Nonproductive complexes

In the next step of the sequence, the authors sought to introduce a hydroxy-methylene substituent at the unsubstituted 7-position of the enone. This bond construction can be carried out by conducting a Baylis-Hillman reaction with formaldehyde. In this instance, the authors used a modification of the Baylis-Hillman reaction which involves the use of a Lewis acid to activate the enone [26]. Under these conditions, the enone 42 is treated with excess paraformaldehyde in the presence of triethylphosphine (1 equiv), lanthanum triflate (5 mol%), and triethanolamine (50 mol%). It is proposed that the lanthanum triflate forms a complex with the triethanolamine. This complex is able to activate the enone toward 1,4-addition of the nucleophilic catalysts (here, triethylphosphine). In the absence of triethanolamine, the Lewis acid catalyst undergoes nonproductive complexation with the nucleophilic catalyst, leading to diminution of catalysis. Under these conditions, the hydroxymethylene derivative 37 was formed in 70 % yield. In the next step of the sequence, the authors sought to conduct a stereoselective epoxidation of the allylic... 

Computer simulations also point to the regulatory potential of these non-productive complexes. See Deadend Complexes Inhibition Nonproductive Complexes Product Inhibition Substrate Inhibition Isotope Trapping Isotope Exchange at Equilibrium Enzyme Regulation... 

On the left-hand side of Fig. 1 are the various mineralforming components in their complexed and uncom-plexed forms. They can directly bind to the crystal growth sites or they can combine to form the crystallization monomers (half-filled squares). These processes can be blocked competitively by the presence of other substances that form nonproductive complexes, thereby depleting the concentrations of precursors through mass action. The diagram also shows a second phase that helps to explain the nature of oriented diffusion and subsequent adsorption of the monomers. This so-called dou-... 

ISOTOPE EXCHANGE AT EQUILIBRIUM ISOTOPE TRAPPING LACTATE DEHYDROGENASE LIGAND EXCLUSION MODEL NONPRODUCTIVE COMPLEXES PRODUCT INHIBITION SUBSTRATE INHIBITION... 

Fig. 11. The slowly hydrolyzed substrate glycyl-L-tyrosine binds to carboxypeptidase A in a nonproductive complex where the amino-terminal glycine complexes the active-site ion (large sphere) to form a five-membered chelate, as in Fig. 10. Protein-bound zinc ligands Glu-72, His-69, and His-196 complete the coordinadon polyhedron of pentacoordinate zinc. Active-site residues are indicated by one-letter abbreviadons and sequence numbers E, glutamate H, hisddine R, arginine Y, tyrosine. [Reprinted with permission from Christianson, D. W., Lipscomb, W. N. (1986) Proc. Natl. Acad. Sci. U.S.A. 83,7568-7572.]...

This attractive hypothesis may prove difficult to substantiate owing to the high probability of forming nonproductive complexes or dimeric chelates11 121. Supporting evidence for a similar pathway in phosphate monoesters will be discussed under Section 2.5. 

Product inhibition (Section A,12) can also provide information about mechanisms. For example, if 1 / v is plotted against 1 / [A] in the presence and absence of the product Q, the product will be found to compete with A and to give a typical family of lines for competitive inhibition. On the other hand, a plot of 1 / v vs 1 / [B] in the presence and absence of Q will indicate noncompetitive inhibition if the binding of substrates is ordered (Eq. 9-43). In other words, only the A-Q pair of substrates are competitive. Product inhibition is also observed with enzymes having ping-pong kinetics (Eq. 9-47) as a result of formation of nonproductive complexes. 

Abortive complexes 466. See also Nonproductive complexes Absorption spectra... 

Noncompetitive inhibition 476,477 Nonheme iron proteins. See Iron-sulfur and diiron proteins Nonlinear equations 460 Nonmetallic ions, ionic radii, table 310 Nonproductive complexes 475 Norepinephrine (noradrenaline) 553,553s in receptor 555s Nuclear envelope 11... 

The second group of arguments involved N3 or its equivalent. Protonation as in U or in C at low pH is acceptable. Methylation blocks activity completely. Sterically interposing a group with a radius of 2 A, 1.4 A from N3 would be expected to lead to a nonproductive complex if any, since N3 or 3 NH are normally H bonded to Thr 45 (at an N-0 distance of 2.9 A) in the bottom of a cleft. Electronically such a substitution would affect the mesomeric system that contains 02. Replacement of N3 with CH in 2-pyridone (504) does not destroy activity. In fact, ks = 0.2 sec-1 and Km decreases to 1.5 mM. The steric argument would be that the blockage is much less severe than N-CH3 (by about 1.2 A) and must be tolerable. The loss of the H bond is not so severe since the hydrophobic interactions would replace it. 

