reviewed
The physiological function for thiaminase II, a thiamin-degrading enzyme, has eluded investigators for more than 50 years. Here, we demonstrate that this enzyme is involved in the regeneration of the ...thiamin pyrimidine rather than in thiamin degradation, and we identify a new pathway involved in the salvage of base-degraded forms of thiamin. This pathway is widely distributed among bacteria, archaea and eukaryotes. In this pathway, thiamin hydrolysis products such as N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine (formylaminopyrimidine; 15) are transported into the cell using the ThiXYZ transport system, deformylated by the ylmB-encoded amidohydrolase and hydrolyzed to 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP; 6)-an intermediate on the de novo thiamin biosynthetic pathway. To our knowledge this is the first example of a thiamin salvage pathway involving thiamin analogs generated by degradation of one of the heterocyclic rings of the cofactor.
Olfactory receptors (ORs) form a large family of G protein-coupled receptor proteins (GPCRs) responsible for sensing the ambient chemical environment. The molecular recognition strategies used by ORs ...to detect and distinguish odorant molecules are unclear. Here, we investigated the variable of odorant carbon chain conformation for an established odorant-OR pair: n-octanal and rat OR-I7. A series of conformationally restricted octanal mimics were tested on live olfactory sensory neurons (OSNs). Our results support a model in which unactivated OR-I7 binds aliphatic aldehydes indiscriminately, and then applies conformational and length filters to distinguish agonists from antagonists. Specific conformers are proposed to activate OR-I7 by steric buttressing of an OR activation pocket. Probing endogenously expressed rat OSNs with octanal and constrained mimics furnished evidence that odorant conformation contributes to an odorant's unique olfactory code signature.
Four new substituted methano-bridged or heteroatom-bridged norbornadienomaleic anhydrides have been prepared and converted to sesquinorbornadiene anhydrides by reaction with cyclic 1,3-dienes. The ...versatility of parity reversal, in conjunction with
N-substituent steric effects, has been used to produce all three possible stereoisomers of the
N,
O-sesquinorbornadiene anhydrides in separate, stereoselective cycloadditions. The anhydrides have been synthesized by cyclization of their diacids (in situ production) or by flash vacuum pyrolysis of their furan adducts (yielding crystalline products); further fragmentation occurs at these or higher temperatures to produce five-membered carbocyclic or heterocyclic anhydrides. Activation energies have been evaluated for the fragmentation and cycloaddition processes using DFT calculations (B3LYP/6-31G∗) and these calculations correctly predict, which reaction can be intercepted at the norbornadienomaleic anhydride stage and preferred stereochemistry of cycloadducts.
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Crosslinking proteins to the nucleic acids they bind affords stable access to otherwise transient regulatory interactions. Photochemical crosslinking provides an attractive alternative to ...formaldehyde‐based protocols, but irradiation with conventional UV sources typically yields inadequate product amounts. Crosslinking with pulsed UV lasers has been heralded as a revolutionary technique to increase photochemical yield, but this method had only been tested on a few protein–nucleic acid complexes. To test the generality of the yield enhancement, we have investigated the benefits of using ∼150 fs UV pulses to crosslink TATA‐binding protein, glucocorticoid receptor and heat shock factor to oligonucleotides in vitro. For these proteins, we find that the quantum yields (and saturating yields) for forming crosslinks using the high‐peak intensity femtosecond laser do not improve on those obtained with low‐intensity continuous wave (CW) UV sources. The photodamage to the oligonucleotides and proteins also has comparable quantum yields. Measurements of the photochemical reaction yields of several small molecules selected to model the crosslinking reactions also exhibit nearly linear dependences on UV intensity instead of the previously predicted quadratic dependence. Unfortunately, these results disprove earlier assertions that femtosecond pulsed laser sources provide significant advantages over CW radiation for protein–nucleic acid crosslinking.
We have synthesized simple model systems to explore the possibility of photo-cross-linking between the pyrimidine bases and the side chains of the aromatic amino acids. Thymine/phenylalanine and ...thymine/tyrosine models gave cross-links, and thymine/tryptophan models gave complex mixtures; the cytosine/phenylalanine model was unreactive. The quantum yields for the model cross-linking reactions were 18−46 times smaller than those for thymine dimer formation. Biphotonic excitation contributes little to the yield of these reactions.
According to climate features and river runoff conditions, Xinjiang could be divided into three research areas： The Altay-Tacheng region, the Tianshan Mountain region and the northern slope of the ...Kunlun Mountains. Utilizing daily observations from 12 sounding stations and the annual runoff dataset from 34 hydrographical stations in Xinjiang for the period 1960-2002, the variance of the summertime 0℃ level height and the changing trends of river runoff are analyzed both qualitatively and quantitatively, through trend contrast of curves processed by a 5-point smoothing procedure and linear correlation. The variance of the summertime 0℃ level height in Xinjiang correlates well with that of the annual river runoff, especially since the early 1990s, but it differs from region to region, with both the average height of the 0℃ level and runoff quantity significantly increasing over time in the Al- tay-Tacheng and Tianshan Mountain regions but decreasing on the northern slope of the Kunlun Mountains. The correlation holds for the whole of Xinjiang as well as the three indi- vidual regions, with a 0.01 significance level. This indicates that in recent years, climate change in Xinjiang has affected not only the surface layer but also the upper levels of the atmosphere, and this raising and lowering of the summertime 0℃ level has a direct impact on the warming and wetting process in Xinjiang and the amount of river runoff. Warming due to climate change increases the height of the 0℃ level, but also speeds up, ice-snow melting in mountain regions, which in turn increases river runoff, leading to a season of plentiful water instead of the more normal low flow period.
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