Active sites anions

The increase of the catalytic activity in the room-temperature oxidation of carbon monoxide, which results from the increase of the temperature of preparation of NiO from 200° to 250°C., is related to the difference in the reactivity of oxygen adsorbed on both surfaces. The interaction between adsorbed oxygen and carbon monoxide has roughly the same velocity on both oxides. But on NiO (200) this interaction yields only C02(ads)> whereas on NiO(250) the same interaction produces C03"(ads) on the most active sites (anionic vacancies) and C02(g) on the less... 

The catalytic properties of M0S2 and WS2 have been compared. In thiophen hds (673 K, 1 atm) the activity per unit area of WS2 was twice that of M0S2. This difference was attributed to the greater ease of reduction of WS2 in H2[AG (1100K) Mo, 120 W, 93.7kJmor ] giving more active sites (anion vacancies). However, in benzothiophen hds (523 K, 100 atm) M0S2 was more active than WS2 (areal activity ratio 2.66). ... 

Z6], The altered relation between T and T2 in the presence of coenzyme may be referred to a retarding effect of coenzyme on the molecular mobility at the active site anion binding region. Thus the correlation time is found to increase from 1... 

Cl relaxation of the perchlorate ion in the presence of various proteins is presently being studied in our laboratory. Of the results obtained so far may be mentioned the demonstration of an effect of CIO on the molecular mobility at the active site anion binding region of fructose-1,6-diphosphate aldolase [1Z6]. Thus addition of fructose-1,6-diphosphate is accompanied by a decreased Cl relaxation rate -6f both Cl and CIO and comparisons of T and... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

We shall examine the solution to this equation in an example below. Frorr the ratio of Eq. (6.106) to Eq. (6.103), note that [BM2 ]/[BMj"] = i7 5. The same sequence of steps outlined in items (3) and (4) can be followec to give the concentrations of n-mer anions resulting from n - 1 additions tc the original active site ... 

P-Lactam antibiotics exert their antibacterial effects via acylation of a serine residue at the active site of the bacterial transpeptidases. Critical to this mechanism of action is a reactive P-lactam ring having a proximate anionic charge that is necessary for positioning the ring within the substrate binding cleft (24). 

The reaction is proposed to proceed from the anion (9) of A/-aminocatbonylaspattic acid [923-37-5] to dehydrooranate (11) via the tetrahedral activated complex (10), which is a highly charged, unstable sp carbon species. In order to design a stable transition-state analogue, the carboxylic acid in dihydrooronate (hexahydro-2,6-dioxo-4-pyrimidinecarboxylic acid) [6202-10-4] was substituted with boronic acid the result is a competitive inhibitor of dibydroorotase witb a iC value of 5 ]lM. Its inhibitory function is supposedly due to tbe formation of tbe charged, but stable, tetrabedral transition-state intermediate (8) at tbe active site of tbe enzyme. 

Ornithine decarboxylase is a pyridoxal dependent enzyme. In its catalytic cycle, it normally converts ornithine (7) to putrisine by decarboxylation. If it starts the process with eflornithine instead, the key imine anion (11) produced by decarboxylation can either alkylate the enzyme directly by displacement of either fluorine atom or it can eject a fluorine atom to produce viny-logue 12 which can alkylate the enzyme by conjugate addidon. In either case, 13 results in which the active site of the enzyme is alkylated and unable to continue processing substrate. The net result is a downturn in the synthesis of cellular polyamine production and a decrease in growth rate. Eflornithine is described as being useful in the treatment of benign prostatic hyperplasia, as an antiprotozoal or an antineoplastic substance [3,4]. 

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). 

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

Bivalent inhibitors of thrombin have been synthesized to bind the anion-binding exosite and active (catalytic) site of thrombin simultaneously. By coupling the carboxy terminal fragment of hirudin to a tripeptide (D-Phe-Pro-Arg) by including a spacer molecule, both the anion exosite and the catalytic site are blocked. An example of such a molecule is Hirulog, which has 20 amino acids and has a Kj of 2 nM (61). Its ability to block the active site has been questioned, since thrombin has been shown to cleave the Arg-Pro bond of Hirulog slowly in vivo (58). In addition to hirudin and hirudin-like compounds, three other classes of site-directed thrombin inhibitors deserve mention. 

To finish with another trend for NO removal consisting in NO direct decomposition, we would like to depict the infrared study of NO adsorption and decomposition over basic lanthanum oxide La203 [78], In this case, the basic oxygens are proposed to lead to N02 and N03 spectator species, whereas the active sites for effective NO decomposition are described as anion vacancies, which are often present in transition metal oxides. This last work makes the transition with the study of DeNO, catalysts from the point of view of their ability to transfer electrons, i.e. their redox properties. 

On reducible supports typically when Pd is deposited on LaCoOj then reduced in H2 at 450°C for obtaining Pd0/CoOx/La2O3, an alternative mechanism would likely occur, which accounts for steps involving the creation of active sites at the metal/support interface. These active sites would be composed of metallic Pd in interaction with anionic vacancies from the support potentially active for the dissociation of NO according to step (26) [54],... 

Water molecules or anions close to the active sites in the protease enzymes, mentioned above, may not be considered circumstantial, but may effectively contribute to the removal of the surplus proton from the imidazolium cation before the actual catalytic event. They could serve well to create the initial ion/neutral form of the Aspl02-His57 couple which is important for the initial step of the catalytic process in most discussions 11611 .13i. such a proton removal may be caused by the productive binding of a true substrate (or inhibitor) of the enzyme to the neighboring recognition clefts of the active site. 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

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