These are positions III and IV of Fig. 20. Position III seems to be preferred in the absence of adenine. It is very exposed to the solvent. In position IV, which is forced by AMP binding, His 119 is against the protein surface and potentially bonded to the carboxyl group of Asp 121. When adenine is present as AMP or in UpcA, His 119 is quite buried. Since these isomers directly involve an active histidine they may be more important than isomerization of His 48. If the shift is too rapid to have been observed it may be insignificant. If it is too slow to have been observed in the dynamic experiments the complex with His 119 in position III, for example, may be a significant nonproductive complex affecting both Km and the turnover number. 

The inhibition effect of poly (vinyl alcohol) on the amylose hydrolysis was investigated. Figure 7 shows Lineweaver-Burk plots of the amylose hydrolysis rates catalyzed by the random copolymer in the presence of poly (vinyl alcohol). The reaction rate is found to decrease with increasing the concentration of poly (vinyl alcohol), and all of the straight lines obtained in the plots cross with each other at a point on the ordinate. This is a feature of the competitive inhibition in the enzymatic reactions. In the present reaction system, however, it is inferred to suggest that the copolymer and poly (vinyl alcohol) molecules competitively absorb the substrate molecules. The elementary reaction can be described in the most simplified form as in Equation 3 where Z, SI, and Kj[ are inhibitor, nonproductive complex, and inhibitor constant, respectively. Then the reaction rate is expressed with Equation 4. 

The classic example of competitive inhibition is inhibition of succinate dehydrogenase, an enzyme, by the compound malonate. Hans Krebs first elucidated the details of the citric acid cycle by adding malonate to minced pigeon muscle tissue and observing which intermediates accumulated after incubation of the mixture with various substrates. The structure of malonate is very similar to that of succinate (see Figure 1). The enzyme will bind malonate but cannot act further on it. That is, the enzyme and inhibitor form a nonproductive complex. We call this competitive inhibition, as succinate and malonate appear to compete for the same site on the enzyme. With competitive inhibition, the percent of inhibition is a function of the ratio between inhibitor and substrate, not the absolute concentration of inhibitor. 

At high concentrations of G3 and G4, condensation reactions were observed with the formation of G6 and G8, respectively, which were then hydrolyzed at their specific bonds to change the product distribution observed for G3 and G4 under dilute conditions. Condensation reactions occurred via formation of nonproductive complexes with G3 and G4 that occur with the subsites to the left of the catalytic groups. Thus, porcine pancreatic a-amylase does not hydrolyze the (1 4)-a-D-glucosidic bonds randomly. 

The precise catalytic roles of Glu 144 and Glu 164 remained uncertain, despite the availability of crystal structures of necessarily nonproductive complexes [31]. These structures pointed to the expected proximity of Glu 164 to carbon-2 and led to the suggestion that it mediates proton transfers to/from carbon. Accordingly, Glu 144 was expected to facilitate attack of water on carbon-3. In addition, the interpretation of the dependence of the kinetic constants for reactions catalyzed by wild type ECH as well as the E144Q and E164Q mutants is uncertain although common sense would require that one be anionic, i.e., a general base, and the other be neutral, i.e., a general acid, no self-consistent, unequivocal support for this expectation could be obtained. 

The most favorable mechanism for achieving specificity in electron transfer reactions would appear to operate at the reactant association stage. Discrimination or proofreading for incorrect redox partners at Steps 2 and 3 may also occur, but this suffers the disadvantage that at least some fraction of the donor and acceptor will be tied up in nonproductive complexes, thus decreasing the concentration of proteins available for productive transfer. 

Terbium(III) binds to metal sites in nucleic acids, but often inhibits function apparently due to its higher charge.Moreover, this metal also has luminescence properties that are useful for investigating metal binding sites of biomolecules. Tb" is suggested to inhibit the hammerhead ribozyme by competing with a functional Mg ion, creating a nonproductive complex. An X-ray... 

The simplest explanation for the competitive inhibition is that the inhibitor binds to the same site on the enzyme as the substrate, forming an abortive, nonproductive complex inhibitor and substrate are mutually exclusive (Fromm, 1979, 1995). In other words, the substrate and inhibitor compete for the same site, so that only one enzyme-inhibitor complex is possible ... 

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