Iron protein high-potential type

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. 

Note A HiPIP stands for high potential iron sulfur protein, a trivial name to indicate that the reduction potential of this type of small ET proteins is relatively high +0. < +0.5 volts. Formally,... 

Figure 5 The most commonly encountered FeS centers (a) the monoiron center of rubredoxin, (b) the FeySy cluster of plant-type ferredoxins, (c) the FeySy cluster of Rieske proteins, (d) the Fe3S4 cluster of ferredoxins, and (e) the Fe4 4 cluster of ferredoxins and high potential iron-sulfur proteins (FliPiPs).

The complete structure of clostridial rubredoxin has been solved by Jensen and his associates using x-ray analysis (49), and the structure of the iron-sulfur center of the high potential iron protein, but not of the polypeptide, has been determined by x-ray analysis by Kraut and associates (50). Both of these are relatively atypical proteins in this class, either having no inorganic sulfide or having a very high redox potential. So far, we still have no solution from x-ray work for the 2-iron-2-sulfur or the 8-iron-8-sulfur type protein. 

High potential iron-sulfur protein (HiPIP) is a special type of Fd which has been isolated from some photosynthetic bacteria and detected by ESR spectroscopy in other bacteria. HiPIP also contains a single 4Fe-4S cluster, but it differs from the other Fd in having a positive standard potential of about-h 350 mV (most Fd have standard potentials in the range of the hydrogen electrode, about -420 mV). Furthermore, the HiPIP from Chromatium is paramagnetic in the oxidized state. 

There are several major types of iron-sulfur proteins, i.e. the rubredoxin (Rb), ferredoxin, Rieske protein, and the high-potential iron-protein (HiPIP). 

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