Ferredoxins defined

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

The two ferredoxins isolated from Dsm. baculatum were investigated by EPR spectroscopy (67). The [4Fe-4S] center of Fdl shows a well-defined rhombic EPR signal in the reduced form at low temperature with g-values at 1.902, 1.937, and 2.068. The as-isolated Dsm. baculatum Fdll shows a weak contribution of a [3Fe-4S] + cluster (5%, Eo = 115 mV). Upon reduction a complex EPR signal appears,... 

The next class of iron-sulfur proteins contains Fe3S4 clusters. The commonest example of such proteins is that of 3Fe ferredoxins, which are found in bacteria. As shown in Figure 23, in these proteins the Fe3S4 cluster belongs to the class previously defined as having an incomplete cuboidal geometry (Chapter 8, Section l.l).46... 

However, in contrast to the cyclic flow of electrons in purple bacteria, some electrons flow from the reaction center to an iron-sulfur protein, ferredoxin, which then passes electrons via ferredoxin NAD reductase to NAD+, producing NADH. The electrons taken from the reaction center to reduce NAD+ are replaced by the oxidation of H2S to elemental S, then to SOf, in the reaction that defines the green sulfur bacteria. This oxidation of H2S by bacteria is chemically analogous to the oxidation of H20 by oxygenic plants. 

When reduced ferredoxin is used as the electron donor for polysulfide reduction, the reaction is monitored by the production of sulfide. The assays are performed in 8-ml serum-stoppered vials under Ar that are shaken at 150 rpm at 80°. The reaction mixture (2 ml) contains 100 vaM EPPS buffer, pH 8.0, 10 mM pyruvate, 2 toM coenzyme A, 150 p,g POR from P. furiosus, 25 lM ferredoxin, 40 p.g FNOR, and a source of elemental sulfur. This is polysulfide (1.5 mA/), sublimed elemental sulfur (0.5%, w/v J. T. Baker, Marietta, GA), or colloidal sulfur (0.05%, w/v Fluka, Ronkonkoma, NY). Note that sublimed elemental sulfur has a very low solubility, whereas colloidal sulfur generates a fine suspension. At 20 min intervals over 2 hr, aliquots of the reaction are removed with a syringe and the amount of sulfide produced is measured by methylene blue formation." One unit of activity is defined as 1 p.mol sulfide produced per min. When reactions are monitored by sulfide production, it is important that control assays be carried out without the addition of enzyme. This is particularly important when poly sulfide is used as the source of elemental sulfur. 

In chloroplasts of higher plants the ferredoxin-thioredoxin system links light-triggered events in thylakoid membranes with the regulation of enzymes in the stroma (1,2). If the conformation of enzymes changes because of modulators action then the surface exposed to the solvent will be different from the native state (3). As a consequence, interactions of modified enzymes with supramolecular structures (membranes, protein complexes) will differ respect to native forms. Since thylakoid membranes are complex structures they are not adequate for uncovering molecular mechanisms that participate in protein interactions(4). In thfe respect, the well-defined structure of micelles of non-ionic detergents constitute model compounds for the analysis of hydrophobic interactions in proteins (5,6). We report herein that chloroplast fructose-1,6-bisphosphatase interacts with micelles of Triton X-114 in a pH-dependent process. 

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