Membranes enzyme-functionalized

The pathway of the metabolic process converting the original nutrients, which are of rather complex composition, to the simple end products of COj and HjO is long and complicated and consists of a large number of intermediate steps. Many of them are associated with electron and proton (or hydrogen-atom) transfer from the reduced species of one redox system to the oxidized species of another redox system. These steps as a rule occur, not homogeneously (in the cytoplasm or intercellular solution) but at the surfaces of special protein molecules, the enzymes, which are built into the intracellular membranes. Enzymes function as specific catalysts for given steps. 

Both sodium and potassium are rather abundant in the Earth s surface (Na, 2.6% K, 2.4%), but potassium, due to its greater solubility and subsequent uptake by plant life, is much less prevalent in the seas. Indeed, potassium is so vital to plants that its major use, usually as the chloride or sulfate, is in fertilizers. This is certainly not a newly recognized technology. Even centuries ago, farmers knew that spreading wood ashes on their lands made crops grow better. We now recognize that the potassium in these ashes was primarily responsible for the effect. Both sodium and potassium ions are present in plants and animals and are essential for normal biochemical functions, particularly for the maintenance of the concentrations of ions across various cellular membranes, enzyme functions, and the firing of nerve impulses. 

TBT and TFT are membrane-active molecules, and their mechanism of action appears to be strongly dependent on organotin(IV) lipophilicity. They function as ionophores and produce hemolysis, release Ca(II) from sarcoplasmic reticulum, alter phosphatodylseiine-induced histamine release, alter mitochondrial membrane permeability and perturb membrane enzymes. Organotin(IV) compounds have been shown to affect cell signaling they activate protein kinase and increase free arachidonic acid through the activation of phospholipase... 

The organization of functionally related enzymes. Functionally related enzymes are organized in three ways (a) as unlinked proteins soluble in the aqueous milieu of the same cellular compartment, (b) as components in a multiprotein complex, or (c) as components on a membrane. 

It appears that insulin and certain growth factors may exert their effects by acting through this type of tyrosine kinase receptor-enzyme system.21,44 Insulin, for example, binds to the extracellular component of a protein located on skeletal muscle cells, thereby initiating activation of this protein s enzymatic activity on the inner surface of the cell membrane. This change in enzyme function causes further changes in cell activity, which ultimately result in increased glucose uptake in the muscle cell. The function of insulin receptors and their role in the cause and treatment of diabetes mellitus are discussed in more detail in Chapter 32. 

It is clear that Mn and Gd can be sensitive EPR probes of structure and function in membrane enzyme systems. If problems of purity and concentration can be overcome, several other membrane enzymes should be amenable to EPR investigations of the type described here. Moreover, the combination of nuclear relaxation studies and EPR studies of free and bound metals can provide sufficient data for the construction of models of active sites for membrane enzymes. For each paramagnetic probe which can be located unambiguously and uniquely at the active site, one more spatial dimension is added to the picture. Thus the identification of a single Mn2+ site on the (Na+ + K+)-ATPase has permitted the determination of three distances between Mn2+ and... 

Among the outer membrane enzymes, OmpT is a special protease that has been implicated in the pathogenicity of bacteria. It is monomeric with the active center pointing to the outside (Vandeputte-Rutten et al., 2001). Another enzyme, the phospholipase A OmpLA, produces holes in the outer membrane when it is activated. The activation process has not yet been clarified, but it is known to require a dimerization of OmpLA in the membrane. The activation by dimer formation has been verified by a crystal structure analysis of an OmpLA dimer that was produced by a reaction with an inhibitor (Snijder et al., 1999). It showed that each active center contained a catalytic triad Ser-His-Asn on one subunit and an ox-anion hole formed by an amide together with a hydrated Ca2+ ion on the other. The active centers are well placed for deacylating lipopolysaccha-rides of the external leaflet of the outer bacterial membrane. OmpLA functions in the secretion of colicins and virulence factors. 

X A new flavoprotein, containing iron and labile sulfide, has been discovered in the mitochondrial inner membrane independently by Ruzicka and Beinert (4SS) and Hatefi et al. (436). The protein contains acid-extractable FAD, and 4 g-atoms of iron and 4 moles of labile sulfide per mole of flavin. The molecular properties and the enzymic function of this iron-sulfur flavoprotein are not clear. 

Thermoplasma acidophilum membranes hydrolyze a variety of phosphate esters most actively at pH 4.6. ATPase activity is stimulated by sulfate and is unaffected by DCCD, azide, or nitrate. Cells extrude sulfate when the intracellular compartment is acidified, and it is suggested that the enzyme functions as a sulfate-translocating ATPase [46]. 

---