Membranes steps involved

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

Although the precise mechanism of the NADH-UQ reductase is not known, the first step involves binding of NADH to the enzyme on the matrix side of the inner mitochondrial membrane, and transfer of electrons from NADH to tightly bound FMN ... 

The steps in the subsequent utilization of muscle LCFAs may be summarized as follows. The free fatty acids, liberated from triglycerides by a neutral triglyceride lipase, are activated to form acyl CoAs by the mediation of LCFAcyl-CoA synthetase which is situated on the outer mitochondrial membrane. The next step involves carnitine palmitoyl transferase I (CPT I, see Figure 9) which is also located on the outer mitochondrial membrane and catalyzes the transfer of LCFAcyl residues from CoA to carnitine (y-trimethyl-amino-P-hydroxybutyrate). LCFAcyl... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

The transfer step involves the passage of iron that has been taken up at the apical, brush border membrane across the mucosal epithelial cell to the basolateral membrane, where it is transferred to the circulation. However, not all the iron taken up from the lumen into the cell is transferred. As a function of the body s... 

The mode of synthesis of alumina membranes through the colloidal suspension route is given in Figure 2.6. The first step involves the preparation of a slip consisting of boehmite particles. These arc plate-shaped in the form of pennies with a diameter of 25-50 nm and a thickness of 3.5-5.5nm (Leenaars ct al. 1984,1985). The synthesis chemistry of the colloidal boehmite (y-AlOOH) solution is described in detail by Leenaars and Yoldas (1975) and to some extent by Anderson, Gieselman and Xu (1988) and by Larbot et al. (1987). 

Membrane-located enzymes in the sER catalyze lipid synthesis. Phospholipid synthesis (see p. 170) is located in the sER, for example, and several steps in cholesterol biosynthesis (see p. 172) also take place there. In endocrine cells that form steroid hormones, a large proportion of the reaction steps involved also take place in the sER (see p. 376). 

E. In contrast to channels, transporters bind and assist in movement of molecules as they cross the membrane and many of the steps involved are analogous to the actions of en2ymes (Figure 4—6). 

The control of enzyme activity by the environment of a polyatomic framework is a vast topic, which I shall not attempt to cover fully in this report. Instead I will concentrate on some selected interactions between and within polypeptide chains that influence enzymatic activity. First, elementary steps involved in ligand-protein, intraprotein, and interprotein interactions are considered. Then enzymes consisting of a single polypeptide chain are discussed, followed by enzymes consisting of multiple polypeptide chains. The concluding sections are concerned with multienzyme complexes and enzymes associated with membranes. 

There is at present no precise information concerning either the control mechanisms that govern wall-biogenesis or the interactions between wall biogenetic-processes and general cellular metabolism. The number of steps involved in the formation of a polysaccharide from a glycosyl-nucleotide is not known, and it is not clear how cellular control is extended beyond the plasma membrane, or how the cell wall is formed from the component polymers. Thus, it appears that the major questions posed by the problem of cell-wall biosynthesis have yet to be answered (see also, Ref. 217). 

In ihe mammalian body, calcium is required to insure the integrity and permeability of cell membranes. 10 regulate nerve and muscle excitubiliiy. to help maintain normal muscular contraction, and to assure cardiac rhvthmicity. Calcium plays an essential role in several of ihe enzymatic steps involved in blood coagulation and also activates certain other enzyme-catalyzed reactions not involved in any of ihe foregoing processes. Calcium is ihe niosi important element of bone sail. Together with phosphate and carbonate, calcium confers on bone most of its mechanical and structural properties. 

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

M Matsuhashi. Utilization of lipid-linked precursors and the formation of peptidoglycan in the process of cell growth and division membrane enzymes involved in the final steps of peptidoglycan synthesis and the mechanism of their regulation. In JM Ghuysen, R Hakenbeck, eds. Bacterial Cell Wall. Amsterdam, The Netherlands Elsevier Science, 1994, pp 55-72. 

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