Mimics of cells

We will learn to produce mimics of enzyme clusters, imitating natural clusters such as gene transcription assemblies. We will learn to produce artificial enzymes that show induced fit, and allosteric control by analogs of hormones. Then we will move to mimics of cells themselves, with their components of many enzymes, to achieve chemical processes more complex than those done by a single enzyme. The biochemistry of life is impressive, but the role of chemistry is not just to admire it. As humans were impelled to invent ways to fly after observing birds, we will learn to create a new area of chemistry - biomimetic reaction chemistry - adding both to our understanding and to our practical abilities. 

Synthetic mimics of cell-surface receptors have been constructed using plasma membrane anchors derived from N -alkyl-3fi-cholesterylamine and related compounds (14). Synthetic receptors comprising protein and other binding motifs linked to this membrane anchor become incorporated rapidly into plasma membranes of mammalian cells. By constimtively cycling between the plasma membrane and the endosomes, cells treated with these compounds gain the capacity to endocytose cell-impermeable hgands. For example, the synthetic Fc receptor shown in Fig. 4 enables human cells that lack Fc receptors... 

Much of the literature uses simpler synthetic mimics of cell membranes, consisting of vesicles (also known as liposomes) and occasionally flat bilayers supported on a substrate. Most contain no protein in order to focus on polymer-lipid interactions. Membranes meant to mimic mammalian membranes generally consist of PC and may include some cholesterol. Red blood cells (RBCs) also serve as a model system, as they do not divide. Gram-positive bacteria mimics generally consist of PC and cardiolipin, while gram-negative bacteria mimics have PE and PC. 

Polymersomes are ideal candidates for a large variety of applications, sucb as (i) drug delivery-containers, " (ii) compartments for contrast agents, (iii) nanoreactors to serve as confined nano-spaces for transport phenomena and chemical transformation, and (iv) artificial organelles or simple mimics of cells. ... 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

Growth characteristics of cells exposed to water stress mimic some of the structural responses of organised plant tissues. A frequently observed response of plants exposed to water stress is a reduction in cell size (Cutler, Rains Loomis, 1977). This cellular phenomenon was observed in tomato cells stressed with PEG (Handa et al., 1983). Concomitantly with a decrease in cell size with increasing osmotic stress was a reduction in fresh weight. In contrast the dry weight was not affected. 

The identification and characterization of cell culture systems (e.g., Caco-2-cells) that mimic in vivo biological barriers (e.g., intestinal mucosa) have afforded pharmaceutical scientists the opportunity to rapidly and efficiently assess the permeability of drugs through these barriers in vitro. The results generated from these types of in vitro studies are generally expressed as effective permeability coefficients (Pe). If Pe is properly corrected to account for the barrier effects of the filter (PF) and the aqueous boundary layer (PAbl) as previously described in Section II.C, the results provide the permeability coefficient for the cell monolayer... 

Marcotrigiano, J., Gingras, A. C., Sonenberg, N., andBurley, S. K. (1999). Cap-dependent translation initiation in eukaryotes is regulated by a molecular mimic of eIF4G. Mol. Cell 3, 707-716. 

All of the above-mentioned examples describe organosiloxane hybrid sheet-like structures. However, cell-mimicry requires spherical structures that can form an inner space as a container. Liposomes and lipid bilayer vesicles are known as models of a spherical cell membrane, which is a direct mimic of a unicellular membrane. However, the limited mechanical stability of conventional lipid vesicles is often disadvantageous for some kinds of practical application. 

The majority of CYP enzymes are located in a hydrophobic environment in the endoplasmic reticulum of cells, although cytosolic enzymes also exist, such as CYP101. In order to mimic the physiological environment of CYP enzymes, a number of groups have used phospholipids to construct biosensors such as DDAB, dimeristoyl-L-a-phosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and distearoylphosphatidylethanolamine (DSPE). Phospholipid layers form stable vesicular dispersions that bear structural relationship with the phospholipid components of biologically important membranes. By this way a membranous environment is created that facilitates electron transfer between the enzyme s redox center and the electrode. 

Crosby, D.G. and R.K. Tucker. 1966. Toxicity of aquatic herbicides to Daphniamagna. Science 154 289-291. Darr, D.J., S. Yanni, and S.R. Pinnell. 1988. Protection of Chinese hamster ovary cells from paraquat-mediated cytotoxicity by a low molecular weight mimic of superoxide dismutase (DF-Mn). Free Radical Biol. Med. 4 357-363. 

For in vitro toxicity studies and assessment of the barrier function, drug transport, cell physiology, and metabolism as well as the development of delivery systems, cell culture models provide powerful systems for scientific research. As the corneal epithelium is the main barrier for ocular penetration, various corneal epithelial cell cultures were established besides the corneal constructs that mimic the whole cornea and serve as reductionist models for the ocular barrier. In general, two types of cell culture models are available primary cell cultures and immortalized, continuous cell lines. 

Besides the synthetic inhibitors, a variety of natural compounds is known to inhibit the CP. One of these natural inhibitors, lactacystin, was discovered by its ability to induce neurite outgrowth in a murine neuroblastoma cell line. Incubation of cells in the presence of radioactive lactacystin leads to the labelling of the yS5 subunit (Fenteany et al. 1995) and to irreversible inhibition of the CP. As shown by X-ray analysis, the inhibitor is covalently attached to subunit fS5 by an ester bond with the N-terminal ThrlO (Groll et al. 1997) (see Figure 10.7A). The subunit selectivity of lactacystin can be attributed to its dimethyl group, which mimics a valine or a leucine side chain and closely interacts with Met45 in the hydrophobic SI pocket of subunit j85. 

The lymph node microenvironment represents a niche where CLL cells interact with different types of cells including monocyte-derived nurse-like cells (NLC), CD3+ CD4+ CD154+ T cells, mesenchymal stromal cells, dendritic cells, and endothelial cells (15). In addition to cell-cell interactions, CLL cells are also exposed to a variety of soluble factors such as antigens, cytokines, and chemokines (2). It is the combination of such signals that renders CLL cells less susceptible to chemotherapy and promotes clonal evolution and drug resistance. Thus, the role of the microenvironment needs to be carefully considered in order to develop novel and more effective therapies for CLL treatment (16). In particular, the efficacy of new drugs must be evaluated under experimental conditions that recapitulate (or at least partially mimic) the CLL microenvironment. 

There are a number of cell culture models available that mimic human epitheha in vivo (Caco-2, HT29, T84, lEC 18, TC7, LLCPKl, MDCK). Some of these cells are derived from human colon carcinomas (e.g., Caco-2, T84, HT29) and have many properties of the normal intestinal epithelium. The Caco-2 cell model is one of the... 

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