HiPIP proteins

High-potential iron-sulfur proteins (HiPIP) form a family of small (—6-10 kDa) soluble electron transport proteins originally only found in photo synthetic representatives of the proteobacteria (for reviews,... 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

Figure 6.4 Absorption spectrum (A) and CD spectrum (B) of the [Fe4S4] cluster of a high-potential iron protein (HiPIP) from Chromatium sp. (From Cowan, 1997. Reproduced with permission from John Wiley Sons., Inc.)...

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

Several models have been proposed for the active center of iron and sulphur in Clostridial ferredoxin in which the cysteine residues in the peptide chain participate in the sulphur bridging. Fig 9 166). Unfortunately X-ray analysis of crystals of these proteins has not been completed. It is difficult to confirm that all the irons are clustered in a single linear array 167, 168). X-ray studies of another non-heme iron protein, the high potential iron protein, hipip, from chromatium, carried out by J. Kraut (personal communication), indicate that the four irons of this molecule may be clustered in a tetrahedral array. 

Another group of related electron carriers, the high-potential iron proteins (HIPIP) contain four labile sulfur and four iron atoms per peptide chain 261-266 X-ray studies showed that the 86-residue polypeptide chain of the HIPIP of Chromatium is wrapped around a single iron-sulfur cluster which contains the side chains of four cysteine residues plus the four iron and four sulfur atoms (Fig. 16-15D)261 This kind of cluster is referred to as [4Fe-4S], or as Fe4S4. Each cysteine sulfur is attached to one atom of Fe, with the four iron atoms forming an irregular tetrahedron with an Fe-Fe... 

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

Figure 17. Schematic diagrams of some representative topologically chiral proteins.79 (a) Condensed schematic drawing of the L subunit of the quinoprotein TV-MADH. The looped line represents the polypeptide backbone with N and C terminals. Cysteine (or half-cystine) residues are numbered, and their a-carbons are indicated by filled circles. Intrachain disulfide bonds are shown as dashed lines joining a pair of filled circles. The heavy line symbolizes an intrachain cofactor link, (b) Chromatium high potential iron protein (HiPIP), one of several Fe4S4 cluster-containing proteins, (c) Toxin II from the scorpion Androctonus australis Hector. Reprinted with permission from C. Liang and K. Mislow, J. Math. Chem. 1994,15,245. Copyright 1994, Baltzer Science Publishers.

High-Potential Iron-Sulfur Protein (HiPIP)... 

The [4 Fe-4 S] cores have been one of the most intriguing inorganic structures involved in biological systems. Carter et al. (1977) 191 demonstrated that the same basic structure is present in the two [4 Fe—4 S] centers of the 8 Fe ferredoxin of Peptococcus aerogenes (E 0 = - 400 mV)2) and in the high potential iron protein (HiPIP) isolated from the purple photosynthetic bacterium Chromatium vinosum... 

Approximate E° values are listed significant changes may be observed for proteins from different sources and with variations in solution conditions. High-potential iron protein, HiPIP bacteriochlorophyll, Bchl bacteriopheophytin, Bphe quinone. Q. 

High-potential Iron Proteins. These proteins (HiPIP s) have redox potentials some 0.7 V more positive than those of the ferredoxins. Chromatium HiPIP, the best characterized one, has a molecular weight of about 10,000, contains four iron atoms, four atoms of inorganic sulfur and four cysteine residues. X-ray investigation34 has shown that the iron and sulfur atoms form the tetrahedral array (Fig. 25-E-3). 

---