Thermophiles membrane function

Like the other macromolecular constituents of the cell (membranes, polynucleotides), the proteins of these extreme thermophiles must also be stable enough to resist heat-induced destruction of conformation and covalent structure. Since virtually all proteins (functional as well as structural proteins) exhibit dynamic properties to fulfill the demands of the living cell, their structure must provide a compromise between rigidity and flexibility, allowing not only stability but also conformational freedom for their biological function at the respective temperature. This means they are not only thermoresistant but require the higher temperature for optimal function. 

It would be of interest to test these hypotheses by examining the methanogens and thermophiles for the presence of FAS and acyl transferases and studying the effect of salt concentration, low pH and high temperature on their activities and on those of the mevalonate enzyme system. It may be noted that preliminary studies with M. thermo-autotrophicum have revealed the presence of a functional FAS producing fatty acids for acylation only of membrane proteins (Pugh and Kates, unpublished data). 

A section on protein structural chemistry in archaea includes Chapters 5 through 7, respectively, by D. Oesterhelt on the structure and function of photoreceptor proteins in the Halobacteriaceae J. Lanyi on the structure and function of ion-transport rhodopsins in extreme halophiles and R. Hensel on proteins of extreme thermophiles. In a section on cell envelopes (Chapters 8-10), O. Kandler and H. Konig discuss the structure and chemistry of archaeal cell walls M. Kates reviews the chemistry and function of membrane lipids of archaea and L.I. Hochstein covers membrane-bound proteins (enzymes) in archaea. 

There are three main reasons to suggest a specific function of subunit III in proton translocation. First, Casey et al. [171] showed that modification of this subunit with dicyclohexylcarbodiimide (DCCD) blocks proton translocation, but has little effect on electron transfer. Similar results have been obtained with the reconstituted oxidase from the thermophilic bacterium PS3 [164]. Prochaska et al. [160] showed that DCCD binds mainly to Glu-90 of the bovine subunit III, which is predicted to lie within the membrane domain and hence to be a site analogous to the DCCD binding site in the membranous fj, sector of the ATP-synthase (Fig. 3.8 see also Ref. 85). Since the latter is a part of a proton-conducting channel in ATP synthase, subunit III was thought to have the same function. However, there is one essential difference between the two phenomena. Modification of the membranous glutamic residue in by DCCD leads also to inhibition of ATP hydrolysis in the complex, as expected for two linked reactions. In contrast, DCCD has little or no effect on electron transfer in cytochrome oxidase under conditions where H translocation is abolished. Hence, DCCD cannot simply be judged to block a proton channel in the oxidase. More appropriately, it decouples proton translocation from electron transfer. 

We close this survey of cell membranes with a remarkable observation that adds support to this novel picture of cytomembrane shape. In Chapter 4 (section 4.13), it was noted that many bacteria are shrouded in a mesh-like protein coat, which often displays a regular, crystallographic form. The most exotic examples of bacteria are the thermophilic archaebacteria, that thrive at temperatures between 70°-105°C, in sulfur-rich hot-springs and mud holes. (So anachronistic are these single-celled organisms, that they are sometimes taxonomically classified as a distinct Kingdom.) It appears that the dimensions of the protein layers in species of these bacteria, Solfolobus solfataricus, are in "precise epitaxial coincidence" with the lattice parameters of a bicontinuous cubic phase, formed in excess water with the membrane lipids predominant in this organism in vitro) [140]. Such a coincidence is indeed difficult to reconcile with the usual notion of a flat, neutral, cytomembrane, whose sole function is to support the real stuff of life, the proteins. 

Thermophiles and extreme thermophUes have characteristic membrane and enzyme systems that allow them to function at temperatures that would otherwise inhibit cellular transport and metaboUc activity. These adaptations include high proportions of saturated lipids in cell membranes to prevent melting, enzyme... 

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