Cell membranes extra-cellular proteins

The non-biodegradable water soluble heavy metals are either oxidized or reduced by the microorganisms and produce less soluble species. The less soluble form of these metals which are formed due to microbial reactions are adsorbed or precipitated/co-precipitated on the surface of the adsorbent and the extra cellular protein of the microorganisms in the biolayer (Srivastava and Majumder, 2008 Vails and Lorenzo, 2002). The methylation of metals is also another important route for bioremediation of heavy metals in water (White et al, 1997). Though the microbial action on metal ion transformation is still a matter of research, it is assumed that there are two paths. In one path oxidation or reduction of heavy metal ions takes place by extra cellular enzymes where the metal ions do not enter into the bacterial cell. In the other path the metal ions are transported into the microbial cells by trans-membrane proteins and are converted to other less soluble forms by metabolic actions of enzymes in the cells followed by subsequent excretion from the cells, yet both the paths are plasmid mediated (Vails and Lorenzo, 2002). Whether the microbial action on a metal ion is performed by only one path or by both the paths is a matter of research. 

Amine hormones include the thyroid hormones and the catecholamines. The thyroid hormones tend to be biologically similar to the steroid hormones. They are mainly insoluble in the blood and are transported predominantly (>99%) bound to proteins. As such, these hormones have longer half-lives (triiodothyronine, t3, = 24 h thyroxine, T4, = 7 days). Furthermore, thyroid hormones cross cell membranes to bind with intracellular receptors and may be administered orally (e.g., synthryoid). In contrast to steroid hormones, however, thyroid hormones have the unique property of being stored extra-cellularly in the thyroid gland as part of the thyroglobulin molecule. 

The bradykinin receptor is a member of a family of receptors for which an intracellular interaction with a G-protein is a critical part of the signal transduction pathway following agonist binding. Structurally, these G-protein-coupled receptors extend from beyond the extracellular boundary of the cell membrane into the cytoplasm. The tertiary structure is such that the protein crosses the bilayer of the cell membrane seven times, thus forming three intracellular loops, three extracellular loops, and giving rise to cytoplasmic C-terminal and extra-cellular N-terminal strands. It is generally presumed that the transmembrane domains of these receptors exist as a bundle of helical strands. This assumption is derived primarily from the known structure of the trans-membrane portions of a structurally related protein, bacteriorhodopsin [40]. 

At mildly acidic pH some annexins are proposed to be capable of undergoing conformational changes that would facilitate their insertion into membranes as pore complexes. It is conceivable that subsequent contortions could release the protein to the extra-cellular membrane. An alternative suggestion is that the proteins are budded-off from the surface of the cell in association with small lipid vesicles which could then lyse and release the protein. Their high affinity for phospholipids in the presence of calcium is consistent with the retention of annexins on the cell surface though there is growing evidence for the presence of non-lipid annexin receptors on certain cell types. 

There are many additional fascinating elements to the cell than are mentioned here mitochondria, ribosomes, protein ion channels, and the nuclear envelope, to name but a few. In this chapter, however, we restrict our discussion to the cell membrane, the cytoskeleton, and the nuclear material (e.g., DNA) as these systems are easily amenable to simple descriptions as soft materials. To learn more detail about the variety of other cellular structures not covered here, some extra sources for further reading are suggested at the end of the chapter. After reading this book, I hope that you will start to think about molecular and cell biology in a different light and consider how cellular structures self-assemble as a result of the interplay between intermolecular forces and thermodynamics. 

Figure 2. Proposed structural organization of connexin proteins into the plasma membrane to form a connexon unit. Panel A Each hemi-connexon is composed of six identical protein units arranged to circle the water filled pore, hemi-connexons dock with equivalent structures in adjacent cells to form an intact gap junction. Panel B topography of the connexin 43 protein in the plasma membrane. There are believed to be four trans-membrane spanning domains, the third of which is believed to line the water filled pore. The two extra-cellular loops are believed to dock with equivalent loops donated by connexins from the adjacent cell. (From [27] with permission).

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