Substrate-detergent interaction

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

When binding of a substrate molecule at an enzyme active site promotes substrate binding at other sites, this is called positive homotropic behavior (one of the allosteric interactions). When this co-operative phenomenon is caused by a compound other than the substrate, the behavior is designated as a positive heterotropic response. Equation (6) explains some of the profile of rate constant vs. detergent concentration. Thus, Piszkiewicz claims that micelle-catalyzed reactions can be conceived as models of allosteric enzymes. A major factor which causes the different kinetic behavior [i.e. (4) vs. (5)] will be the hydrophobic nature of substrate. If a substrate molecule does not perturb the micellar structure extensively, the classical formulation of (4) is derived. On the other hand, the allosteric kinetics of (5) will be found if a hydrophobic substrate molecule can induce micellization. 

Topical spermicides such as nonoxynol-9 (N9) and benzalkonium chloride act on sperm membranes through a detergent effect, namely, hydrophobe-hydrophobe interaction between the active and substrate (spermatozoa). The idea was to optimize the cationic/hydrophobic polymer in the drug delivery system so epithelial cells were protected without sacrificing the drug s spermicidal activity. One of the questions that needed to be answered in designing an optimum cationic/hydrophobe modified polymer was the effect of the hydrophobe on the drug activity (N9 initially, and other actives subsequently). 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

Deinococcu radiodurans, with 2-3 nm pores and widths of 5-6 mn (Fig. 13.11). The surface layer proteins were bound to the cells by hydrophobic interactions and were extracted using a detergent, sodium dodecyl sulfate (SDS). The resulting templates have been randomly dispersed on conductive substrates for through-mask deposition of Cu O, Ni, Pt, Pd, and Co [82]. 

A mechanistic model has been proposed for PPIase catalysis in which a twisted peptide bond, a structure involving substrate strain, is stabilized by noncovalent interaction with the enzyme [156], However, catalytic antibodies generated to transition state analogs containing twisted carbonyl moieties do not show a PPIase-like catalytic efficiency [157,158], Consequently, small detergent micelles and phosphatidylcholine membranes are able to catalyze CTI of typical PPIase substrates in a manner reminiscent of that observed for catalytic antibodies [159]. Apparently, sequestration of hydrophobic substrates within the enzyme may account for both a small portion of the catalytic power of FKBP and the acceleration of CTI by catalytic antibodies. Despite overall amino acid sequence dissimilarity the structural features making up the active sites of prototypic enzymes such as Cypl8 and ParlO proved to be similar (Fig. 10.6). 

We simultaneously incorporate both lipid and protein by using dialysis to remove detergent from a solubilized lipid-protein mixture in the presence of the alkylsilanated substrate. Under our conditions, from the evidence in this paper and elsewhere (9), the surface structures appear to be single bilayer membranes. Our hypothesis is that the hydrocarbon chains attached to the surface serve as initiation sites for a lipid bilayer membrane to form as the detergent is slowly removed. The model is of a membrane that is anchored to the surface by hydrophobic interactions with the surface-bound hydrocarbon layer. Integral membrane proteins are retained in these structures by their interaction with the hydrophobic core of the membrane without being directly attached to the electrode surface. 

The adsorption of plasma proteins to polymers precedes the interaction of blood cells with the surfaces, and therefore, is likely to be an important initial event in the response of blood to polymers (25, 29). At present, however, little is known about the adsorbed protein layer, even though it has been studied in some detail in recent years (30-36). Because protein adsorption from blood plasma is a competitive process, differences in the adsorbed layer on different polymer substrates could be a primary cause of differences in thrombogenicity. Previous studies of the composition of the adsorbed protein layer have employed 12oI-labeled protein added to plasma (37-39), antibody binding (34) to detect individual proteins, or electrophoretic analysis of detergent-elutable proteins (17, 33, 35). The procedure used in this study does not require the large surface areas used in previous work (35), nor does it rely on incorporation of radiolabels (36) into adsorbed protein. Instead, a staining method at least 100-fold more sensitive than these other techniques has been used. 

Neuraminidase isolated from the culture filtrate of Arthrobacter ureafaciens has been characterized in detail with respect to its action on glycolipids. Strong electrolytes had a reversible inhibitory effect on the action of the enzyme on brain gangliosides in accordance with Debye-Hiickel effect of ionic environment on ionic activity, and resulted in an acidic shift and a broadening of the pH optimum. Both ionic and non-ionic detergents markedly enhanced the activity of the thiol-sensitive enzyme on the gangliosides, and caused an acidic shift of the pH optimum. It was suggested that the hydrophobic ceramide moiety increases affinity of the lipid substrate to the enzyme, but inhibits hydrolysis of the substrate, possibly due to its hydrophobic interaction with hydrophobic portions of the enzyme molecule. 

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