Protein diffusion mechanisms

Studies of the effect of permeant s size on the translational diffusion in membranes suggest that a free-volume model is appropriate for the description of diffusion processes in the bilayers [93]. The dynamic motion of the chains of the membrane lipids and proteins may result in the formation of transient pockets of free volume or cavities into which a permeant molecule can enter. Diffusion occurs when a permeant jumps from a donor to an acceptor cavity. Results from recent molecular dynamics simulations suggest that the free volume transport mechanism is more likely to be operative in the core of the bilayer [84]. In the more ordered region of the bilayer, a kink shift diffusion mechanism is more likely to occur [84,94]. Kinks may be pictured as dynamic structural defects representing small, mobile free volumes in the hydrocarbon phase of the membrane, i.e., conformational kink g tg ) isomers of the hydrocarbon chains resulting from thermal motion [52] (Fig. 8). Small molecules can enter the small free volumes of the kinks and migrate across the membrane together with the kinks. 

Cefadroxyl and cefaclor are beta-lactam antibiotics which show high affinity for the PepTl carrier system, whereas the other two beta-lactams, cephalotin and cef-metazole, are not recognized by PepTl protein and are not actively transported in the intestine. However, as the VolSurf Caco-2 model predicts that all the beta-lactams are nonpenetrating compounds, it is very probable that, as they rely only the diffusion mechanism, cefadroxyl and cefaclor will not cross the cell monolayer. 

The question is How much can one infer from Dr. Thomas diffusion experiments on the diffusion mechanism in large (bulk) artificial bilayer membranes about mechanisms on bilayer membranes of phospholipids with proteins inclusion Some part of the formulation may break down because we pass from bulk to surface only, from macro- to microdescription. 

A sustained drug release is favourable for drugs with short elimination half-life. It can be controlled by hydration and diffusion mechanisms or ionic interactions between the drug and the polymeric carrier. In the case of diffusion control the stability of the carrier system is essential, as its disintegration leads to a burst release. Therefore, the cohesiveness of the polymer network plays a crucial role in order to control the release over several hours. Due to the formation of disulphide bonds within the network thiomers offer adequate cohesive stability. Almost zero-order release kinetics could be shown for insulin embedded in thiolated polycarbophil matrices (Clausen and Bernkop-Schnurch 2001). In the case of peptide and protein drugs release can be controlled via ionic interactions. An anionic or cationic polymer has to be chosen depending... 

Until recently, experiments that probe protein diffusion relied on fluorescent antibody-based probes, and thus were limited to plasma membrane proteins with extracellular epitopes. In contrast, the mechanisms that regulate diffusion of intracellular membrane proteins remained unexplored because of their inaccessibility to labeling. With the development of genetically encoded fluorescent probes, such studies have become tractable because proteins targeted to a particular organelle can be flu-orescently labeled directly with GEP. In addition, improved technology has now made it possible to monitor the movement of multiple types of proteins or lipids, tagged with different markers simultaneously. Some examples of the types of questions it is now possible to address include ... 

An easy control, if the diffusion mechanism is the one controlling the adsorption process, is to determine the so-called induction time, i.e. the time at which the surface tension starts to decrease. According to the diffusion law this time should increase with the square root of the protein bulk concentration. In Figure 7 the data obtained from the dynamic surface tensions are shown in a double logarithmic plot. 

The basis of RAMs is the simultaneous size exclusion of macromolecules and extraction/enrichment of low-molecular-weight compounds into the interior phase via partition. The outer surface of the particles, which is in contact with biological matrix components such as proteins and nucleic acids, possesses a special chemistry to prevent adsorption of these molecules. Macromolecules can be excluded by a physical barrier, the pore diameter, or by a chemical diffusion barrier created by a protein (or polymer) network at the outer surface of the particle. RAMs can be classified according to the protein exclusion mechanism used into the following two groups RAM with a... 

The appearance of several peaks on the /(D) curves for certain samples (Figure 5.48) can be explained by the mechanism of protein diffusion through the CG hydrogel (without or with interactions with the pore walls) and aggregation of proteins in the solution and at the pore walls. However, the position of the main/(D) peaks for all the proteins studied reveals that the D values at the /(D) maxima are close to the Dq values shown earlier. Consequently, the diffusion of the... 

Flavin reductases having a diffusible fiavin can be also be grouped with what is referred to as class I enzymes, exemplified by the Fre NADPH flavin reductase ofE. coli (81). Flavin reduction in class I enzymes follows a sequential, ordered mechanism in which NADPH binds to the enzyme followed by flavin, which is then reduced and released (82). This type of diffusible mechanism implies that reduced flavin is supplied to the desulfurization monooxygenases (DszC and A) without the need for protein-to-protein interaction with the reductase (DszD). It is thus not surprising that reduced flavin can be provided to the monoo genases by a variety of reductases, as demonstrated by Lei and Tu (83) and others (84). 

The fat-soluble vitamins A, D, E and K pass through the intestinal mucosa mainly by the same passive diffusion mechanism as for fats.AMthin the cells they may combine with proteins and enter the general circulation as lipoproteins. 

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