Membrane-bound proteins and enzymes

Although it is known that the lipid composition of cell membranes determines membrane fluidity as well as the function and activity of membrane-bound proteins and enzymes (24), it is not known what effect changes in the relative proportions of specific fatty acids of phospholipids have on the capacitive properties of membranes. The maintenance of an electrical potential across membranes is essential to cell function and survival and may be altered in severe disease states (25). 

However, the reaction of NP with thiols may be a necessary but not sufficient cause for the release of NO from the ion as there are many thiols in frog heart tissue and NP is a vasodilator only under illumination. Furthermore Sogo et al. [50] could not detect NO generation from NP in human plasma containing cysteine, glutathione, homocysteine and reduced cysteine residues. Therefore, there must be a unique component of mammalian tissues which is involved in the release of NO from NP, and this reaction comes after reaction with thiol. Kowaluk et al. [51] report that NP is readily metabolised to NO in subcellular fractions of bovine coronary arterial smooth muscle and that the dominant site of metabolism is in the membrane fraction. This led to the isolation of a small membrane-bound protein or enzyme that can convert NP into NO. The mechanism shown in Scheme 8.2 combines the thiol reaction and that with an enzyme. 

Iron-sulfur proteins contain non-heme iron and inorganic (acid-labile) sulfur in their active centers as 4Fe-4S or 2Fe-2S or, in the case of rubredoxin, as one iron alone. The iron is always bonded to cysteine sulfur. They catalyze redox reactions between +350 and —600 mV (hydrogen electrode = —420 mV). They are usually of low molecular weight (6000-15,000 Daltons) but can form complex enzymes with molybdenum and flavin. They occur as soluble or membrane-bound proteins and catalyze key reactions in photosynthesis, oxidative phosphorylation, nitrogen fixation, H2 metabolism, steroid hydroxylation, carbon and sulfur metabolism, etc. They occur in all organisms so far investigated and may... 

The mitochondrion is bounded by two pho-spholipid membranes. The outer membrane is freely permeable to molecules, including water, with a molecular weight of up to about 5000. The inner membrane is rich in membrane-bound proteins and consists, in terms of membrane area, of 50% phospholipid and 50% protein (Lenaz, 1988). Pyruvate dehydn>genase, a mitochondrial enzyme, is water soluble. The proteins of the respiratory chain, as well as ATP synthase, are all bound to the inner mitochondrial membrane. The enzymes of the Krebs cycle are water soluble, with the exception of succinate dehydrogenase. This enzyme is bound to the mitochondrial membrane, where it directly funnels electrons, via HAD, to the respiratory chain. 

Selenium is a component of all three members of the deiodinase enzyme family, the enzymes responsible for deiodination of the thyroid hormones (Kohrle 1994 St. Germain and Galton 1997). The deiodinases contain a selenocysteine at the active site, which is required for catalytic activity. There are three types of deiodinases and they differ in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization, and sensitivity to inhibitors. The first to be recognized as a selenoprotein was type I iodothyronine 5 -deiodinase which converts the prohormone thyroxine (T4) to the active form, triiodothyronine (T3) and to date, studies of the effects of excess selenium have focused on this protein. Under normal circumstances the human thyroid produces only 20-30% of its hormone as T3 the remainder is T4 (a minute amount of reverse T3 (rT3) is also produced), which is largely converted to active T3 by type I deiodinase located within the liver, euthyroid pituitary, kidney, thyroid, and brain. Type I deiodinase is a membrane bound protein and, thus, its activity has not been directly measured in studies of humans supplemented with selenium. Human studies have instead measured serum levels of T3, rT3, T4, and TSH. 

More than 400 arene cis-dihydrodiols derived from a wide variety of aromatic substrates have now been reported. For a more in-depth coverage, the reader is directed to several excellent reviews that have appeared recently [23]. Bicychc and heterocychc arenes are viable substrates for the transformation, although when 5-membered heteroarenes (furan, thiophene) undergo this transformation, the products are not always stable. Arene dioxygenases are membrane-bound proteins, and as such, this biotransformation is not carried out using isolated enzymes. Rather, whole-cell fermentation approaches are employed. While these are not as operationally simple as the use of isolated enzymes, they can nevertheless be carried out without recourse to any particularly unusual... 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

Unlike iNOS and nNOS, the eNOS protein is post-translationally modified by the attachment of fatty acids, myristate or palmitate. This modification is important because the fatty acids help to attach the enzyme, in an inactive form, to the inner face of plasma membrane of endothelial cells or platelets. Several mechanisms serve to release eNOS from its membrane bound state and thus activate the enzyme. 

Figure 9.15 Enzymes in aqueous (light-coloured) and hydrophobic (shaded) phases. (A) A protein in the periplasm (PP) of a cell (OM = outer membrane, CM = cytoplasmic membrane) (B) membrane-bound protein in a lipid bilayer (C) hydrophilic protein in an inverted micelle (D) interaction between enzyme and substrates in aqueous micelles (E) graph of catalytic activity as a function of micelle concentration.

Recently there has been much interest in the possible role of the family of protein kinases which translate information from the second messenger to the membrane proteins. Many of these kinases are controlled by free calcium ions within the cell. It is now established that some serotonin (5-HT) receptors, for example, are linked via G proteins to the phosphatidyl inositol pathway which, by mobilizing membrane-bound diacylglycerol and free calcium ions, can activate a specific protein kinase C. This enzyme affects the concentration of calmodulin, a calcium sequestering protein that plays a key role in many intracellular processes. 

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