Binding, covalent

The main advantage of this method is high operational stability as a result of the interaction with a low leakage from the lipase and the support even if the process temperature, pH, or solvent is changed. This is a crucial feature in the feasibility of any industrial process. Hence, it is considered better than adsorption, as diffusion restrictions are fewer and the bond strength prevents the enzyme from leaking into the solution. Rodrigues et al. (2002) studied the covalent immobilization of Chromobacterium 

Supercritical Fluids Technology in Lipase Catalyzed Processes 

FIGURE 3.2 Electron microscope images of (a) surface-modified mesoporous activated carbon and (b) lipase immobilized on surface-modified mesoporous activated carbon. (From Ramani, K., R. Boopathy, A. B. Mandal, and G. Sekaran, 2011, Catalysis Communications 14 (l) 82-88. With permission.) 

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Organosilanes, such as trichlorosilanes or trimethylsilanes, can establish SA monolayers on hydroxylated surfaces. Apart from their (covalent) binding to the surface these molecules can also establish a covalent intennolecular network, resulting in an enlranced mechanical stability of the films (figure C2.4.11). In 1980, work was published on the fonnation of SAMs of octadecyltrichlorosilane (OTS) 11171. Subsequently, the use of this material was extended to the fonnation of multilayers 11341. 

Elucidating Mechanisms for the Inhibition of Enzyme Catalysis An inhibitor interacts with an enzyme in a manner that decreases the enzyme s catalytic efficiency. Examples of inhibitors include some drugs and poisons. Irreversible inhibitors covalently bind to the enzyme s active site, producing a permanent loss in catalytic efficiency even when the inhibitor s concentration is decreased. Reversible inhibitors form noncovalent complexes with the enzyme, thereby causing a temporary de-... 

Covalent binding of chemicals to biological macromolecules can also cause toxicity. During biotransformation and metabolic activation, chemical compounds can be changed to free radicals, which have an unpaired... 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

The metabolic control is exercised on certain key regulatory enzymes of a pathway called allosteric enzymes. These are enzymes whose catalytic activity is modulated through non-covalent binding of a specific metabolite at a site on the protein other than the catalytic site. Such enzymes may be allosterically inhibited by ATP or allosterically activated by ATP (some by ADP and/or AMP). 

To develop a continuous process, the immobilisation of aminoacylase of Aspergillus oryzae by a variety of methods was studied, for example ionic binding to DEAE-Sephadex, covalent binding to iodo-acetyl cellulose and entrapment in polyacrylamide gel. Ionic binding to DEAE-Sephadex was chosen because the method of preparation was easy, activity was high and stable, and regeneration was possible. 

The resolution of these columns for protein mixtures, however, was comparably poor. The peak capacity for human serum albumin was near 3 during 20 min gradient elution. Improvement has been reached by covalent binding of PEI (M = 400-600) onto a 330 A silica of 5 pm particle size [38], The peak capacities of ovalbumin and 2a -arid glycoprotein were 30-40 (tgradienl = 20 min). Enhanced peak capacity and resolution probably were due to the more diffuse structure of PEI coupled to silane moieties than that of strictly adsorbed on silica and cross-linked (see Sect, 2.2). Other applications of covalently adsorbed PEI are discussed in Sect. 4.1. 

Aggregation of floe for formation of cross-linking Covalent binding... 

Irreversible metabolic inhibition caused by covalent binding of the inhibitor to the enzyme after being metabolized by the same enzyme. The inhibitory effect remains after elimination of the inhibitor from the body. 

Since a substituted benzimidazole was first reported to inhibit the H,K-ATPase by covalent binding [ 1 ], many PPIs have been synthesized and are in clinical use. These all have a similar core structure, 2-pyridy lmethylsulfinyl benzimidazole moiety except tenatoprazole. Tenato-prazole has 2-pyridylmethylsulfinyl pyridoimidazole moiety. 

FIGURE 4-17 Preconcentrating surfaces based on covalent binding of the ligand to a polymer backbone. Q = charge A = electrode area T = surface coverage. (Reproduced with permission from reference 52.)... 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

Covalent binding of peptides to polymer surfaces is now a standard method to improve their biocompatibility. The primary amino acid sequence of a peptide can be chosen to mimic the putative... 

De Matters, R (1974). Covalent binding of snlphnr to microsomes and loss of cytochrome P450 dnring oxidative desnlphuration of several chemicals. Molecular Pharmacology 10, 849. 

Hyaluronic acid is a linear polysaccharide found in the highest concentrations in soft connective tissues where it fills an important structural role in the organization of the extracellular matrix (23,24). It has been used in ophthalmic preparations to enhance ocular absorption of timolol, a beta blocker used for the treatment of glaucoma (25), and in a viscoelastic tear formulation for conjunctivitis (26). The covalent binding of adriamycin and daunomycin to sodium hy-aluronate to produce water-soluble conjugates was recently reported (27). 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

FIG. 8 Electron micrographs of freeze-etched preparations of whole cells from (a, b) Bacillus sphaericus CCM 2120 exhibiting a square S-layer lattice or from (c, d) Thermoanaerobacter ther-mohydrosulfuricus Llll-69 carrying a hexagonally ordered S-layer lattice, (a, c) Native S-layer lattices (b, d) S-layer lattices after covalent binding of ferritin to carbodiknide-activated carboxylic acid groups of the S-layer protein. Bars, 100 nm. 

The first is cell injury (cytotoxicity), which can be severe enough to result in cell death. There are many mechanisms by which xenobiotics injure cells. The one considered here is covalent binding to cell macromol-ecules of reactive species of xenobiotics produced by metabolism. These macromolecular targets include DNA, RNA, and protein. If the macromolecule to which the reactive xenobiotic binds is essential for short-term cell survival, eg, a protein or enzyme involved in some critical cellular function such as oxidative phosphorylation or regulation of the permeability of the plasma membrane, then severe effects on cellular function could become evident quite rapidly. 

Hatcher PG, JM Bortiatynski, RD Minard, J Dec, J-M Bollag (1993) Use of high-resolution NMR to examine the enzymatic covalent binding of C-labeled 2,4-dichlorophenol to humic substances. Environ Sci Technol 21 2098-2103. 

Thorn K A, KR Kennedy (2002) N NMR investigation of the covalent binding of reduced TNT amines to soil humic acid, model compounds, and lignocellulose. Environ Sci Technol 36 3787-3796. 

Thorn KA, PJ Pettigrew, WS Goldenberg, EJ Weber (1996) Covalent binding of aniline to humic substances. 2. N NMR studies of nucleophilic addition reactions. Environ Sci Technol 30 1764-1775. 

Achnich A, E Fernandes, J-M Bollag, H-J Knackmuss, H Lenke (1999) Covalent binding of reduced metabolites of [ NjJTNT to soil organic matter during a bioremediation process analyzed by NMR spectroscopy. Environ Sci Technol 33 4448-4456. 

In a discussion of papers by Rice Harris (1954) and Harris Rice (1954), Van Wazer (1954) suggested that there could be covalent binding as well as electrostatic interaction and that cations could be held at specific sites by complex formation. 

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