Histidine bridging

The sequence His-Cys-His has an extended chain conformation. In this sequence, the cysteine binds type 1 and the two histidines, two separate copper atoms of the trinuclear cluster, thus making the shortest distance from the type 1 copper to the trinuclear cluster 13 A. Within the cluster, the Cu-Cu distances are 3.4, 3.9, and 4 A. It should be noted that none of the histidines bridges the copper atoms, unlike SOD, in which a histidine bridges the copper and zinc. The pattern of histidine binding to the trinuclear cluster is rather clever two His-X-His pairs from separate domains, His-Cys-His (507-509) and His-Gly-His (106-108), bind the... 

Almost normal activity has been determined for the zinc-depleted enzyme (CU2E2SOD E = empty) (332), indicating that the presence of the histidine bridge is not essential for catalysis. 

Fig. 2. Reaction of diphosphoglycerate (2,3-DPG) and deoxyhemoglobin. The molecule fits into the central cavity of hemoglobin and forms salt bridges with valine NA(I)p, histidines NA2(2)p, H2I(I43)p, and lysine EF6(82)p. A, E, and E correspond to specific hemoglobin hehces and NA is the sequence...

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Early mutational studies of the Rieske protein from 6ci complexes have been performed with the intention of identifying the ligands of the Rieske cluster. These studies have shown that the four conserved cysteine residues as well as the two conserved histidine residues are essential for the insertion of the [2Fe-2S] cluster (44, 45). Small amounts of a Rieske cluster with altered properties were obtained in Rhodobacter capsulatus when the second cysteine in the cluster binding loop II (Cys 155, corresponding to Cys 160 in the bovine ISF) was replaced by serine (45). The fact that all four cysteine residues are essential in Rieske clusters from be complexes, but that only two cysteines are conserved in Rieske-type clusters, led to the suggestion that the Rieske protein may contain a disulfide bridge the disulfide bridge was finally shown to exist in the X-ray structure (9). 

The structure of the enzymatic site of superoxide dismutase. The proximity, with a histidine side chain (color screened) acting as a bridge between the metals. 

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